assertions.js

/*!
 * chai
 * http://chaijs.com
 * Copyright(c) 2011-2014 Jake Luer <jake@alogicalparadox.com>
 * MIT Licensed
 */

import {Assertion} from '../assertion.js';
import {AssertionError} from 'assertion-error';
import * as _ from '../utils/index.js';

const {flag} = _;

/**
 * ### Language Chains
 *
 * The following are provided as chainable getters to improve the readability
 * of your assertions.
 *
 * **Chains**
 *
 * - to
 * - be
 * - been
 * - is
 * - that
 * - which
 * - and
 * - has
 * - have
 * - with
 * - at
 * - of
 * - same
 * - but
 * - does
 * - still
 * - also
 *
 * @name language chains
 * @namespace BDD
 * @public
 */

[ 'to', 'be', 'been', 'is'
, 'and', 'has', 'have', 'with'
, 'that', 'which', 'at', 'of'
, 'same', 'but', 'does', 'still', "also" ].forEach(function (chain) {
  Assertion.addProperty(chain);
});

/**
 * ### .not
 *
 * Negates all assertions that follow in the chain.
 *
 *     expect(function () {}).to.not.throw();
 *     expect({a: 1}).to.not.have.property('b');
 *     expect([1, 2]).to.be.an('array').that.does.not.include(3);
 *
 * Just because you can negate any assertion with `.not` doesn't mean you
 * should. With great power comes great responsibility. It's often best to
 * assert that the one expected output was produced, rather than asserting
 * that one of countless unexpected outputs wasn't produced. See individual
 * assertions for specific guidance.
 *
 *     expect(2).to.equal(2); // Recommended
 *     expect(2).to.not.equal(1); // Not recommended
 *
 * @name not
 * @namespace BDD
 * @public
 */

Assertion.addProperty('not', function () {
  flag(this, 'negate', true);
});

/**
 * ### .deep
 *
 * Causes all `.equal`, `.include`, `.members`, `.keys`, and `.property`
 * assertions that follow in the chain to use deep equality instead of strict
 * (`===`) equality. See the `deep-eql` project page for info on the deep
 * equality algorithm: https://github.com/chaijs/deep-eql.
 *
 *     // Target object deeply (but not strictly) equals `{a: 1}`
 *     expect({a: 1}).to.deep.equal({a: 1});
 *     expect({a: 1}).to.not.equal({a: 1});
 *
 *     // Target array deeply (but not strictly) includes `{a: 1}`
 *     expect([{a: 1}]).to.deep.include({a: 1});
 *     expect([{a: 1}]).to.not.include({a: 1});
 *
 *     // Target object deeply (but not strictly) includes `x: {a: 1}`
 *     expect({x: {a: 1}}).to.deep.include({x: {a: 1}});
 *     expect({x: {a: 1}}).to.not.include({x: {a: 1}});
 *
 *     // Target array deeply (but not strictly) has member `{a: 1}`
 *     expect([{a: 1}]).to.have.deep.members([{a: 1}]);
 *     expect([{a: 1}]).to.not.have.members([{a: 1}]);
 *
 *     // Target set deeply (but not strictly) has key `{a: 1}`
 *     expect(new Set([{a: 1}])).to.have.deep.keys([{a: 1}]);
 *     expect(new Set([{a: 1}])).to.not.have.keys([{a: 1}]);
 *
 *     // Target object deeply (but not strictly) has property `x: {a: 1}`
 *     expect({x: {a: 1}}).to.have.deep.property('x', {a: 1});
 *     expect({x: {a: 1}}).to.not.have.property('x', {a: 1});
 *
 * @name deep
 * @namespace BDD
 * @public
 */

Assertion.addProperty('deep', function () {
  flag(this, 'deep', true);
});

/**
 * ### .nested
 *
 * Enables dot- and bracket-notation in all `.property` and `.include`
 * assertions that follow in the chain.
 *
 *     expect({a: {b: ['x', 'y']}}).to.have.nested.property('a.b[1]');
 *     expect({a: {b: ['x', 'y']}}).to.nested.include({'a.b[1]': 'y'});
 *
 * If `.` or `[]` are part of an actual property name, they can be escaped by
 * adding two backslashes before them.
 *
 *     expect({'.a': {'[b]': 'x'}}).to.have.nested.property('\\.a.\\[b\\]');
 *     expect({'.a': {'[b]': 'x'}}).to.nested.include({'\\.a.\\[b\\]': 'x'});
 *
 * `.nested` cannot be combined with `.own`.
 *
 * @name nested
 * @namespace BDD
 * @public
 */

Assertion.addProperty('nested', function () {
  flag(this, 'nested', true);
});

/**
 * ### .own
 *
 * Causes all `.property` and `.include` assertions that follow in the chain
 * to ignore inherited properties.
 *
 *     Object.prototype.b = 2;
 *
 *     expect({a: 1}).to.have.own.property('a');
 *     expect({a: 1}).to.have.property('b');
 *     expect({a: 1}).to.not.have.own.property('b');
 *
 *     expect({a: 1}).to.own.include({a: 1});
 *     expect({a: 1}).to.include({b: 2}).but.not.own.include({b: 2});
 *
 * `.own` cannot be combined with `.nested`.
 *
 * @name own
 * @namespace BDD
 * @public
 */

Assertion.addProperty('own', function () {
  flag(this, 'own', true);
});

/**
 * ### .ordered
 *
 * Causes all `.members` assertions that follow in the chain to require that
 * members be in the same order.
 *
 *     expect([1, 2]).to.have.ordered.members([1, 2])
 *       .but.not.have.ordered.members([2, 1]);
 *
 * When `.include` and `.ordered` are combined, the ordering begins at the
 * start of both arrays.
 *
 *     expect([1, 2, 3]).to.include.ordered.members([1, 2])
 *       .but.not.include.ordered.members([2, 3]);
 *
 * @name ordered
 * @namespace BDD
 * @public
 */

Assertion.addProperty('ordered', function () {
  flag(this, 'ordered', true);
});

/**
 * ### .any
 *
 * Causes all `.keys` assertions that follow in the chain to only require that
 * the target have at least one of the given keys. This is the opposite of
 * `.all`, which requires that the target have all of the given keys.
 *
 *     expect({a: 1, b: 2}).to.not.have.any.keys('c', 'd');
 *
 * See the `.keys` doc for guidance on when to use `.any` or `.all`.
 *
 * @name any
 * @namespace BDD
 * @public
 */

Assertion.addProperty('any', function () {
  flag(this, 'any', true);
  flag(this, 'all', false);
});

/**
 * ### .all
 *
 * Causes all `.keys` assertions that follow in the chain to require that the
 * target have all of the given keys. This is the opposite of `.any`, which
 * only requires that the target have at least one of the given keys.
 *
 *     expect({a: 1, b: 2}).to.have.all.keys('a', 'b');
 *
 * Note that `.all` is used by default when neither `.all` nor `.any` are
 * added earlier in the chain. However, it's often best to add `.all` anyway
 * because it improves readability.
 *
 * See the `.keys` doc for guidance on when to use `.any` or `.all`.
 *
 * @name all
 * @namespace BDD
 * @public
 */

Assertion.addProperty('all', function () {
  flag(this, 'all', true);
  flag(this, 'any', false);
});

const functionTypes = {
  'function': ['function', 'asyncfunction', 'generatorfunction', 'asyncgeneratorfunction'],
  'asyncfunction': ['asyncfunction', 'asyncgeneratorfunction'],
  'generatorfunction': ['generatorfunction', 'asyncgeneratorfunction'],
  'asyncgeneratorfunction': ['asyncgeneratorfunction']
}

/**
 * ### .a(type[, msg])
 *
 * Asserts that the target's type is equal to the given string `type`. Types
 * are case insensitive. See the utility file `./type-detect.js` for info on the
 * type detection algorithm.
 *
 *     expect('foo').to.be.a('string');
 *     expect({a: 1}).to.be.an('object');
 *     expect(null).to.be.a('null');
 *     expect(undefined).to.be.an('undefined');
 *     expect(new Error).to.be.an('error');
 *     expect(Promise.resolve()).to.be.a('promise');
 *     expect(new Float32Array).to.be.a('float32array');
 *     expect(Symbol()).to.be.a('symbol');
 *
 * `.a` supports objects that have a custom type set via `Symbol.toStringTag`.
 *
 *     var myObj = {
 *         [Symbol.toStringTag]: 'myCustomType'
 *     };
 *
 *     expect(myObj).to.be.a('myCustomType').but.not.an('object');
 *
 * It's often best to use `.a` to check a target's type before making more
 * assertions on the same target. That way, you avoid unexpected behavior from
 * any assertion that does different things based on the target's type.
 *
 *     expect([1, 2, 3]).to.be.an('array').that.includes(2);
 *     expect([]).to.be.an('array').that.is.empty;
 *
 * Add `.not` earlier in the chain to negate `.a`. However, it's often best to
 * assert that the target is the expected type, rather than asserting that it
 * isn't one of many unexpected types.
 *
 *     expect('foo').to.be.a('string'); // Recommended
 *     expect('foo').to.not.be.an('array'); // Not recommended
 *
 * `.a` accepts an optional `msg` argument which is a custom error message to
 * show when the assertion fails. The message can also be given as the second
 * argument to `expect`.
 *
 *     expect(1).to.be.a('string', 'nooo why fail??');
 *     expect(1, 'nooo why fail??').to.be.a('string');
 *
 * `.a` can also be used as a language chain to improve the readability of
 * your assertions.
 *
 *     expect({b: 2}).to.have.a.property('b');
 *
 * The alias `.an` can be used interchangeably with `.a`.
 *
 * @name a
 * @alias an
 * @param {string} type
 * @param {string} msg _optional_
 * @namespace BDD
 * @public
 */
function an (type, msg) {
  if (msg) flag(this, 'message', msg);
  type = type.toLowerCase();
  var obj = flag(this, 'object')
    , article = ~[ 'a', 'e', 'i', 'o', 'u' ].indexOf(type.charAt(0)) ? 'an ' : 'a ';

  const detectedType = _.type(obj).toLowerCase();

  if (functionTypes['function'].includes(type)) {
    this.assert(
        functionTypes[type].includes(detectedType)
      , 'expected #{this} to be ' + article + type
      , 'expected #{this} not to be ' + article + type
    );
  } else {
    this.assert(
      type === detectedType
      , 'expected #{this} to be ' + article + type
      , 'expected #{this} not to be ' + article + type
    );
  }
}

Assertion.addChainableMethod('an', an);
Assertion.addChainableMethod('a', an);

/**
 *
 * @param {unknown} a
 * @param {unknown} b
 * @returns {boolean}
 */
function SameValueZero(a, b) {
  return (_.isNaN(a) && _.isNaN(b)) || a === b;
}

/**
 *
 */
function includeChainingBehavior () {
  flag(this, 'contains', true);
}

/**
 * ### .include(val[, msg])
 *
 * When the target is a string, `.include` asserts that the given string `val`
 * is a substring of the target.
 *
 *     expect('foobar').to.include('foo');
 *
 * When the target is an array, `.include` asserts that the given `val` is a
 * member of the target.
 *
 *     expect([1, 2, 3]).to.include(2);
 *
 * When the target is an object, `.include` asserts that the given object
 * `val`'s properties are a subset of the target's properties.
 *
 *     expect({a: 1, b: 2, c: 3}).to.include({a: 1, b: 2});
 *
 * When the target is a Set or WeakSet, `.include` asserts that the given `val` is a
 * member of the target. SameValueZero equality algorithm is used.
 *
 *     expect(new Set([1, 2])).to.include(2);
 *
 * When the target is a Map, `.include` asserts that the given `val` is one of
 * the values of the target. SameValueZero equality algorithm is used.
 *
 *     expect(new Map([['a', 1], ['b', 2]])).to.include(2);
 *
 * Because `.include` does different things based on the target's type, it's
 * important to check the target's type before using `.include`. See the `.a`
 * doc for info on testing a target's type.
 *
 *     expect([1, 2, 3]).to.be.an('array').that.includes(2);
 *
 * By default, strict (`===`) equality is used to compare array members and
 * object properties. Add `.deep` earlier in the chain to use deep equality
 * instead (WeakSet targets are not supported). See the `deep-eql` project
 * page for info on the deep equality algorithm: https://github.com/chaijs/deep-eql.
 *
 *     // Target array deeply (but not strictly) includes `{a: 1}`
 *     expect([{a: 1}]).to.deep.include({a: 1});
 *     expect([{a: 1}]).to.not.include({a: 1});
 *
 *     // Target object deeply (but not strictly) includes `x: {a: 1}`
 *     expect({x: {a: 1}}).to.deep.include({x: {a: 1}});
 *     expect({x: {a: 1}}).to.not.include({x: {a: 1}});
 *
 * By default, all of the target's properties are searched when working with
 * objects. This includes properties that are inherited and/or non-enumerable.
 * Add `.own` earlier in the chain to exclude the target's inherited
 * properties from the search.
 *
 *     Object.prototype.b = 2;
 *
 *     expect({a: 1}).to.own.include({a: 1});
 *     expect({a: 1}).to.include({b: 2}).but.not.own.include({b: 2});
 *
 * Note that a target object is always only searched for `val`'s own
 * enumerable properties.
 *
 * `.deep` and `.own` can be combined.
 *
 *     expect({a: {b: 2}}).to.deep.own.include({a: {b: 2}});
 *
 * Add `.nested` earlier in the chain to enable dot- and bracket-notation when
 * referencing nested properties.
 *
 *     expect({a: {b: ['x', 'y']}}).to.nested.include({'a.b[1]': 'y'});
 *
 * If `.` or `[]` are part of an actual property name, they can be escaped by
 * adding two backslashes before them.
 *
 *     expect({'.a': {'[b]': 2}}).to.nested.include({'\\.a.\\[b\\]': 2});
 *
 * `.deep` and `.nested` can be combined.
 *
 *     expect({a: {b: [{c: 3}]}}).to.deep.nested.include({'a.b[0]': {c: 3}});
 *
 * `.own` and `.nested` cannot be combined.
 *
 * Add `.not` earlier in the chain to negate `.include`.
 *
 *     expect('foobar').to.not.include('taco');
 *     expect([1, 2, 3]).to.not.include(4);
 *
 * However, it's dangerous to negate `.include` when the target is an object.
 * The problem is that it creates uncertain expectations by asserting that the
 * target object doesn't have all of `val`'s key/value pairs but may or may
 * not have some of them. It's often best to identify the exact output that's
 * expected, and then write an assertion that only accepts that exact output.
 *
 * When the target object isn't even expected to have `val`'s keys, it's
 * often best to assert exactly that.
 *
 *     expect({c: 3}).to.not.have.any.keys('a', 'b'); // Recommended
 *     expect({c: 3}).to.not.include({a: 1, b: 2}); // Not recommended
 *
 * When the target object is expected to have `val`'s keys, it's often best to
 * assert that each of the properties has its expected value, rather than
 * asserting that each property doesn't have one of many unexpected values.
 *
 *     expect({a: 3, b: 4}).to.include({a: 3, b: 4}); // Recommended
 *     expect({a: 3, b: 4}).to.not.include({a: 1, b: 2}); // Not recommended
 *
 * `.include` accepts an optional `msg` argument which is a custom error
 * message to show when the assertion fails. The message can also be given as
 * the second argument to `expect`.
 *
 *     expect([1, 2, 3]).to.include(4, 'nooo why fail??');
 *     expect([1, 2, 3], 'nooo why fail??').to.include(4);
 *
 * `.include` can also be used as a language chain, causing all `.members` and
 * `.keys` assertions that follow in the chain to require the target to be a
 * superset of the expected set, rather than an identical set. Note that
 * `.members` ignores duplicates in the subset when `.include` is added.
 *
 *     // Target object's keys are a superset of ['a', 'b'] but not identical
 *     expect({a: 1, b: 2, c: 3}).to.include.all.keys('a', 'b');
 *     expect({a: 1, b: 2, c: 3}).to.not.have.all.keys('a', 'b');
 *
 *     // Target array is a superset of [1, 2] but not identical
 *     expect([1, 2, 3]).to.include.members([1, 2]);
 *     expect([1, 2, 3]).to.not.have.members([1, 2]);
 *
 *     // Duplicates in the subset are ignored
 *     expect([1, 2, 3]).to.include.members([1, 2, 2, 2]);
 *
 * Note that adding `.any` earlier in the chain causes the `.keys` assertion
 * to ignore `.include`.
 *
 *     // Both assertions are identical
 *     expect({a: 1}).to.include.any.keys('a', 'b');
 *     expect({a: 1}).to.have.any.keys('a', 'b');
 *
 * The aliases `.includes`, `.contain`, and `.contains` can be used
 * interchangeably with `.include`.
 *
 * @name include
 * @alias contain
 * @alias includes
 * @alias contains
 * @param {unknown} val
 * @param {string} msg _optional_
 * @namespace BDD
 * @public
 */
function include (val, msg) {
  if (msg) flag(this, 'message', msg);

  var obj = flag(this, 'object')
    , objType = _.type(obj).toLowerCase()
    , flagMsg = flag(this, 'message')
    , negate = flag(this, 'negate')
    , ssfi = flag(this, 'ssfi')
    , isDeep = flag(this, 'deep')
    , descriptor = isDeep ? 'deep ' : ''
    , isEql = isDeep ? flag(this, 'eql') : SameValueZero;

  flagMsg = flagMsg ? flagMsg + ': ' : '';

  var included = false;

  switch (objType) {
    case 'string':
      included = obj.indexOf(val) !== -1;
      break;

    case 'weakset':
      if (isDeep) {
        throw new AssertionError(
          flagMsg + 'unable to use .deep.include with WeakSet',
          undefined,
          ssfi
        );
      }

      included = obj.has(val);
      break;

    case 'map':
      obj.forEach(function (item) {
        included = included || isEql(item, val);
      });
      break;

    case 'set':
      if (isDeep) {
        obj.forEach(function (item) {
          included = included || isEql(item, val);
        });
      } else {
        included = obj.has(val);
      }
      break;

    case 'array':
      if (isDeep) {
        included = obj.some(function (item) {
          return isEql(item, val);
        })
      } else {
        included = obj.indexOf(val) !== -1;
      }
      break;

    default:
      // This block is for asserting a subset of properties in an object.
      // `_.expectTypes` isn't used here because `.include` should work with
      // objects with a custom `@@toStringTag`.
      if (val !== Object(val)) {
        throw new AssertionError(
          flagMsg + 'the given combination of arguments ('
          + objType + ' and '
          + _.type(val).toLowerCase() + ')'
          + ' is invalid for this assertion. '
          + 'You can use an array, a map, an object, a set, a string, '
          + 'or a weakset instead of a '
          + _.type(val).toLowerCase(),
          undefined,
          ssfi
        );
      }

      var props = Object.keys(val)
        , firstErr = null
        , numErrs = 0;

      props.forEach(function (prop) {
        var propAssertion = new Assertion(obj);
        _.transferFlags(this, propAssertion, true);
        flag(propAssertion, 'lockSsfi', true);

        if (!negate || props.length === 1) {
          propAssertion.property(prop, val[prop]);
          return;
        }

        try {
          propAssertion.property(prop, val[prop]);
        } catch (err) {
          if (!_.checkError.compatibleConstructor(err, AssertionError)) {
            throw err;
          }
          if (firstErr === null) firstErr = err;
          numErrs++;
        }
      }, this);

      // When validating .not.include with multiple properties, we only want
      // to throw an assertion error if all of the properties are included,
      // in which case we throw the first property assertion error that we
      // encountered.
      if (negate && props.length > 1 && numErrs === props.length) {
        throw firstErr;
      }
      return;
  }

  // Assert inclusion in collection or substring in a string.
  this.assert(
    included
    , 'expected #{this} to ' + descriptor + 'include ' + _.inspect(val)
    , 'expected #{this} to not ' + descriptor + 'include ' + _.inspect(val));
}

Assertion.addChainableMethod('include', include, includeChainingBehavior);
Assertion.addChainableMethod('contain', include, includeChainingBehavior);
Assertion.addChainableMethod('contains', include, includeChainingBehavior);
Assertion.addChainableMethod('includes', include, includeChainingBehavior);

/**
 * ### .ok
 *
 * Asserts that the target is a truthy value (considered `true` in boolean context).
 * However, it's often best to assert that the target is strictly (`===`) or
 * deeply equal to its expected value.
 *
 *     expect(1).to.equal(1); // Recommended
 *     expect(1).to.be.ok; // Not recommended
 *
 *     expect(true).to.be.true; // Recommended
 *     expect(true).to.be.ok; // Not recommended
 *
 * Add `.not` earlier in the chain to negate `.ok`.
 *
 *     expect(0).to.equal(0); // Recommended
 *     expect(0).to.not.be.ok; // Not recommended
 *
 *     expect(false).to.be.false; // Recommended
 *     expect(false).to.not.be.ok; // Not recommended
 *
 *     expect(null).to.be.null; // Recommended
 *     expect(null).to.not.be.ok; // Not recommended
 *
 *     expect(undefined).to.be.undefined; // Recommended
 *     expect(undefined).to.not.be.ok; // Not recommended
 *
 * A custom error message can be given as the second argument to `expect`.
 *
 *     expect(false, 'nooo why fail??').to.be.ok;
 *
 * @name ok
 * @namespace BDD
 * @public
 */
Assertion.addProperty('ok', function () {
  this.assert(
      flag(this, 'object')
    , 'expected #{this} to be truthy'
    , 'expected #{this} to be falsy');
});

/**
 * ### .true
 *
 * Asserts that the target is strictly (`===`) equal to `true`.
 *
 *     expect(true).to.be.true;
 *
 * Add `.not` earlier in the chain to negate `.true`. However, it's often best
 * to assert that the target is equal to its expected value, rather than not
 * equal to `true`.
 *
 *     expect(false).to.be.false; // Recommended
 *     expect(false).to.not.be.true; // Not recommended
 *
 *     expect(1).to.equal(1); // Recommended
 *     expect(1).to.not.be.true; // Not recommended
 *
 * A custom error message can be given as the second argument to `expect`.
 *
 *     expect(false, 'nooo why fail??').to.be.true;
 *
 * @name true
 * @namespace BDD
 * @public
 */
Assertion.addProperty('true', function () {
  this.assert(
      true === flag(this, 'object')
    , 'expected #{this} to be true'
    , 'expected #{this} to be false'
    , flag(this, 'negate') ? false : true
  );
});

/**
 * ### .callable
 *
 * Asserts that the target a callable function.
 *
 *     expect(console.log).to.be.callable;
 *
 * A custom error message can be given as the second argument to `expect`.
 *
 *     expect('not a function', 'nooo why fail??').to.be.callable;
 *
 * @name callable
 * @namespace BDD
 * @public
 */
Assertion.addProperty('callable', function () {
  const val = flag(this, 'object')
  const ssfi = flag(this, 'ssfi')
  const message = flag(this, 'message')
  const msg = message ? `${message}: ` : ''
  const negate = flag(this, 'negate');

  const assertionMessage = negate ?
    `${msg}expected ${_.inspect(val)} not to be a callable function` : 
    `${msg}expected ${_.inspect(val)} to be a callable function`;

  const isCallable = ['Function', 'AsyncFunction', 'GeneratorFunction', 'AsyncGeneratorFunction'].includes(_.type(val));

  if ((isCallable && negate) || (!isCallable && !negate)) {
    throw new AssertionError(
      assertionMessage,
      undefined,
      ssfi
    );
  }
});

/**
 * ### .false
 *
 * Asserts that the target is strictly (`===`) equal to `false`.
 *
 *     expect(false).to.be.false;
 *
 * Add `.not` earlier in the chain to negate `.false`. However, it's often
 * best to assert that the target is equal to its expected value, rather than
 * not equal to `false`.
 *
 *     expect(true).to.be.true; // Recommended
 *     expect(true).to.not.be.false; // Not recommended
 *
 *     expect(1).to.equal(1); // Recommended
 *     expect(1).to.not.be.false; // Not recommended
 *
 * A custom error message can be given as the second argument to `expect`.
 *
 *     expect(true, 'nooo why fail??').to.be.false;
 *
 * @name false
 * @namespace BDD
 * @public
 */
Assertion.addProperty('false', function () {
  this.assert(
      false === flag(this, 'object')
    , 'expected #{this} to be false'
    , 'expected #{this} to be true'
    , flag(this, 'negate') ? true : false
  );
});

/**
 * ### .null
 *
 * Asserts that the target is strictly (`===`) equal to `null`.
 *
 *     expect(null).to.be.null;
 *
 * Add `.not` earlier in the chain to negate `.null`. However, it's often best
 * to assert that the target is equal to its expected value, rather than not
 * equal to `null`.
 *
 *     expect(1).to.equal(1); // Recommended
 *     expect(1).to.not.be.null; // Not recommended
 *
 * A custom error message can be given as the second argument to `expect`.
 *
 *     expect(42, 'nooo why fail??').to.be.null;
 *
 * @name null
 * @namespace BDD
 * @public
 */
Assertion.addProperty('null', function () {
  this.assert(
      null === flag(this, 'object')
    , 'expected #{this} to be null'
    , 'expected #{this} not to be null'
  );
});

/**
 * ### .undefined
 *
 * Asserts that the target is strictly (`===`) equal to `undefined`.
 *
 *     expect(undefined).to.be.undefined;
 *
 * Add `.not` earlier in the chain to negate `.undefined`. However, it's often
 * best to assert that the target is equal to its expected value, rather than
 * not equal to `undefined`.
 *
 *     expect(1).to.equal(1); // Recommended
 *     expect(1).to.not.be.undefined; // Not recommended
 *
 * A custom error message can be given as the second argument to `expect`.
 *
 *     expect(42, 'nooo why fail??').to.be.undefined;
 *
 * @name undefined
 * @namespace BDD
 * @public
 */
Assertion.addProperty('undefined', function () {
  this.assert(
      undefined === flag(this, 'object')
    , 'expected #{this} to be undefined'
    , 'expected #{this} not to be undefined'
  );
});

/**
 * ### .NaN
 *
 * Asserts that the target is exactly `NaN`.
 *
 *     expect(NaN).to.be.NaN;
 *
 * Add `.not` earlier in the chain to negate `.NaN`. However, it's often best
 * to assert that the target is equal to its expected value, rather than not
 * equal to `NaN`.
 *
 *     expect('foo').to.equal('foo'); // Recommended
 *     expect('foo').to.not.be.NaN; // Not recommended
 *
 * A custom error message can be given as the second argument to `expect`.
 *
 *     expect(42, 'nooo why fail??').to.be.NaN;
 *
 * @name NaN
 * @namespace BDD
 * @public
 */
Assertion.addProperty('NaN', function () {
  this.assert(
      _.isNaN(flag(this, 'object'))
      , 'expected #{this} to be NaN'
      , 'expected #{this} not to be NaN'
  );
});

/**
 * ### .exist
 *
 * Asserts that the target is not strictly (`===`) equal to either `null` or
 * `undefined`. However, it's often best to assert that the target is equal to
 * its expected value.
 *
 *     expect(1).to.equal(1); // Recommended
 *     expect(1).to.exist; // Not recommended
 *
 *     expect(0).to.equal(0); // Recommended
 *     expect(0).to.exist; // Not recommended
 *
 * Add `.not` earlier in the chain to negate `.exist`.
 *
 *     expect(null).to.be.null; // Recommended
 *     expect(null).to.not.exist; // Not recommended
 *
 *     expect(undefined).to.be.undefined; // Recommended
 *     expect(undefined).to.not.exist; // Not recommended
 *
 * A custom error message can be given as the second argument to `expect`.
 *
 *     expect(null, 'nooo why fail??').to.exist;
 *
 * The alias `.exists` can be used interchangeably with `.exist`.
 *
 * @name exist
 * @alias exists
 * @namespace BDD
 * @public
 */
function assertExist () {
  var val = flag(this, 'object');
  this.assert(
      val !== null && val !== undefined
    , 'expected #{this} to exist'
    , 'expected #{this} to not exist'
  );
}

Assertion.addProperty('exist', assertExist);
Assertion.addProperty('exists', assertExist);

/**
 * ### .empty
 *
 * When the target is a string or array, `.empty` asserts that the target's
 * `length` property is strictly (`===`) equal to `0`.
 *
 *     expect([]).to.be.empty;
 *     expect('').to.be.empty;
 *
 * When the target is a map or set, `.empty` asserts that the target's `size`
 * property is strictly equal to `0`.
 *
 *     expect(new Set()).to.be.empty;
 *     expect(new Map()).to.be.empty;
 *
 * When the target is a non-function object, `.empty` asserts that the target
 * doesn't have any own enumerable properties. Properties with Symbol-based
 * keys are excluded from the count.
 *
 *     expect({}).to.be.empty;
 *
 * Because `.empty` does different things based on the target's type, it's
 * important to check the target's type before using `.empty`. See the `.a`
 * doc for info on testing a target's type.
 *
 *     expect([]).to.be.an('array').that.is.empty;
 *
 * Add `.not` earlier in the chain to negate `.empty`. However, it's often
 * best to assert that the target contains its expected number of values,
 * rather than asserting that it's not empty.
 *
 *     expect([1, 2, 3]).to.have.lengthOf(3); // Recommended
 *     expect([1, 2, 3]).to.not.be.empty; // Not recommended
 *
 *     expect(new Set([1, 2, 3])).to.have.property('size', 3); // Recommended
 *     expect(new Set([1, 2, 3])).to.not.be.empty; // Not recommended
 *
 *     expect(Object.keys({a: 1})).to.have.lengthOf(1); // Recommended
 *     expect({a: 1}).to.not.be.empty; // Not recommended
 *
 * A custom error message can be given as the second argument to `expect`.
 *
 *     expect([1, 2, 3], 'nooo why fail??').to.be.empty;
 *
 * @name empty
 * @namespace BDD
 * @public
 */
Assertion.addProperty('empty', function () {
  var val = flag(this, 'object')
    , ssfi = flag(this, 'ssfi')
    , flagMsg = flag(this, 'message')
    , itemsCount;

  flagMsg = flagMsg ? flagMsg + ': ' : '';

  switch (_.type(val).toLowerCase()) {
    case 'array':
    case 'string':
      itemsCount = val.length;
      break;
    case 'map':
    case 'set':
      itemsCount = val.size;
      break;
    case 'weakmap':
    case 'weakset':
      throw new AssertionError(
        flagMsg + '.empty was passed a weak collection',
        undefined,
        ssfi
      );
    case 'function':
      var msg = flagMsg + '.empty was passed a function ' + _.getName(val);
      throw new AssertionError(msg.trim(), undefined, ssfi);
    default:
      if (val !== Object(val)) {
        throw new AssertionError(
          flagMsg + '.empty was passed non-string primitive ' + _.inspect(val),
          undefined,
          ssfi
        );
      }
      itemsCount = Object.keys(val).length;
  }

  this.assert(
      0 === itemsCount
    , 'expected #{this} to be empty'
    , 'expected #{this} not to be empty'
  );
});

/**
 * ### .arguments
 *
 * Asserts that the target is an `arguments` object.
 *
 *     function test () {
 *         expect(arguments).to.be.arguments;
 *     }
 *
 *     test();
 *
 * Add `.not` earlier in the chain to negate `.arguments`. However, it's often
 * best to assert which type the target is expected to be, rather than
 * asserting that it’s not an `arguments` object.
 *
 *     expect('foo').to.be.a('string'); // Recommended
 *     expect('foo').to.not.be.arguments; // Not recommended
 *
 * A custom error message can be given as the second argument to `expect`.
 *
 *     expect({}, 'nooo why fail??').to.be.arguments;
 *
 * The alias `.Arguments` can be used interchangeably with `.arguments`.
 *
 * @name arguments
 * @alias Arguments
 * @namespace BDD
 * @public
 */
function checkArguments () {
  var obj = flag(this, 'object')
    , type = _.type(obj);
  this.assert(
      'Arguments' === type
    , 'expected #{this} to be arguments but got ' + type
    , 'expected #{this} to not be arguments'
  );
}

Assertion.addProperty('arguments', checkArguments);
Assertion.addProperty('Arguments', checkArguments);

/**
 * ### .equal(val[, msg])
 *
 * Asserts that the target is strictly (`===`) equal to the given `val`.
 *
 *     expect(1).to.equal(1);
 *     expect('foo').to.equal('foo');
 *
 * Add `.deep` earlier in the chain to use deep equality instead. See the
 * `deep-eql` project page for info on the deep equality algorithm:
 * https://github.com/chaijs/deep-eql.
 *
 *     // Target object deeply (but not strictly) equals `{a: 1}`
 *     expect({a: 1}).to.deep.equal({a: 1});
 *     expect({a: 1}).to.not.equal({a: 1});
 *
 *     // Target array deeply (but not strictly) equals `[1, 2]`
 *     expect([1, 2]).to.deep.equal([1, 2]);
 *     expect([1, 2]).to.not.equal([1, 2]);
 *
 * Add `.not` earlier in the chain to negate `.equal`. However, it's often
 * best to assert that the target is equal to its expected value, rather than
 * not equal to one of countless unexpected values.
 *
 *     expect(1).to.equal(1); // Recommended
 *     expect(1).to.not.equal(2); // Not recommended
 *
 * `.equal` accepts an optional `msg` argument which is a custom error message
 * to show when the assertion fails. The message can also be given as the
 * second argument to `expect`.
 *
 *     expect(1).to.equal(2, 'nooo why fail??');
 *     expect(1, 'nooo why fail??').to.equal(2);
 *
 * The aliases `.equals` and `eq` can be used interchangeably with `.equal`.
 *
 * @name equal
 * @alias equals
 * @alias eq
 * @param {unknown} val
 * @param {string} msg _optional_
 * @namespace BDD
 * @public
 */
function assertEqual (val, msg) {
  if (msg) flag(this, 'message', msg);
  var obj = flag(this, 'object');
  if (flag(this, 'deep')) {
    var prevLockSsfi = flag(this, 'lockSsfi');
    flag(this, 'lockSsfi', true);
    this.eql(val);
    flag(this, 'lockSsfi', prevLockSsfi);
  } else {
    this.assert(
        val === obj
      , 'expected #{this} to equal #{exp}'
      , 'expected #{this} to not equal #{exp}'
      , val
      , this._obj
      , true
    );
  }
}

Assertion.addMethod('equal', assertEqual);
Assertion.addMethod('equals', assertEqual);
Assertion.addMethod('eq', assertEqual);

/**
 * ### .eql(obj[, msg])
 *
 * Asserts that the target is deeply equal to the given `obj`. See the
 * `deep-eql` project page for info on the deep equality algorithm:
 * https://github.com/chaijs/deep-eql.
 *
 *     // Target object is deeply (but not strictly) equal to {a: 1}
 *     expect({a: 1}).to.eql({a: 1}).but.not.equal({a: 1});
 *
 *     // Target array is deeply (but not strictly) equal to [1, 2]
 *     expect([1, 2]).to.eql([1, 2]).but.not.equal([1, 2]);
 *
 * Add `.not` earlier in the chain to negate `.eql`. However, it's often best
 * to assert that the target is deeply equal to its expected value, rather
 * than not deeply equal to one of countless unexpected values.
 *
 *     expect({a: 1}).to.eql({a: 1}); // Recommended
 *     expect({a: 1}).to.not.eql({b: 2}); // Not recommended
 *
 * `.eql` accepts an optional `msg` argument which is a custom error message
 * to show when the assertion fails. The message can also be given as the
 * second argument to `expect`.
 *
 *     expect({a: 1}).to.eql({b: 2}, 'nooo why fail??');
 *     expect({a: 1}, 'nooo why fail??').to.eql({b: 2});
 *
 * The alias `.eqls` can be used interchangeably with `.eql`.
 *
 * The `.deep.equal` assertion is almost identical to `.eql` but with one
 * difference: `.deep.equal` causes deep equality comparisons to also be used
 * for any other assertions that follow in the chain.
 *
 * @name eql
 * @alias eqls
 * @param {unknown} obj
 * @param {string} msg _optional_
 * @namespace BDD
 * @public
 */
function assertEql(obj, msg) {
  if (msg) flag(this, 'message', msg);
  var eql = flag(this, 'eql');
  this.assert(
      eql(obj, flag(this, 'object'))
    , 'expected #{this} to deeply equal #{exp}'
    , 'expected #{this} to not deeply equal #{exp}'
    , obj
    , this._obj
    , true
  );
}

Assertion.addMethod('eql', assertEql);
Assertion.addMethod('eqls', assertEql);

/**
 * ### .above(n[, msg])
 *
 * Asserts that the target is a number or a date greater than the given number or date `n` respectively.
 * However, it's often best to assert that the target is equal to its expected
 * value.
 *
 *     expect(2).to.equal(2); // Recommended
 *     expect(2).to.be.above(1); // Not recommended
 *
 * Add `.lengthOf` earlier in the chain to assert that the target's `length`
 * or `size` is greater than the given number `n`.
 *
 *     expect('foo').to.have.lengthOf(3); // Recommended
 *     expect('foo').to.have.lengthOf.above(2); // Not recommended
 *
 *     expect([1, 2, 3]).to.have.lengthOf(3); // Recommended
 *     expect([1, 2, 3]).to.have.lengthOf.above(2); // Not recommended
 *
 * Add `.not` earlier in the chain to negate `.above`.
 *
 *     expect(2).to.equal(2); // Recommended
 *     expect(1).to.not.be.above(2); // Not recommended
 *
 * `.above` accepts an optional `msg` argument which is a custom error message
 * to show when the assertion fails. The message can also be given as the
 * second argument to `expect`.
 *
 *     expect(1).to.be.above(2, 'nooo why fail??');
 *     expect(1, 'nooo why fail??').to.be.above(2);
 *
 * The aliases `.gt` and `.greaterThan` can be used interchangeably with
 * `.above`.
 *
 * @name above
 * @alias gt
 * @alias greaterThan
 * @param {number} n
 * @param {string} msg _optional_
 * @namespace BDD
 * @public
 */
function assertAbove (n, msg) {
  if (msg) flag(this, 'message', msg);
  var obj = flag(this, 'object')
    , doLength = flag(this, 'doLength')
    , flagMsg = flag(this, 'message')
    , msgPrefix = ((flagMsg) ? flagMsg + ': ' : '')
    , ssfi = flag(this, 'ssfi')
    , objType = _.type(obj).toLowerCase()
    , nType = _.type(n).toLowerCase()
    , errorMessage
    , shouldThrow = true;

  if (doLength && objType !== 'map' && objType !== 'set') {
    new Assertion(obj, flagMsg, ssfi, true).to.have.property('length');
  }

  if (!doLength && (objType === 'date' && nType !== 'date')) {
    errorMessage = msgPrefix + 'the argument to above must be a date';
  } else if (nType !== 'number' && (doLength || objType === 'number')) {
    errorMessage = msgPrefix + 'the argument to above must be a number';
  } else if (!doLength && (objType !== 'date' && objType !== 'number')) {
    var printObj = (objType === 'string') ? "'" + obj + "'" : obj;
    errorMessage = msgPrefix + 'expected ' + printObj + ' to be a number or a date';
  } else {
    shouldThrow = false;
  }

  if (shouldThrow) {
    throw new AssertionError(errorMessage, undefined, ssfi);
  }

  if (doLength) {
    var descriptor = 'length'
      , itemsCount;
    if (objType === 'map' || objType === 'set') {
      descriptor = 'size';
      itemsCount = obj.size;
    } else {
      itemsCount = obj.length;
    }
    this.assert(
        itemsCount > n
      , 'expected #{this} to have a ' + descriptor + ' above #{exp} but got #{act}'
      , 'expected #{this} to not have a ' + descriptor + ' above #{exp}'
      , n
      , itemsCount
    );
  } else {
    this.assert(
        obj > n
      , 'expected #{this} to be above #{exp}'
      , 'expected #{this} to be at most #{exp}'
      , n
    );
  }
}

Assertion.addMethod('above', assertAbove);
Assertion.addMethod('gt', assertAbove);
Assertion.addMethod('greaterThan', assertAbove);

/**
 * ### .least(n[, msg])
 *
 * Asserts that the target is a number or a date greater than or equal to the given
 * number or date `n` respectively. However, it's often best to assert that the target is equal to
 * its expected value.
 *
 *     expect(2).to.equal(2); // Recommended
 *     expect(2).to.be.at.least(1); // Not recommended
 *     expect(2).to.be.at.least(2); // Not recommended
 *
 * Add `.lengthOf` earlier in the chain to assert that the target's `length`
 * or `size` is greater than or equal to the given number `n`.
 *
 *     expect('foo').to.have.lengthOf(3); // Recommended
 *     expect('foo').to.have.lengthOf.at.least(2); // Not recommended
 *
 *     expect([1, 2, 3]).to.have.lengthOf(3); // Recommended
 *     expect([1, 2, 3]).to.have.lengthOf.at.least(2); // Not recommended
 *
 * Add `.not` earlier in the chain to negate `.least`.
 *
 *     expect(1).to.equal(1); // Recommended
 *     expect(1).to.not.be.at.least(2); // Not recommended
 *
 * `.least` accepts an optional `msg` argument which is a custom error message
 * to show when the assertion fails. The message can also be given as the
 * second argument to `expect`.
 *
 *     expect(1).to.be.at.least(2, 'nooo why fail??');
 *     expect(1, 'nooo why fail??').to.be.at.least(2);
 *
 * The aliases `.gte` and `.greaterThanOrEqual` can be used interchangeably with
 * `.least`.
 *
 * @name least
 * @alias gte
 * @alias greaterThanOrEqual
 * @param {number} n
 * @param {string} msg _optional_
 * @namespace BDD
 * @public
 */
function assertLeast (n, msg) {
  if (msg) flag(this, 'message', msg);
  var obj = flag(this, 'object')
    , doLength = flag(this, 'doLength')
    , flagMsg = flag(this, 'message')
    , msgPrefix = ((flagMsg) ? flagMsg + ': ' : '')
    , ssfi = flag(this, 'ssfi')
    , objType = _.type(obj).toLowerCase()
    , nType = _.type(n).toLowerCase()
    , errorMessage
    , shouldThrow = true;

  if (doLength && objType !== 'map' && objType !== 'set') {
    new Assertion(obj, flagMsg, ssfi, true).to.have.property('length');
  }

  if (!doLength && (objType === 'date' && nType !== 'date')) {
    errorMessage = msgPrefix + 'the argument to least must be a date';
  } else if (nType !== 'number' && (doLength || objType === 'number')) {
    errorMessage = msgPrefix + 'the argument to least must be a number';
  } else if (!doLength && (objType !== 'date' && objType !== 'number')) {
    var printObj = (objType === 'string') ? "'" + obj + "'" : obj;
    errorMessage = msgPrefix + 'expected ' + printObj + ' to be a number or a date';
  } else {
    shouldThrow = false;
  }

  if (shouldThrow) {
    throw new AssertionError(errorMessage, undefined, ssfi);
  }

  if (doLength) {
    var descriptor = 'length'
      , itemsCount;
    if (objType === 'map' || objType === 'set') {
      descriptor = 'size';
      itemsCount = obj.size;
    } else {
      itemsCount = obj.length;
    }
    this.assert(
        itemsCount >= n
      , 'expected #{this} to have a ' + descriptor + ' at least #{exp} but got #{act}'
      , 'expected #{this} to have a ' + descriptor + ' below #{exp}'
      , n
      , itemsCount
    );
  } else {
    this.assert(
        obj >= n
      , 'expected #{this} to be at least #{exp}'
      , 'expected #{this} to be below #{exp}'
      , n
    );
  }
}

Assertion.addMethod('least', assertLeast);
Assertion.addMethod('gte', assertLeast);
Assertion.addMethod('greaterThanOrEqual', assertLeast);

/**
 * ### .below(n[, msg])
 *
 * Asserts that the target is a number or a date less than the given number or date `n` respectively.
 * However, it's often best to assert that the target is equal to its expected
 * value.
 *
 *     expect(1).to.equal(1); // Recommended
 *     expect(1).to.be.below(2); // Not recommended
 *
 * Add `.lengthOf` earlier in the chain to assert that the target's `length`
 * or `size` is less than the given number `n`.
 *
 *     expect('foo').to.have.lengthOf(3); // Recommended
 *     expect('foo').to.have.lengthOf.below(4); // Not recommended
 *
 *     expect([1, 2, 3]).to.have.length(3); // Recommended
 *     expect([1, 2, 3]).to.have.lengthOf.below(4); // Not recommended
 *
 * Add `.not` earlier in the chain to negate `.below`.
 *
 *     expect(2).to.equal(2); // Recommended
 *     expect(2).to.not.be.below(1); // Not recommended
 *
 * `.below` accepts an optional `msg` argument which is a custom error message
 * to show when the assertion fails. The message can also be given as the
 * second argument to `expect`.
 *
 *     expect(2).to.be.below(1, 'nooo why fail??');
 *     expect(2, 'nooo why fail??').to.be.below(1);
 *
 * The aliases `.lt` and `.lessThan` can be used interchangeably with
 * `.below`.
 *
 * @name below
 * @alias lt
 * @alias lessThan
 * @param {number} n
 * @param {string} msg _optional_
 * @namespace BDD
 * @public
 */
function assertBelow (n, msg) {
  if (msg) flag(this, 'message', msg);
  var obj = flag(this, 'object')
    , doLength = flag(this, 'doLength')
    , flagMsg = flag(this, 'message')
    , msgPrefix = ((flagMsg) ? flagMsg + ': ' : '')
    , ssfi = flag(this, 'ssfi')
    , objType = _.type(obj).toLowerCase()
    , nType = _.type(n).toLowerCase()
    , errorMessage
    , shouldThrow = true;

  if (doLength && objType !== 'map' && objType !== 'set') {
    new Assertion(obj, flagMsg, ssfi, true).to.have.property('length');
  }

  if (!doLength && (objType === 'date' && nType !== 'date')) {
    errorMessage = msgPrefix + 'the argument to below must be a date';
  } else if (nType !== 'number' && (doLength || objType === 'number')) {
    errorMessage = msgPrefix + 'the argument to below must be a number';
  } else if (!doLength && (objType !== 'date' && objType !== 'number')) {
    var printObj = (objType === 'string') ? "'" + obj + "'" : obj;
    errorMessage = msgPrefix + 'expected ' + printObj + ' to be a number or a date';
  } else {
    shouldThrow = false;
  }

  if (shouldThrow) {
    throw new AssertionError(errorMessage, undefined, ssfi);
  }

  if (doLength) {
    var descriptor = 'length'
      , itemsCount;
    if (objType === 'map' || objType === 'set') {
      descriptor = 'size';
      itemsCount = obj.size;
    } else {
      itemsCount = obj.length;
    }
    this.assert(
        itemsCount < n
      , 'expected #{this} to have a ' + descriptor + ' below #{exp} but got #{act}'
      , 'expected #{this} to not have a ' + descriptor + ' below #{exp}'
      , n
      , itemsCount
    );
  } else {
    this.assert(
        obj < n
      , 'expected #{this} to be below #{exp}'
      , 'expected #{this} to be at least #{exp}'
      , n
    );
  }
}

Assertion.addMethod('below', assertBelow);
Assertion.addMethod('lt', assertBelow);
Assertion.addMethod('lessThan', assertBelow);

/**
 * ### .most(n[, msg])
 *
 * Asserts that the target is a number or a date less than or equal to the given number
 * or date `n` respectively. However, it's often best to assert that the target is equal to its
 * expected value.
 *
 *     expect(1).to.equal(1); // Recommended
 *     expect(1).to.be.at.most(2); // Not recommended
 *     expect(1).to.be.at.most(1); // Not recommended
 *
 * Add `.lengthOf` earlier in the chain to assert that the target's `length`
 * or `size` is less than or equal to the given number `n`.
 *
 *     expect('foo').to.have.lengthOf(3); // Recommended
 *     expect('foo').to.have.lengthOf.at.most(4); // Not recommended
 *
 *     expect([1, 2, 3]).to.have.lengthOf(3); // Recommended
 *     expect([1, 2, 3]).to.have.lengthOf.at.most(4); // Not recommended
 *
 * Add `.not` earlier in the chain to negate `.most`.
 *
 *     expect(2).to.equal(2); // Recommended
 *     expect(2).to.not.be.at.most(1); // Not recommended
 *
 * `.most` accepts an optional `msg` argument which is a custom error message
 * to show when the assertion fails. The message can also be given as the
 * second argument to `expect`.
 *
 *     expect(2).to.be.at.most(1, 'nooo why fail??');
 *     expect(2, 'nooo why fail??').to.be.at.most(1);
 *
 * The aliases `.lte` and `.lessThanOrEqual` can be used interchangeably with
 * `.most`.
 *
 * @name most
 * @alias lte
 * @alias lessThanOrEqual
 * @param {number} n
 * @param {string} msg _optional_
 * @namespace BDD
 * @public
 */
function assertMost (n, msg) {
  if (msg) flag(this, 'message', msg);
  var obj = flag(this, 'object')
    , doLength = flag(this, 'doLength')
    , flagMsg = flag(this, 'message')
    , msgPrefix = ((flagMsg) ? flagMsg + ': ' : '')
    , ssfi = flag(this, 'ssfi')
    , objType = _.type(obj).toLowerCase()
    , nType = _.type(n).toLowerCase()
    , errorMessage
    , shouldThrow = true;

  if (doLength && objType !== 'map' && objType !== 'set') {
    new Assertion(obj, flagMsg, ssfi, true).to.have.property('length');
  }

  if (!doLength && (objType === 'date' && nType !== 'date')) {
    errorMessage = msgPrefix + 'the argument to most must be a date';
  } else if (nType !== 'number' && (doLength || objType === 'number')) {
    errorMessage = msgPrefix + 'the argument to most must be a number';
  } else if (!doLength && (objType !== 'date' && objType !== 'number')) {
    var printObj = (objType === 'string') ? "'" + obj + "'" : obj;
    errorMessage = msgPrefix + 'expected ' + printObj + ' to be a number or a date';
  } else {
    shouldThrow = false;
  }

  if (shouldThrow) {
    throw new AssertionError(errorMessage, undefined, ssfi);
  }

  if (doLength) {
    var descriptor = 'length'
      , itemsCount;
    if (objType === 'map' || objType === 'set') {
      descriptor = 'size';
      itemsCount = obj.size;
    } else {
      itemsCount = obj.length;
    }
    this.assert(
        itemsCount <= n
      , 'expected #{this} to have a ' + descriptor + ' at most #{exp} but got #{act}'
      , 'expected #{this} to have a ' + descriptor + ' above #{exp}'
      , n
      , itemsCount
    );
  } else {
    this.assert(
        obj <= n
      , 'expected #{this} to be at most #{exp}'
      , 'expected #{this} to be above #{exp}'
      , n
    );
  }
}

Assertion.addMethod('most', assertMost);
Assertion.addMethod('lte', assertMost);
Assertion.addMethod('lessThanOrEqual', assertMost);

/**
 * ### .within(start, finish[, msg])
 *
 * Asserts that the target is a number or a date greater than or equal to the given
 * number or date `start`, and less than or equal to the given number or date `finish` respectively.
 * However, it's often best to assert that the target is equal to its expected
 * value.
 *
 *     expect(2).to.equal(2); // Recommended
 *     expect(2).to.be.within(1, 3); // Not recommended
 *     expect(2).to.be.within(2, 3); // Not recommended
 *     expect(2).to.be.within(1, 2); // Not recommended
 *
 * Add `.lengthOf` earlier in the chain to assert that the target's `length`
 * or `size` is greater than or equal to the given number `start`, and less
 * than or equal to the given number `finish`.
 *
 *     expect('foo').to.have.lengthOf(3); // Recommended
 *     expect('foo').to.have.lengthOf.within(2, 4); // Not recommended
 *
 *     expect([1, 2, 3]).to.have.lengthOf(3); // Recommended
 *     expect([1, 2, 3]).to.have.lengthOf.within(2, 4); // Not recommended
 *
 * Add `.not` earlier in the chain to negate `.within`.
 *
 *     expect(1).to.equal(1); // Recommended
 *     expect(1).to.not.be.within(2, 4); // Not recommended
 *
 * `.within` accepts an optional `msg` argument which is a custom error
 * message to show when the assertion fails. The message can also be given as
 * the second argument to `expect`.
 *
 *     expect(4).to.be.within(1, 3, 'nooo why fail??');
 *     expect(4, 'nooo why fail??').to.be.within(1, 3);
 *
 * @name within
 * @param {number} start lower bound inclusive
 * @param {number} finish upper bound inclusive
 * @param {string} msg _optional_
 * @namespace BDD
 * @public
 */
Assertion.addMethod('within', function (start, finish, msg) {
  if (msg) flag(this, 'message', msg);
  var obj = flag(this, 'object')
    , doLength = flag(this, 'doLength')
    , flagMsg = flag(this, 'message')
    , msgPrefix = ((flagMsg) ? flagMsg + ': ' : '')
    , ssfi = flag(this, 'ssfi')
    , objType = _.type(obj).toLowerCase()
    , startType = _.type(start).toLowerCase()
    , finishType = _.type(finish).toLowerCase()
    , errorMessage
    , shouldThrow = true
    , range = (startType === 'date' && finishType === 'date')
        ? start.toISOString() + '..' + finish.toISOString()
        : start + '..' + finish;

  if (doLength && objType !== 'map' && objType !== 'set') {
    new Assertion(obj, flagMsg, ssfi, true).to.have.property('length');
  }

  if (!doLength && (objType === 'date' && (startType !== 'date' || finishType !== 'date'))) {
    errorMessage = msgPrefix + 'the arguments to within must be dates';
  } else if ((startType !== 'number' || finishType !== 'number') && (doLength || objType === 'number')) {
    errorMessage = msgPrefix + 'the arguments to within must be numbers';
  } else if (!doLength && (objType !== 'date' && objType !== 'number')) {
    var printObj = (objType === 'string') ? "'" + obj + "'" : obj;
    errorMessage = msgPrefix + 'expected ' + printObj + ' to be a number or a date';
  } else {
    shouldThrow = false;
  }

  if (shouldThrow) {
    throw new AssertionError(errorMessage, undefined, ssfi);
  }

  if (doLength) {
    var descriptor = 'length'
      , itemsCount;
    if (objType === 'map' || objType === 'set') {
      descriptor = 'size';
      itemsCount = obj.size;
    } else {
      itemsCount = obj.length;
    }
    this.assert(
        itemsCount >= start && itemsCount <= finish
      , 'expected #{this} to have a ' + descriptor + ' within ' + range
      , 'expected #{this} to not have a ' + descriptor + ' within ' + range
    );
  } else {
    this.assert(
        obj >= start && obj <= finish
      , 'expected #{this} to be within ' + range
      , 'expected #{this} to not be within ' + range
    );
  }
});

/**
 * ### .instanceof(constructor[, msg])
 *
 * Asserts that the target is an instance of the given `constructor`.
 *
 *     function Cat () { }
 *
 *     expect(new Cat()).to.be.an.instanceof(Cat);
 *     expect([1, 2]).to.be.an.instanceof(Array);
 *
 * Add `.not` earlier in the chain to negate `.instanceof`.
 *
 *     expect({a: 1}).to.not.be.an.instanceof(Array);
 *
 * `.instanceof` accepts an optional `msg` argument which is a custom error
 * message to show when the assertion fails. The message can also be given as
 * the second argument to `expect`.
 *
 *     expect(1).to.be.an.instanceof(Array, 'nooo why fail??');
 *     expect(1, 'nooo why fail??').to.be.an.instanceof(Array);
 *
 * Due to limitations in ES5, `.instanceof` may not always work as expected
 * when using a transpiler such as Babel or TypeScript. In particular, it may
 * produce unexpected results when subclassing built-in object such as
 * `Array`, `Error`, and `Map`. See your transpiler's docs for details:
 *
 * - ([Babel](https://babeljs.io/docs/usage/caveats/#classes))
 * - ([TypeScript](https://github.com/Microsoft/TypeScript/wiki/Breaking-Changes#extending-built-ins-like-error-array-and-map-may-no-longer-work))
 *
 * The alias `.instanceOf` can be used interchangeably with `.instanceof`.
 *
 * @name instanceof
 * @param {unknown} constructor
 * @param {string} msg _optional_
 * @alias instanceOf
 * @namespace BDD
 * @public
 */
function assertInstanceOf (constructor, msg) {
  if (msg) flag(this, 'message', msg);

  var target = flag(this, 'object')
  var ssfi = flag(this, 'ssfi');
  var flagMsg = flag(this, 'message');

  try {
    var isInstanceOf = target instanceof constructor;
  } catch (err) {
    if (err instanceof TypeError) {
      flagMsg = flagMsg ? flagMsg + ': ' : '';
      throw new AssertionError(
        flagMsg + 'The instanceof assertion needs a constructor but '
          + _.type(constructor) + ' was given.',
        undefined,
        ssfi
      );
    }
    throw err;
  }

  var name = _.getName(constructor);
  if (name == null) {
    name = 'an unnamed constructor';
  }

  this.assert(
      isInstanceOf
    , 'expected #{this} to be an instance of ' + name
    , 'expected #{this} to not be an instance of ' + name
  );
};

Assertion.addMethod('instanceof', assertInstanceOf);
Assertion.addMethod('instanceOf', assertInstanceOf);

/**
 * ### .property(name[, val[, msg]])
 *
 * Asserts that the target has a property with the given key `name`.
 *
 *     expect({a: 1}).to.have.property('a');
 *
 * When `val` is provided, `.property` also asserts that the property's value
 * is equal to the given `val`.
 *
 *     expect({a: 1}).to.have.property('a', 1);
 *
 * By default, strict (`===`) equality is used. Add `.deep` earlier in the
 * chain to use deep equality instead. See the `deep-eql` project page for
 * info on the deep equality algorithm: https://github.com/chaijs/deep-eql.
 *
 *     // Target object deeply (but not strictly) has property `x: {a: 1}`
 *     expect({x: {a: 1}}).to.have.deep.property('x', {a: 1});
 *     expect({x: {a: 1}}).to.not.have.property('x', {a: 1});
 *
 * The target's enumerable and non-enumerable properties are always included
 * in the search. By default, both own and inherited properties are included.
 * Add `.own` earlier in the chain to exclude inherited properties from the
 * search.
 *
 *     Object.prototype.b = 2;
 *
 *     expect({a: 1}).to.have.own.property('a');
 *     expect({a: 1}).to.have.own.property('a', 1);
 *     expect({a: 1}).to.have.property('b');
 *     expect({a: 1}).to.not.have.own.property('b');
 *
 * `.deep` and `.own` can be combined.
 *
 *     expect({x: {a: 1}}).to.have.deep.own.property('x', {a: 1});
 *
 * Add `.nested` earlier in the chain to enable dot- and bracket-notation when
 * referencing nested properties.
 *
 *     expect({a: {b: ['x', 'y']}}).to.have.nested.property('a.b[1]');
 *     expect({a: {b: ['x', 'y']}}).to.have.nested.property('a.b[1]', 'y');
 *
 * If `.` or `[]` are part of an actual property name, they can be escaped by
 * adding two backslashes before them.
 *
 *     expect({'.a': {'[b]': 'x'}}).to.have.nested.property('\\.a.\\[b\\]');
 *
 * `.deep` and `.nested` can be combined.
 *
 *     expect({a: {b: [{c: 3}]}})
 *       .to.have.deep.nested.property('a.b[0]', {c: 3});
 *
 * `.own` and `.nested` cannot be combined.
 *
 * Add `.not` earlier in the chain to negate `.property`.
 *
 *     expect({a: 1}).to.not.have.property('b');
 *
 * However, it's dangerous to negate `.property` when providing `val`. The
 * problem is that it creates uncertain expectations by asserting that the
 * target either doesn't have a property with the given key `name`, or that it
 * does have a property with the given key `name` but its value isn't equal to
 * the given `val`. It's often best to identify the exact output that's
 * expected, and then write an assertion that only accepts that exact output.
 *
 * When the target isn't expected to have a property with the given key
 * `name`, it's often best to assert exactly that.
 *
 *     expect({b: 2}).to.not.have.property('a'); // Recommended
 *     expect({b: 2}).to.not.have.property('a', 1); // Not recommended
 *
 * When the target is expected to have a property with the given key `name`,
 * it's often best to assert that the property has its expected value, rather
 * than asserting that it doesn't have one of many unexpected values.
 *
 *     expect({a: 3}).to.have.property('a', 3); // Recommended
 *     expect({a: 3}).to.not.have.property('a', 1); // Not recommended
 *
 * `.property` changes the target of any assertions that follow in the chain
 * to be the value of the property from the original target object.
 *
 *     expect({a: 1}).to.have.property('a').that.is.a('number');
 *
 * `.property` accepts an optional `msg` argument which is a custom error
 * message to show when the assertion fails. The message can also be given as
 * the second argument to `expect`. When not providing `val`, only use the
 * second form.
 *
 *     // Recommended
 *     expect({a: 1}).to.have.property('a', 2, 'nooo why fail??');
 *     expect({a: 1}, 'nooo why fail??').to.have.property('a', 2);
 *     expect({a: 1}, 'nooo why fail??').to.have.property('b');
 *
 *     // Not recommended
 *     expect({a: 1}).to.have.property('b', undefined, 'nooo why fail??');
 *
 * The above assertion isn't the same thing as not providing `val`. Instead,
 * it's asserting that the target object has a `b` property that's equal to
 * `undefined`.
 *
 * The assertions `.ownProperty` and `.haveOwnProperty` can be used
 * interchangeably with `.own.property`.
 *
 * @name property
 * @param {string} name
 * @param {unknown} val (optional)
 * @param {string} msg _optional_
 * @namespace BDD
 * @public
 */
function assertProperty (name, val, msg) {
  if (msg) flag(this, 'message', msg);

  var isNested = flag(this, 'nested')
    , isOwn = flag(this, 'own')
    , flagMsg = flag(this, 'message')
    , obj = flag(this, 'object')
    , ssfi = flag(this, 'ssfi')
    , nameType = typeof name;

  flagMsg = flagMsg ? flagMsg + ': ' : '';

  if (isNested) {
    if (nameType !== 'string') {
      throw new AssertionError(
        flagMsg + 'the argument to property must be a string when using nested syntax',
        undefined,
        ssfi
      );
    }
  } else {
    if (nameType !== 'string' && nameType !== 'number' && nameType !== 'symbol') {
      throw new AssertionError(
        flagMsg + 'the argument to property must be a string, number, or symbol',
        undefined,
        ssfi
      );
    }
  }

  if (isNested && isOwn) {
    throw new AssertionError(
      flagMsg + 'The "nested" and "own" flags cannot be combined.',
      undefined,
      ssfi
    );
  }

  if (obj === null || obj === undefined) {
    throw new AssertionError(
      flagMsg + 'Target cannot be null or undefined.',
      undefined,
      ssfi
    );
  }

  var isDeep = flag(this, 'deep')
    , negate = flag(this, 'negate')
    , pathInfo = isNested ? _.getPathInfo(obj, name) : null
    , value = isNested ? pathInfo.value : obj[name]
    , isEql = isDeep ? flag(this, 'eql') : (val1, val2) => val1 === val2;

  var descriptor = '';
  if (isDeep) descriptor += 'deep ';
  if (isOwn) descriptor += 'own ';
  if (isNested) descriptor += 'nested ';
  descriptor += 'property ';

  var hasProperty;
  if (isOwn) hasProperty = Object.prototype.hasOwnProperty.call(obj, name);
  else if (isNested) hasProperty = pathInfo.exists;
  else hasProperty = _.hasProperty(obj, name);

  // When performing a negated assertion for both name and val, merely having
  // a property with the given name isn't enough to cause the assertion to
  // fail. It must both have a property with the given name, and the value of
  // that property must equal the given val. Therefore, skip this assertion in
  // favor of the next.
  if (!negate || arguments.length === 1) {
    this.assert(
        hasProperty
      , 'expected #{this} to have ' + descriptor + _.inspect(name)
      , 'expected #{this} to not have ' + descriptor + _.inspect(name));
  }

  if (arguments.length > 1) {
    this.assert(
      hasProperty && isEql(val, value)
      , 'expected #{this} to have ' + descriptor + _.inspect(name) + ' of #{exp}, but got #{act}'
      , 'expected #{this} to not have ' + descriptor + _.inspect(name) + ' of #{act}'
      , val
      , value
    );
  }

  flag(this, 'object', value);
}

Assertion.addMethod('property', assertProperty);

/**
 *
 * @param {unknown} name
 * @param {unknown} value
 * @param {string} msg
 */
function assertOwnProperty (name, value, msg) {
  flag(this, 'own', true);
  assertProperty.apply(this, arguments);
}

Assertion.addMethod('ownProperty', assertOwnProperty);
Assertion.addMethod('haveOwnProperty', assertOwnProperty);

/**
 * ### .ownPropertyDescriptor(name[, descriptor[, msg]])
 *
 * Asserts that the target has its own property descriptor with the given key
 * `name`. Enumerable and non-enumerable properties are included in the
 * search.
 *
 *     expect({a: 1}).to.have.ownPropertyDescriptor('a');
 *
 * When `descriptor` is provided, `.ownPropertyDescriptor` also asserts that
 * the property's descriptor is deeply equal to the given `descriptor`. See
 * the `deep-eql` project page for info on the deep equality algorithm:
 * https://github.com/chaijs/deep-eql.
 *
 *     expect({a: 1}).to.have.ownPropertyDescriptor('a', {
 *         configurable: true,
 *         enumerable: true,
 *         writable: true,
 *         value: 1,
 *     });
 *
 * Add `.not` earlier in the chain to negate `.ownPropertyDescriptor`.
 *
 *     expect({a: 1}).to.not.have.ownPropertyDescriptor('b');
 *
 * However, it's dangerous to negate `.ownPropertyDescriptor` when providing
 * a `descriptor`. The problem is that it creates uncertain expectations by
 * asserting that the target either doesn't have a property descriptor with
 * the given key `name`, or that it does have a property descriptor with the
 * given key `name` but it’s not deeply equal to the given `descriptor`. It's
 * often best to identify the exact output that's expected, and then write an
 * assertion that only accepts that exact output.
 *
 * When the target isn't expected to have a property descriptor with the given
 * key `name`, it's often best to assert exactly that.
 *
 *     // Recommended
 *     expect({b: 2}).to.not.have.ownPropertyDescriptor('a');
 *
 *     // Not recommended
 *     expect({b: 2}).to.not.have.ownPropertyDescriptor('a', {
 *         configurable: true,
 *         enumerable: true,
 *         writable: true,
 *         value: 1,
 *     });
 *
 * When the target is expected to have a property descriptor with the given
 * key `name`, it's often best to assert that the property has its expected
 * descriptor, rather than asserting that it doesn't have one of many
 * unexpected descriptors.
 *
 *     // Recommended
 *     expect({a: 3}).to.have.ownPropertyDescriptor('a', {
 *         configurable: true,
 *         enumerable: true,
 *         writable: true,
 *         value: 3,
 *     });
 *
 *     // Not recommended
 *     expect({a: 3}).to.not.have.ownPropertyDescriptor('a', {
 *         configurable: true,
 *         enumerable: true,
 *         writable: true,
 *         value: 1,
 *     });
 *
 * `.ownPropertyDescriptor` changes the target of any assertions that follow
 * in the chain to be the value of the property descriptor from the original
 * target object.
 *
 *     expect({a: 1}).to.have.ownPropertyDescriptor('a')
 *       .that.has.property('enumerable', true);
 *
 * `.ownPropertyDescriptor` accepts an optional `msg` argument which is a
 * custom error message to show when the assertion fails. The message can also
 * be given as the second argument to `expect`. When not providing
 * `descriptor`, only use the second form.
 *
 *     // Recommended
 *     expect({a: 1}).to.have.ownPropertyDescriptor('a', {
 *         configurable: true,
 *         enumerable: true,
 *         writable: true,
 *         value: 2,
 *     }, 'nooo why fail??');
 *
 *     // Recommended
 *     expect({a: 1}, 'nooo why fail??').to.have.ownPropertyDescriptor('a', {
 *         configurable: true,
 *         enumerable: true,
 *         writable: true,
 *         value: 2,
 *     });
 *
 *     // Recommended
 *     expect({a: 1}, 'nooo why fail??').to.have.ownPropertyDescriptor('b');
 *
 *     // Not recommended
 *     expect({a: 1})
 *       .to.have.ownPropertyDescriptor('b', undefined, 'nooo why fail??');
 *
 * The above assertion isn't the same thing as not providing `descriptor`.
 * Instead, it's asserting that the target object has a `b` property
 * descriptor that's deeply equal to `undefined`.
 *
 * The alias `.haveOwnPropertyDescriptor` can be used interchangeably with
 * `.ownPropertyDescriptor`.
 *
 * @name ownPropertyDescriptor
 * @alias haveOwnPropertyDescriptor
 * @param {string} name
 * @param {object} descriptor _optional_
 * @param {string} msg _optional_
 * @namespace BDD
 * @public
 */
function assertOwnPropertyDescriptor (name, descriptor, msg) {
  if (typeof descriptor === 'string') {
    msg = descriptor;
    descriptor = null;
  }
  if (msg) flag(this, 'message', msg);
  var obj = flag(this, 'object');
  var actualDescriptor = Object.getOwnPropertyDescriptor(Object(obj), name);
  var eql = flag(this, 'eql');
  if (actualDescriptor && descriptor) {
    this.assert(
        eql(descriptor, actualDescriptor)
      , 'expected the own property descriptor for ' + _.inspect(name) + ' on #{this} to match ' + _.inspect(descriptor) + ', got ' + _.inspect(actualDescriptor)
      , 'expected the own property descriptor for ' + _.inspect(name) + ' on #{this} to not match ' + _.inspect(descriptor)
      , descriptor
      , actualDescriptor
      , true
    );
  } else {
    this.assert(
        actualDescriptor
      , 'expected #{this} to have an own property descriptor for ' + _.inspect(name)
      , 'expected #{this} to not have an own property descriptor for ' + _.inspect(name)
    );
  }
  flag(this, 'object', actualDescriptor);
}

Assertion.addMethod('ownPropertyDescriptor', assertOwnPropertyDescriptor);
Assertion.addMethod('haveOwnPropertyDescriptor', assertOwnPropertyDescriptor);

/**
 *
 */
function assertLengthChain () {
  flag(this, 'doLength', true);
}

/**
 * ### .lengthOf(n[, msg])
 *
 * Asserts that the target's `length` or `size` is equal to the given number
 * `n`.
 *
 *     expect([1, 2, 3]).to.have.lengthOf(3);
 *     expect('foo').to.have.lengthOf(3);
 *     expect(new Set([1, 2, 3])).to.have.lengthOf(3);
 *     expect(new Map([['a', 1], ['b', 2], ['c', 3]])).to.have.lengthOf(3);
 *
 * Add `.not` earlier in the chain to negate `.lengthOf`. However, it's often
 * best to assert that the target's `length` property is equal to its expected
 * value, rather than not equal to one of many unexpected values.
 *
 *     expect('foo').to.have.lengthOf(3); // Recommended
 *     expect('foo').to.not.have.lengthOf(4); // Not recommended
 *
 * `.lengthOf` accepts an optional `msg` argument which is a custom error
 * message to show when the assertion fails. The message can also be given as
 * the second argument to `expect`.
 *
 *     expect([1, 2, 3]).to.have.lengthOf(2, 'nooo why fail??');
 *     expect([1, 2, 3], 'nooo why fail??').to.have.lengthOf(2);
 *
 * `.lengthOf` can also be used as a language chain, causing all `.above`,
 * `.below`, `.least`, `.most`, and `.within` assertions that follow in the
 * chain to use the target's `length` property as the target. However, it's
 * often best to assert that the target's `length` property is equal to its
 * expected length, rather than asserting that its `length` property falls
 * within some range of values.
 *
 *     // Recommended
 *     expect([1, 2, 3]).to.have.lengthOf(3);
 *
 *     // Not recommended
 *     expect([1, 2, 3]).to.have.lengthOf.above(2);
 *     expect([1, 2, 3]).to.have.lengthOf.below(4);
 *     expect([1, 2, 3]).to.have.lengthOf.at.least(3);
 *     expect([1, 2, 3]).to.have.lengthOf.at.most(3);
 *     expect([1, 2, 3]).to.have.lengthOf.within(2,4);
 *
 * Due to a compatibility issue, the alias `.length` can't be chained directly
 * off of an uninvoked method such as `.a`. Therefore, `.length` can't be used
 * interchangeably with `.lengthOf` in every situation. It's recommended to
 * always use `.lengthOf` instead of `.length`.
 *
 *     expect([1, 2, 3]).to.have.a.length(3); // incompatible; throws error
 *     expect([1, 2, 3]).to.have.a.lengthOf(3);  // passes as expected
 *
 * @name lengthOf
 * @alias length
 * @param {number} n
 * @param {string} msg _optional_
 * @namespace BDD
 * @public
 */
function assertLength (n, msg) {
  if (msg) flag(this, 'message', msg);
  var obj = flag(this, 'object')
    , objType = _.type(obj).toLowerCase()
    , flagMsg = flag(this, 'message')
    , ssfi = flag(this, 'ssfi')
    , descriptor = 'length'
    , itemsCount;

  switch (objType) {
    case 'map':
    case 'set':
      descriptor = 'size';
      itemsCount = obj.size;
      break;
    default:
      new Assertion(obj, flagMsg, ssfi, true).to.have.property('length');
      itemsCount = obj.length;
  }

  this.assert(
      itemsCount == n
    , 'expected #{this} to have a ' + descriptor + ' of #{exp} but got #{act}'
    , 'expected #{this} to not have a ' + descriptor + ' of #{act}'
    , n
    , itemsCount
  );
}

Assertion.addChainableMethod('length', assertLength, assertLengthChain);
Assertion.addChainableMethod('lengthOf', assertLength, assertLengthChain);

/**
 * ### .match(re[, msg])
 *
 * Asserts that the target matches the given regular expression `re`.
 *
 *     expect('foobar').to.match(/^foo/);
 *
 * Add `.not` earlier in the chain to negate `.match`.
 *
 *     expect('foobar').to.not.match(/taco/);
 *
 * `.match` accepts an optional `msg` argument which is a custom error message
 * to show when the assertion fails. The message can also be given as the
 * second argument to `expect`.
 *
 *     expect('foobar').to.match(/taco/, 'nooo why fail??');
 *     expect('foobar', 'nooo why fail??').to.match(/taco/);
 *
 * The alias `.matches` can be used interchangeably with `.match`.
 *
 * @name match
 * @alias matches
 * @param {RegExp} re
 * @param {string} msg _optional_
 * @namespace BDD
 * @public
 */
function assertMatch(re, msg) {
  if (msg) flag(this, 'message', msg);
  var obj = flag(this, 'object');
  this.assert(
      re.exec(obj)
    , 'expected #{this} to match ' + re
    , 'expected #{this} not to match ' + re
  );
}

Assertion.addMethod('match', assertMatch);
Assertion.addMethod('matches', assertMatch);

/**
 * ### .string(str[, msg])
 *
 * Asserts that the target string contains the given substring `str`.
 *
 *     expect('foobar').to.have.string('bar');
 *
 * Add `.not` earlier in the chain to negate `.string`.
 *
 *     expect('foobar').to.not.have.string('taco');
 *
 * `.string` accepts an optional `msg` argument which is a custom error
 * message to show when the assertion fails. The message can also be given as
 * the second argument to `expect`.
 *
 *     expect('foobar').to.have.string('taco', 'nooo why fail??');
 *     expect('foobar', 'nooo why fail??').to.have.string('taco');
 *
 * @name string
 * @param {string} str
 * @param {string} msg _optional_
 * @namespace BDD
 * @public
 */
Assertion.addMethod('string', function (str, msg) {
  if (msg) flag(this, 'message', msg);
  var obj = flag(this, 'object')
    , flagMsg = flag(this, 'message')
    , ssfi = flag(this, 'ssfi');
  new Assertion(obj, flagMsg, ssfi, true).is.a('string');

  this.assert(
      ~obj.indexOf(str)
    , 'expected #{this} to contain ' + _.inspect(str)
    , 'expected #{this} to not contain ' + _.inspect(str)
  );
});

/**
 * ### .keys(key1[, key2[, ...]])
 *
 * Asserts that the target object, array, map, or set has the given keys. Only
 * the target's own inherited properties are included in the search.
 *
 * When the target is an object or array, keys can be provided as one or more
 * string arguments, a single array argument, or a single object argument. In
 * the latter case, only the keys in the given object matter; the values are
 * ignored.
 *
 *     expect({a: 1, b: 2}).to.have.all.keys('a', 'b');
 *     expect(['x', 'y']).to.have.all.keys(0, 1);
 *
 *     expect({a: 1, b: 2}).to.have.all.keys(['a', 'b']);
 *     expect(['x', 'y']).to.have.all.keys([0, 1]);
 *
 *     expect({a: 1, b: 2}).to.have.all.keys({a: 4, b: 5}); // ignore 4 and 5
 *     expect(['x', 'y']).to.have.all.keys({0: 4, 1: 5}); // ignore 4 and 5
 *
 * When the target is a map or set, each key must be provided as a separate
 * argument.
 *
 *     expect(new Map([['a', 1], ['b', 2]])).to.have.all.keys('a', 'b');
 *     expect(new Set(['a', 'b'])).to.have.all.keys('a', 'b');
 *
 * Because `.keys` does different things based on the target's type, it's
 * important to check the target's type before using `.keys`. See the `.a` doc
 * for info on testing a target's type.
 *
 *     expect({a: 1, b: 2}).to.be.an('object').that.has.all.keys('a', 'b');
 *
 * By default, strict (`===`) equality is used to compare keys of maps and
 * sets. Add `.deep` earlier in the chain to use deep equality instead. See
 * the `deep-eql` project page for info on the deep equality algorithm:
 * https://github.com/chaijs/deep-eql.
 *
 *     // Target set deeply (but not strictly) has key `{a: 1}`
 *     expect(new Set([{a: 1}])).to.have.all.deep.keys([{a: 1}]);
 *     expect(new Set([{a: 1}])).to.not.have.all.keys([{a: 1}]);
 *
 * By default, the target must have all of the given keys and no more. Add
 * `.any` earlier in the chain to only require that the target have at least
 * one of the given keys. Also, add `.not` earlier in the chain to negate
 * `.keys`. It's often best to add `.any` when negating `.keys`, and to use
 * `.all` when asserting `.keys` without negation.
 *
 * When negating `.keys`, `.any` is preferred because `.not.any.keys` asserts
 * exactly what's expected of the output, whereas `.not.all.keys` creates
 * uncertain expectations.
 *
 *     // Recommended; asserts that target doesn't have any of the given keys
 *     expect({a: 1, b: 2}).to.not.have.any.keys('c', 'd');
 *
 *     // Not recommended; asserts that target doesn't have all of the given
 *     // keys but may or may not have some of them
 *     expect({a: 1, b: 2}).to.not.have.all.keys('c', 'd');
 *
 * When asserting `.keys` without negation, `.all` is preferred because
 * `.all.keys` asserts exactly what's expected of the output, whereas
 * `.any.keys` creates uncertain expectations.
 *
 *     // Recommended; asserts that target has all the given keys
 *     expect({a: 1, b: 2}).to.have.all.keys('a', 'b');
 *
 *     // Not recommended; asserts that target has at least one of the given
 *     // keys but may or may not have more of them
 *     expect({a: 1, b: 2}).to.have.any.keys('a', 'b');
 *
 * Note that `.all` is used by default when neither `.all` nor `.any` appear
 * earlier in the chain. However, it's often best to add `.all` anyway because
 * it improves readability.
 *
 *     // Both assertions are identical
 *     expect({a: 1, b: 2}).to.have.all.keys('a', 'b'); // Recommended
 *     expect({a: 1, b: 2}).to.have.keys('a', 'b'); // Not recommended
 *
 * Add `.include` earlier in the chain to require that the target's keys be a
 * superset of the expected keys, rather than identical sets.
 *
 *     // Target object's keys are a superset of ['a', 'b'] but not identical
 *     expect({a: 1, b: 2, c: 3}).to.include.all.keys('a', 'b');
 *     expect({a: 1, b: 2, c: 3}).to.not.have.all.keys('a', 'b');
 *
 * However, if `.any` and `.include` are combined, only the `.any` takes
 * effect. The `.include` is ignored in this case.
 *
 *     // Both assertions are identical
 *     expect({a: 1}).to.have.any.keys('a', 'b');
 *     expect({a: 1}).to.include.any.keys('a', 'b');
 *
 * A custom error message can be given as the second argument to `expect`.
 *
 *     expect({a: 1}, 'nooo why fail??').to.have.key('b');
 *
 * The alias `.key` can be used interchangeably with `.keys`.
 *
 * @name keys
 * @alias key
 * @param {...string | Array | object} keys
 * @namespace BDD
 * @public
 */
function assertKeys (keys) {
  var obj = flag(this, 'object')
    , objType = _.type(obj)
    , keysType = _.type(keys)
    , ssfi = flag(this, 'ssfi')
    , isDeep = flag(this, 'deep')
    , str
    , deepStr = ''
    , actual
    , ok = true
    , flagMsg = flag(this, 'message');

  flagMsg = flagMsg ? flagMsg + ': ' : '';
  var mixedArgsMsg = flagMsg + 'when testing keys against an object or an array you must give a single Array|Object|String argument or multiple String arguments';

  if (objType === 'Map' || objType === 'Set') {
    deepStr = isDeep ? 'deeply ' : '';
    actual = [];

    // Map and Set '.keys' aren't supported in IE 11. Therefore, use .forEach.
    obj.forEach(function (val, key) { actual.push(key) });

    if (keysType !== 'Array') {
      keys = Array.prototype.slice.call(arguments);
    }
  } else {
    actual = _.getOwnEnumerableProperties(obj);

    switch (keysType) {
      case 'Array':
        if (arguments.length > 1) {
          throw new AssertionError(mixedArgsMsg, undefined, ssfi);
        }
        break;
      case 'Object':
        if (arguments.length > 1) {
          throw new AssertionError(mixedArgsMsg, undefined, ssfi);
        }
        keys = Object.keys(keys);
        break;
      default:
        keys = Array.prototype.slice.call(arguments);
    }

    // Only stringify non-Symbols because Symbols would become "Symbol()"
    keys = keys.map(function (val) {
      return typeof val === 'symbol' ? val : String(val);
    });
  }

  if (!keys.length) {
    throw new AssertionError(flagMsg + 'keys required', undefined, ssfi);
  }

  var len = keys.length
    , any = flag(this, 'any')
    , all = flag(this, 'all')
    , expected = keys
    , isEql = isDeep ? flag(this, 'eql') : (val1, val2) => val1 === val2;

  if (!any && !all) {
    all = true;
  }

  // Has any
  if (any) {
    ok = expected.some(function(expectedKey) {
      return actual.some(function(actualKey) {
        return isEql(expectedKey, actualKey);
      });
    });
  }

  // Has all
  if (all) {
    ok = expected.every(function(expectedKey) {
      return actual.some(function(actualKey) {
        return isEql(expectedKey, actualKey);
      });
    });

    if (!flag(this, 'contains')) {
      ok = ok && keys.length == actual.length;
    }
  }

  // Key string
  if (len > 1) {
    keys = keys.map(function(key) {
      return _.inspect(key);
    });
    var last = keys.pop();
    if (all) {
      str = keys.join(', ') + ', and ' + last;
    }
    if (any) {
      str = keys.join(', ') + ', or ' + last;
    }
  } else {
    str = _.inspect(keys[0]);
  }

  // Form
  str = (len > 1 ? 'keys ' : 'key ') + str;

  // Have / include
  str = (flag(this, 'contains') ? 'contain ' : 'have ') + str;

  // Assertion
  this.assert(
      ok
    , 'expected #{this} to ' + deepStr + str
    , 'expected #{this} to not ' + deepStr + str
    , expected.slice(0).sort(_.compareByInspect)
    , actual.sort(_.compareByInspect)
    , true
  );
}

Assertion.addMethod('keys', assertKeys);
Assertion.addMethod('key', assertKeys);

/**
 * ### .throw([errorLike], [errMsgMatcher], [msg])
 *
 * When no arguments are provided, `.throw` invokes the target function and
 * asserts that an error is thrown.
 *
 *     var badFn = function () { throw new TypeError('Illegal salmon!'); };
 *     expect(badFn).to.throw();
 *
 * When one argument is provided, and it's an error constructor, `.throw`
 * invokes the target function and asserts that an error is thrown that's an
 * instance of that error constructor.
 *
 *     var badFn = function () { throw new TypeError('Illegal salmon!'); };
 *     expect(badFn).to.throw(TypeError);
 *
 * When one argument is provided, and it's an error instance, `.throw` invokes
 * the target function and asserts that an error is thrown that's strictly
 * (`===`) equal to that error instance.
 *
 *     var err = new TypeError('Illegal salmon!');
 *     var badFn = function () { throw err; };
 *
 *     expect(badFn).to.throw(err);
 *
 * When one argument is provided, and it's a string, `.throw` invokes the
 * target function and asserts that an error is thrown with a message that
 * contains that string.
 *
 *     var badFn = function () { throw new TypeError('Illegal salmon!'); };
 *     expect(badFn).to.throw('salmon');
 *
 * When one argument is provided, and it's a regular expression, `.throw`
 * invokes the target function and asserts that an error is thrown with a
 * message that matches that regular expression.
 *
 *     var badFn = function () { throw new TypeError('Illegal salmon!'); };
 *     expect(badFn).to.throw(/salmon/);
 *
 * When two arguments are provided, and the first is an error instance or
 * constructor, and the second is a string or regular expression, `.throw`
 * invokes the function and asserts that an error is thrown that fulfills both
 * conditions as described above.
 *
 *     var err = new TypeError('Illegal salmon!');
 *     var badFn = function () { throw err; };
 *
 *     expect(badFn).to.throw(TypeError, 'salmon');
 *     expect(badFn).to.throw(TypeError, /salmon/);
 *     expect(badFn).to.throw(err, 'salmon');
 *     expect(badFn).to.throw(err, /salmon/);
 *
 * Add `.not` earlier in the chain to negate `.throw`.
 *
 *     var goodFn = function () {};
 *     expect(goodFn).to.not.throw();
 *
 * However, it's dangerous to negate `.throw` when providing any arguments.
 * The problem is that it creates uncertain expectations by asserting that the
 * target either doesn't throw an error, or that it throws an error but of a
 * different type than the given type, or that it throws an error of the given
 * type but with a message that doesn't include the given string. It's often
 * best to identify the exact output that's expected, and then write an
 * assertion that only accepts that exact output.
 *
 * When the target isn't expected to throw an error, it's often best to assert
 * exactly that.
 *
 *     var goodFn = function () {};
 *
 *     expect(goodFn).to.not.throw(); // Recommended
 *     expect(goodFn).to.not.throw(ReferenceError, 'x'); // Not recommended
 *
 * When the target is expected to throw an error, it's often best to assert
 * that the error is of its expected type, and has a message that includes an
 * expected string, rather than asserting that it doesn't have one of many
 * unexpected types, and doesn't have a message that includes some string.
 *
 *     var badFn = function () { throw new TypeError('Illegal salmon!'); };
 *
 *     expect(badFn).to.throw(TypeError, 'salmon'); // Recommended
 *     expect(badFn).to.not.throw(ReferenceError, 'x'); // Not recommended
 *
 * `.throw` changes the target of any assertions that follow in the chain to
 * be the error object that's thrown.
 *
 *     var err = new TypeError('Illegal salmon!');
 *     err.code = 42;
 *     var badFn = function () { throw err; };
 *
 *     expect(badFn).to.throw(TypeError).with.property('code', 42);
 *
 * `.throw` accepts an optional `msg` argument which is a custom error message
 * to show when the assertion fails. The message can also be given as the
 * second argument to `expect`. When not providing two arguments, always use
 * the second form.
 *
 *     var goodFn = function () {};
 *
 *     expect(goodFn).to.throw(TypeError, 'x', 'nooo why fail??');
 *     expect(goodFn, 'nooo why fail??').to.throw();
 *
 * Due to limitations in ES5, `.throw` may not always work as expected when
 * using a transpiler such as Babel or TypeScript. In particular, it may
 * produce unexpected results when subclassing the built-in `Error` object and
 * then passing the subclassed constructor to `.throw`. See your transpiler's
 * docs for details:
 *
 * - ([Babel](https://babeljs.io/docs/usage/caveats/#classes))
 * - ([TypeScript](https://github.com/Microsoft/TypeScript/wiki/Breaking-Changes#extending-built-ins-like-error-array-and-map-may-no-longer-work))
 *
 * Beware of some common mistakes when using the `throw` assertion. One common
 * mistake is to accidentally invoke the function yourself instead of letting
 * the `throw` assertion invoke the function for you. For example, when
 * testing if a function named `fn` throws, provide `fn` instead of `fn()` as
 * the target for the assertion.
 *
 *     expect(fn).to.throw();     // Good! Tests `fn` as desired
 *     expect(fn()).to.throw();   // Bad! Tests result of `fn()`, not `fn`
 *
 * If you need to assert that your function `fn` throws when passed certain
 * arguments, then wrap a call to `fn` inside of another function.
 *
 *     expect(function () { fn(42); }).to.throw();  // Function expression
 *     expect(() => fn(42)).to.throw();             // ES6 arrow function
 *
 * Another common mistake is to provide an object method (or any stand-alone
 * function that relies on `this`) as the target of the assertion. Doing so is
 * problematic because the `this` context will be lost when the function is
 * invoked by `.throw`; there's no way for it to know what `this` is supposed
 * to be. There are two ways around this problem. One solution is to wrap the
 * method or function call inside of another function. Another solution is to
 * use `bind`.
 *
 *     expect(function () { cat.meow(); }).to.throw();  // Function expression
 *     expect(() => cat.meow()).to.throw();             // ES6 arrow function
 *     expect(cat.meow.bind(cat)).to.throw();           // Bind
 *
 * Finally, it's worth mentioning that it's a best practice in JavaScript to
 * only throw `Error` and derivatives of `Error` such as `ReferenceError`,
 * `TypeError`, and user-defined objects that extend `Error`. No other type of
 * value will generate a stack trace when initialized. With that said, the
 * `throw` assertion does technically support any type of value being thrown,
 * not just `Error` and its derivatives.
 *
 * The aliases `.throws` and `.Throw` can be used interchangeably with
 * `.throw`.
 *
 * @name throw
 * @alias throws
 * @alias Throw
 * @param {Error} errorLike
 * @param {string | RegExp} errMsgMatcher error message
 * @param {string} msg _optional_
 * @see https://developer.mozilla.org/en/JavaScript/Reference/Global_Objects/Error#Error_types
 * @returns {void} error for chaining (null if no error)
 * @namespace BDD
 * @public
 */
function assertThrows (errorLike, errMsgMatcher, msg) {
  if (msg) flag(this, 'message', msg);
  var obj = flag(this, 'object')
    , ssfi = flag(this, 'ssfi')
    , flagMsg = flag(this, 'message')
    , negate = flag(this, 'negate') || false;
  new Assertion(obj, flagMsg, ssfi, true).is.a('function');

  if (_.isRegExp(errorLike) || typeof errorLike === 'string') {
    errMsgMatcher = errorLike;
    errorLike = null;
  }

  let caughtErr;
  let errorWasThrown = false;
  try {
    obj();
  } catch (err) {
    errorWasThrown = true;
    caughtErr = err;
  }

  // If we have the negate flag enabled and at least one valid argument it means we do expect an error
  // but we want it to match a given set of criteria
  var everyArgIsUndefined = errorLike === undefined && errMsgMatcher === undefined;

  // If we've got the negate flag enabled and both args, we should only fail if both aren't compatible
  // See Issue #551 and PR #683@GitHub
  var everyArgIsDefined = Boolean(errorLike && errMsgMatcher);
  var errorLikeFail = false;
  var errMsgMatcherFail = false;

  // Checking if error was thrown
  if (everyArgIsUndefined || !everyArgIsUndefined && !negate) {
    // We need this to display results correctly according to their types
    var errorLikeString = 'an error';
    if (errorLike instanceof Error) {
      errorLikeString = '#{exp}';
    } else if (errorLike) {
      errorLikeString = _.checkError.getConstructorName(errorLike);
    }

    let actual = caughtErr;
    if (caughtErr instanceof Error) {
      actual = caughtErr.toString();
    } else if (typeof caughtErr === 'string') {
      actual = caughtErr;
    } else if (caughtErr && (typeof caughtErr === 'object' || typeof caughtErr === 'function')) {
      try {
        actual = _.checkError.getConstructorName(caughtErr);
      } catch (_err) {
        // somehow wasn't a constructor, maybe we got a function thrown
        // or similar
      }
    }

    this.assert(
      errorWasThrown
      , 'expected #{this} to throw ' + errorLikeString
      , 'expected #{this} to not throw an error but #{act} was thrown'
      , errorLike && errorLike.toString()
      , actual
    );
  }

  if (errorLike && caughtErr) {
    // We should compare instances only if `errorLike` is an instance of `Error`
    if (errorLike instanceof Error) {
      var isCompatibleInstance = _.checkError.compatibleInstance(caughtErr, errorLike);

      if (isCompatibleInstance === negate) {
        // These checks were created to ensure we won't fail too soon when we've got both args and a negate
        // See Issue #551 and PR #683@GitHub
        if (everyArgIsDefined && negate) {
          errorLikeFail = true;
        } else {
          this.assert(
              negate
            , 'expected #{this} to throw #{exp} but #{act} was thrown'
            , 'expected #{this} to not throw #{exp}' + (caughtErr && !negate ? ' but #{act} was thrown' : '')
            , errorLike.toString()
            , caughtErr.toString()
          );
        }
      }
    }

    var isCompatibleConstructor = _.checkError.compatibleConstructor(caughtErr, errorLike);
    if (isCompatibleConstructor === negate) {
      if (everyArgIsDefined && negate) {
          errorLikeFail = true;
      } else {
        this.assert(
            negate
          , 'expected #{this} to throw #{exp} but #{act} was thrown'
          , 'expected #{this} to not throw #{exp}' + (caughtErr ? ' but #{act} was thrown' : '')
          , (errorLike instanceof Error ? errorLike.toString() : errorLike && _.checkError.getConstructorName(errorLike))
          , (caughtErr instanceof Error ? caughtErr.toString() : caughtErr && _.checkError.getConstructorName(caughtErr))
        );
      }
    }
  }

  if (caughtErr && errMsgMatcher !== undefined && errMsgMatcher !== null) {
    // Here we check compatible messages
    var placeholder = 'including';
    if (_.isRegExp(errMsgMatcher)) {
      placeholder = 'matching'
    }

    var isCompatibleMessage = _.checkError.compatibleMessage(caughtErr, errMsgMatcher);
    if (isCompatibleMessage === negate) {
      if (everyArgIsDefined && negate) {
          errMsgMatcherFail = true;
      } else {
        this.assert(
          negate
          , 'expected #{this} to throw error ' + placeholder + ' #{exp} but got #{act}'
          , 'expected #{this} to throw error not ' + placeholder + ' #{exp}'
          ,  errMsgMatcher
          ,  _.checkError.getMessage(caughtErr)
        );
      }
    }
  }

  // If both assertions failed and both should've matched we throw an error
  if (errorLikeFail && errMsgMatcherFail) {
    this.assert(
      negate
      , 'expected #{this} to throw #{exp} but #{act} was thrown'
      , 'expected #{this} to not throw #{exp}' + (caughtErr ? ' but #{act} was thrown' : '')
      , (errorLike instanceof Error ? errorLike.toString() : errorLike && _.checkError.getConstructorName(errorLike))
      , (caughtErr instanceof Error ? caughtErr.toString() : caughtErr && _.checkError.getConstructorName(caughtErr))
    );
  }

  flag(this, 'object', caughtErr);
};

Assertion.addMethod('throw', assertThrows);
Assertion.addMethod('throws', assertThrows);
Assertion.addMethod('Throw', assertThrows);

/**
 * ### .respondTo(method[, msg])
 *
 * When the target is a non-function object, `.respondTo` asserts that the
 * target has a method with the given name `method`. The method can be own or
 * inherited, and it can be enumerable or non-enumerable.
 *
 *     function Cat () {}
 *     Cat.prototype.meow = function () {};
 *
 *     expect(new Cat()).to.respondTo('meow');
 *
 * When the target is a function, `.respondTo` asserts that the target's
 * `prototype` property has a method with the given name `method`. Again, the
 * method can be own or inherited, and it can be enumerable or non-enumerable.
 *
 *     function Cat () {}
 *     Cat.prototype.meow = function () {};
 *
 *     expect(Cat).to.respondTo('meow');
 *
 * Add `.itself` earlier in the chain to force `.respondTo` to treat the
 * target as a non-function object, even if it's a function. Thus, it asserts
 * that the target has a method with the given name `method`, rather than
 * asserting that the target's `prototype` property has a method with the
 * given name `method`.
 *
 *     function Cat () {}
 *     Cat.prototype.meow = function () {};
 *     Cat.hiss = function () {};
 *
 *     expect(Cat).itself.to.respondTo('hiss').but.not.respondTo('meow');
 *
 * When not adding `.itself`, it's important to check the target's type before
 * using `.respondTo`. See the `.a` doc for info on checking a target's type.
 *
 *     function Cat () {}
 *     Cat.prototype.meow = function () {};
 *
 *     expect(new Cat()).to.be.an('object').that.respondsTo('meow');
 *
 * Add `.not` earlier in the chain to negate `.respondTo`.
 *
 *     function Dog () {}
 *     Dog.prototype.bark = function () {};
 *
 *     expect(new Dog()).to.not.respondTo('meow');
 *
 * `.respondTo` accepts an optional `msg` argument which is a custom error
 * message to show when the assertion fails. The message can also be given as
 * the second argument to `expect`.
 *
 *     expect({}).to.respondTo('meow', 'nooo why fail??');
 *     expect({}, 'nooo why fail??').to.respondTo('meow');
 *
 * The alias `.respondsTo` can be used interchangeably with `.respondTo`.
 *
 * @name respondTo
 * @alias respondsTo
 * @param {string} method
 * @param {string} msg _optional_
 * @namespace BDD
 * @public
 */
function respondTo (method, msg) {
  if (msg) flag(this, 'message', msg);
  var obj = flag(this, 'object')
    , itself = flag(this, 'itself')
    , context = ('function' === typeof obj && !itself)
      ? obj.prototype[method]
      : obj[method];

  this.assert(
      'function' === typeof context
    , 'expected #{this} to respond to ' + _.inspect(method)
    , 'expected #{this} to not respond to ' + _.inspect(method)
  );
}

Assertion.addMethod('respondTo', respondTo);
Assertion.addMethod('respondsTo', respondTo);

/**
 * ### .itself
 *
 * Forces all `.respondTo` assertions that follow in the chain to behave as if
 * the target is a non-function object, even if it's a function. Thus, it
 * causes `.respondTo` to assert that the target has a method with the given
 * name, rather than asserting that the target's `prototype` property has a
 * method with the given name.
 *
 *     function Cat () {}
 *     Cat.prototype.meow = function () {};
 *     Cat.hiss = function () {};
 *
 *     expect(Cat).itself.to.respondTo('hiss').but.not.respondTo('meow');
 *
 * @name itself
 * @namespace BDD
 * @public
 */
Assertion.addProperty('itself', function () {
  flag(this, 'itself', true);
});

/**
 * ### .satisfy(matcher[, msg])
 *
 * Invokes the given `matcher` function with the target being passed as the
 * first argument, and asserts that the value returned is truthy.
 *
 *     expect(1).to.satisfy(function(num) {
 *         return num > 0;
 *     });
 *
 * Add `.not` earlier in the chain to negate `.satisfy`.
 *
 *     expect(1).to.not.satisfy(function(num) {
 *         return num > 2;
 *     });
 *
 * `.satisfy` accepts an optional `msg` argument which is a custom error
 * message to show when the assertion fails. The message can also be given as
 * the second argument to `expect`.
 *
 *     expect(1).to.satisfy(function(num) {
 *         return num > 2;
 *     }, 'nooo why fail??');
 *
 *     expect(1, 'nooo why fail??').to.satisfy(function(num) {
 *         return num > 2;
 *     });
 *
 * The alias `.satisfies` can be used interchangeably with `.satisfy`.
 *
 * @name satisfy
 * @alias satisfies
 * @param {Function} matcher
 * @param {string} msg _optional_
 * @namespace BDD
 * @public
 */
function satisfy (matcher, msg) {
  if (msg) flag(this, 'message', msg);
  var obj = flag(this, 'object');
  var result = matcher(obj);
  this.assert(
      result
    , 'expected #{this} to satisfy ' + _.objDisplay(matcher)
    , 'expected #{this} to not satisfy' + _.objDisplay(matcher)
    , flag(this, 'negate') ? false : true
    , result
  );
}

Assertion.addMethod('satisfy', satisfy);
Assertion.addMethod('satisfies', satisfy);

/**
 * ### .closeTo(expected, delta[, msg])
 *
 * Asserts that the target is a number that's within a given +/- `delta` range
 * of the given number `expected`. However, it's often best to assert that the
 * target is equal to its expected value.
 *
 *     // Recommended
 *     expect(1.5).to.equal(1.5);
 *
 *     // Not recommended
 *     expect(1.5).to.be.closeTo(1, 0.5);
 *     expect(1.5).to.be.closeTo(2, 0.5);
 *     expect(1.5).to.be.closeTo(1, 1);
 *
 * Add `.not` earlier in the chain to negate `.closeTo`.
 *
 *     expect(1.5).to.equal(1.5); // Recommended
 *     expect(1.5).to.not.be.closeTo(3, 1); // Not recommended
 *
 * `.closeTo` accepts an optional `msg` argument which is a custom error
 * message to show when the assertion fails. The message can also be given as
 * the second argument to `expect`.
 *
 *     expect(1.5).to.be.closeTo(3, 1, 'nooo why fail??');
 *     expect(1.5, 'nooo why fail??').to.be.closeTo(3, 1);
 *
 * The alias `.approximately` can be used interchangeably with `.closeTo`.
 *
 * @name closeTo
 * @alias approximately
 * @param {number} expected
 * @param {number} delta
 * @param {string} msg _optional_
 * @namespace BDD
 * @public
 */
function closeTo(expected, delta, msg) {
  if (msg) flag(this, 'message', msg);
  var obj = flag(this, 'object')
    , flagMsg = flag(this, 'message')
    , ssfi = flag(this, 'ssfi');

  new Assertion(obj, flagMsg, ssfi, true).is.a('number');
  if (typeof expected !== 'number' || typeof delta !== 'number') {
    flagMsg = flagMsg ? flagMsg + ': ' : '';
    var deltaMessage = delta === undefined ? ", and a delta is required" : "";
    throw new AssertionError(
        flagMsg + 'the arguments to closeTo or approximately must be numbers' + deltaMessage,
        undefined,
        ssfi
    );
  }

  this.assert(
      Math.abs(obj - expected) <= delta
    , 'expected #{this} to be close to ' + expected + ' +/- ' + delta
    , 'expected #{this} not to be close to ' + expected + ' +/- ' + delta
  );
}

Assertion.addMethod('closeTo', closeTo);
Assertion.addMethod('approximately', closeTo);

/**
 * @param {unknown} _subset
 * @param {unknown} _superset
 * @param {unknown} cmp
 * @param {unknown} contains
 * @param {unknown} ordered
 * @returns {boolean}
 */
function isSubsetOf(_subset, _superset, cmp, contains, ordered) {
  let superset = Array.from(_superset);
  let subset = Array.from(_subset);
  if (!contains) {
    if (subset.length !== superset.length) return false;
    superset = superset.slice();
  }

  return subset.every(function(elem, idx) {
    if (ordered) return cmp ? cmp(elem, superset[idx]) : elem === superset[idx];

    if (!cmp) {
      var matchIdx = superset.indexOf(elem);
      if (matchIdx === -1) return false;

      // Remove match from superset so not counted twice if duplicate in subset.
      if (!contains) superset.splice(matchIdx, 1);
      return true;
    }

    return superset.some(function(elem2, matchIdx) {
      if (!cmp(elem, elem2)) return false;

      // Remove match from superset so not counted twice if duplicate in subset.
      if (!contains) superset.splice(matchIdx, 1);
      return true;
    });
  });
}

/**
 * ### .members(set[, msg])
 *
 * Asserts that the target array has the same members as the given array
 * `set`.
 *
 *     expect([1, 2, 3]).to.have.members([2, 1, 3]);
 *     expect([1, 2, 2]).to.have.members([2, 1, 2]);
 *
 * By default, members are compared using strict (`===`) equality. Add `.deep`
 * earlier in the chain to use deep equality instead. See the `deep-eql`
 * project page for info on the deep equality algorithm:
 * https://github.com/chaijs/deep-eql.
 *
 *     // Target array deeply (but not strictly) has member `{a: 1}`
 *     expect([{a: 1}]).to.have.deep.members([{a: 1}]);
 *     expect([{a: 1}]).to.not.have.members([{a: 1}]);
 *
 * By default, order doesn't matter. Add `.ordered` earlier in the chain to
 * require that members appear in the same order.
 *
 *     expect([1, 2, 3]).to.have.ordered.members([1, 2, 3]);
 *     expect([1, 2, 3]).to.have.members([2, 1, 3])
 *       .but.not.ordered.members([2, 1, 3]);
 *
 * By default, both arrays must be the same size. Add `.include` earlier in
 * the chain to require that the target's members be a superset of the
 * expected members. Note that duplicates are ignored in the subset when
 * `.include` is added.
 *
 *     // Target array is a superset of [1, 2] but not identical
 *     expect([1, 2, 3]).to.include.members([1, 2]);
 *     expect([1, 2, 3]).to.not.have.members([1, 2]);
 *
 *     // Duplicates in the subset are ignored
 *     expect([1, 2, 3]).to.include.members([1, 2, 2, 2]);
 *
 * `.deep`, `.ordered`, and `.include` can all be combined. However, if
 * `.include` and `.ordered` are combined, the ordering begins at the start of
 * both arrays.
 *
 *     expect([{a: 1}, {b: 2}, {c: 3}])
 *       .to.include.deep.ordered.members([{a: 1}, {b: 2}])
 *       .but.not.include.deep.ordered.members([{b: 2}, {c: 3}]);
 *
 * Add `.not` earlier in the chain to negate `.members`. However, it's
 * dangerous to do so. The problem is that it creates uncertain expectations
 * by asserting that the target array doesn't have all of the same members as
 * the given array `set` but may or may not have some of them. It's often best
 * to identify the exact output that's expected, and then write an assertion
 * that only accepts that exact output.
 *
 *     expect([1, 2]).to.not.include(3).and.not.include(4); // Recommended
 *     expect([1, 2]).to.not.have.members([3, 4]); // Not recommended
 *
 * `.members` accepts an optional `msg` argument which is a custom error
 * message to show when the assertion fails. The message can also be given as
 * the second argument to `expect`.
 *
 *     expect([1, 2]).to.have.members([1, 2, 3], 'nooo why fail??');
 *     expect([1, 2], 'nooo why fail??').to.have.members([1, 2, 3]);
 *
 * @name members
 * @param {Array} set
 * @param {string} msg _optional_
 * @namespace BDD
 * @public
 */
Assertion.addMethod('members', function (subset, msg) {
  if (msg) flag(this, 'message', msg);
  var obj = flag(this, 'object')
    , flagMsg = flag(this, 'message')
    , ssfi = flag(this, 'ssfi');

  new Assertion(obj, flagMsg, ssfi, true).to.be.iterable;
  new Assertion(subset, flagMsg, ssfi, true).to.be.iterable;

  var contains = flag(this, 'contains');
  var ordered = flag(this, 'ordered');

  var subject, failMsg, failNegateMsg;

  if (contains) {
    subject = ordered ? 'an ordered superset' : 'a superset';
    failMsg = 'expected #{this} to be ' + subject + ' of #{exp}';
    failNegateMsg = 'expected #{this} to not be ' + subject + ' of #{exp}';
  } else {
    subject = ordered ? 'ordered members' : 'members';
    failMsg = 'expected #{this} to have the same ' + subject + ' as #{exp}';
    failNegateMsg = 'expected #{this} to not have the same ' + subject + ' as #{exp}';
  }

  var cmp = flag(this, 'deep') ? flag(this, 'eql') : undefined;

  this.assert(
      isSubsetOf(subset, obj, cmp, contains, ordered)
    , failMsg
    , failNegateMsg
    , subset
    , obj
    , true
  );
});

/**
 * ### .iterable
 *
 * Asserts that the target is an iterable, which means that it has a iterator.
 *
 *     expect([1, 2]).to.be.iterable;
 *     expect("foobar").to.be.iterable;
 *
 * Add `.not` earlier in the chain to negate `.iterable`.
 *
 *     expect(1).to.not.be.iterable;
 *     expect(true).to.not.be.iterable;
 *
 * A custom error message can be given as the second argument to `expect`.
 *
 *     expect(1, 'nooo why fail??').to.be.iterable;
 *
 * @name iterable
 * @namespace BDD
 * @public
 */
Assertion.addProperty('iterable', function(msg) {
  if (msg) flag(this, 'message', msg);
  var obj = flag(this, 'object');

  this.assert(
    obj != undefined && obj[Symbol.iterator]
    , 'expected #{this} to be an iterable'
    , 'expected #{this} to not be an iterable'
    , obj
  );
});

/**
 * ### .oneOf(list[, msg])
 *
 * Asserts that the target is a member of the given array `list`. However,
 * it's often best to assert that the target is equal to its expected value.
 *
 *     expect(1).to.equal(1); // Recommended
 *     expect(1).to.be.oneOf([1, 2, 3]); // Not recommended
 *
 * Comparisons are performed using strict (`===`) equality.
 *
 * Add `.not` earlier in the chain to negate `.oneOf`.
 *
 *     expect(1).to.equal(1); // Recommended
 *     expect(1).to.not.be.oneOf([2, 3, 4]); // Not recommended
 *
 * It can also be chained with `.contain` or `.include`, which will work with
 * both arrays and strings:
 *
 *     expect('Today is sunny').to.contain.oneOf(['sunny', 'cloudy'])
 *     expect('Today is rainy').to.not.contain.oneOf(['sunny', 'cloudy'])
 *     expect([1,2,3]).to.contain.oneOf([3,4,5])
 *     expect([1,2,3]).to.not.contain.oneOf([4,5,6])
 *
 * `.oneOf` accepts an optional `msg` argument which is a custom error message
 * to show when the assertion fails. The message can also be given as the
 * second argument to `expect`.
 *
 *     expect(1).to.be.oneOf([2, 3, 4], 'nooo why fail??');
 *     expect(1, 'nooo why fail??').to.be.oneOf([2, 3, 4]);
 *
 * @name oneOf
 * @param {Array<*>} list
 * @param {string} msg _optional_
 * @namespace BDD
 * @public
 */
function oneOf (list, msg) {
  if (msg) flag(this, 'message', msg);
  var expected = flag(this, 'object')
    , flagMsg = flag(this, 'message')
    , ssfi = flag(this, 'ssfi')
    , contains = flag(this, 'contains')
    , isDeep = flag(this, 'deep')
    , eql = flag(this, 'eql');
  new Assertion(list, flagMsg, ssfi, true).to.be.an('array');

  if (contains) {
    this.assert(
      list.some(function(possibility) { return expected.indexOf(possibility) > -1 })
      , 'expected #{this} to contain one of #{exp}'
      , 'expected #{this} to not contain one of #{exp}'
      , list
      , expected
    );
  } else {
    if (isDeep) {
      this.assert(
        list.some(function(possibility) { return eql(expected, possibility) })
        , 'expected #{this} to deeply equal one of #{exp}'
        , 'expected #{this} to deeply equal one of #{exp}'
        , list
        , expected
      );
    } else {
      this.assert(
        list.indexOf(expected) > -1
        , 'expected #{this} to be one of #{exp}'
        , 'expected #{this} to not be one of #{exp}'
        , list
        , expected
      );
    }
  }
}

Assertion.addMethod('oneOf', oneOf);

/**
 * ### .change(subject[, prop[, msg]])
 *
 * When one argument is provided, `.change` asserts that the given function
 * `subject` returns a different value when it's invoked before the target
 * function compared to when it's invoked afterward. However, it's often best
 * to assert that `subject` is equal to its expected value.
 *
 *     var dots = ''
 *     , addDot = function () { dots += '.'; }
 *     , getDots = function () { return dots; };
 *
 *     // Recommended
 *     expect(getDots()).to.equal('');
 *     addDot();
 *     expect(getDots()).to.equal('.');
 *
 *     // Not recommended
 *     expect(addDot).to.change(getDots);
 *
 * When two arguments are provided, `.change` asserts that the value of the
 * given object `subject`'s `prop` property is different before invoking the
 * target function compared to afterward.
 *
 *     var myObj = {dots: ''}
 *     , addDot = function () { myObj.dots += '.'; };
 *
 *     // Recommended
 *     expect(myObj).to.have.property('dots', '');
 *     addDot();
 *     expect(myObj).to.have.property('dots', '.');
 *
 *     // Not recommended
 *     expect(addDot).to.change(myObj, 'dots');
 *
 * Strict (`===`) equality is used to compare before and after values.
 *
 * Add `.not` earlier in the chain to negate `.change`.
 *
 *     var dots = ''
 *     , noop = function () {}
 *     , getDots = function () { return dots; };
 *
 *     expect(noop).to.not.change(getDots);
 *
 *     var myObj = {dots: ''}
 *     , noop = function () {};
 *
 *     expect(noop).to.not.change(myObj, 'dots');
 *
 * `.change` accepts an optional `msg` argument which is a custom error
 * message to show when the assertion fails. The message can also be given as
 * the second argument to `expect`. When not providing two arguments, always
 * use the second form.
 *
 *     var myObj = {dots: ''}
 *     , addDot = function () { myObj.dots += '.'; };
 *
 *     expect(addDot).to.not.change(myObj, 'dots', 'nooo why fail??');
 *
 *     var dots = ''
 *     , addDot = function () { dots += '.'; }
 *     , getDots = function () { return dots; };
 *
 *     expect(addDot, 'nooo why fail??').to.not.change(getDots);
 *
 * `.change` also causes all `.by` assertions that follow in the chain to
 * assert how much a numeric subject was increased or decreased by. However,
 * it's dangerous to use `.change.by`. The problem is that it creates
 * uncertain expectations by asserting that the subject either increases by
 * the given delta, or that it decreases by the given delta. It's often best
 * to identify the exact output that's expected, and then write an assertion
 * that only accepts that exact output.
 *
 *     var myObj = {val: 1}
 *     , addTwo = function () { myObj.val += 2; }
 *     , subtractTwo = function () { myObj.val -= 2; };
 *
 *     expect(addTwo).to.increase(myObj, 'val').by(2); // Recommended
 *     expect(addTwo).to.change(myObj, 'val').by(2); // Not recommended
 *
 *     expect(subtractTwo).to.decrease(myObj, 'val').by(2); // Recommended
 *     expect(subtractTwo).to.change(myObj, 'val').by(2); // Not recommended
 *
 * The alias `.changes` can be used interchangeably with `.change`.
 *
 * @name change
 * @alias changes
 * @param {string} subject
 * @param {string} prop name _optional_
 * @param {string} msg _optional_
 * @namespace BDD
 * @public
 */
function assertChanges (subject, prop, msg) {
  if (msg) flag(this, 'message', msg);
  var fn = flag(this, 'object')
    , flagMsg = flag(this, 'message')
    , ssfi = flag(this, 'ssfi');
  new Assertion(fn, flagMsg, ssfi, true).is.a('function');

  var initial;
  if (!prop) {
    new Assertion(subject, flagMsg, ssfi, true).is.a('function');
    initial = subject();
  } else {
    new Assertion(subject, flagMsg, ssfi, true).to.have.property(prop);
    initial = subject[prop];
  }

  fn();

  var final = prop === undefined || prop === null ? subject() : subject[prop];
  var msgObj = prop === undefined || prop === null ? initial : '.' + prop;

  // This gets flagged because of the .by(delta) assertion
  flag(this, 'deltaMsgObj', msgObj);
  flag(this, 'initialDeltaValue', initial);
  flag(this, 'finalDeltaValue', final);
  flag(this, 'deltaBehavior', 'change');
  flag(this, 'realDelta', final !== initial);

  this.assert(
    initial !== final
    , 'expected ' + msgObj + ' to change'
    , 'expected ' + msgObj + ' to not change'
  );
}

Assertion.addMethod('change', assertChanges);
Assertion.addMethod('changes', assertChanges);

/**
 * ### .increase(subject[, prop[, msg]])
 *
 * When one argument is provided, `.increase` asserts that the given function
 * `subject` returns a greater number when it's invoked after invoking the
 * target function compared to when it's invoked beforehand. `.increase` also
 * causes all `.by` assertions that follow in the chain to assert how much
 * greater of a number is returned. It's often best to assert that the return
 * value increased by the expected amount, rather than asserting it increased
 * by any amount.
 *
 *     var val = 1
 *     , addTwo = function () { val += 2; }
 *     , getVal = function () { return val; };
 *
 *     expect(addTwo).to.increase(getVal).by(2); // Recommended
 *     expect(addTwo).to.increase(getVal); // Not recommended
 *
 * When two arguments are provided, `.increase` asserts that the value of the
 * given object `subject`'s `prop` property is greater after invoking the
 * target function compared to beforehand.
 *
 *     var myObj = {val: 1}
 *     , addTwo = function () { myObj.val += 2; };
 *
 *     expect(addTwo).to.increase(myObj, 'val').by(2); // Recommended
 *     expect(addTwo).to.increase(myObj, 'val'); // Not recommended
 *
 * Add `.not` earlier in the chain to negate `.increase`. However, it's
 * dangerous to do so. The problem is that it creates uncertain expectations
 * by asserting that the subject either decreases, or that it stays the same.
 * It's often best to identify the exact output that's expected, and then
 * write an assertion that only accepts that exact output.
 *
 * When the subject is expected to decrease, it's often best to assert that it
 * decreased by the expected amount.
 *
 *     var myObj = {val: 1}
 *     , subtractTwo = function () { myObj.val -= 2; };
 *
 *     expect(subtractTwo).to.decrease(myObj, 'val').by(2); // Recommended
 *     expect(subtractTwo).to.not.increase(myObj, 'val'); // Not recommended
 *
 * When the subject is expected to stay the same, it's often best to assert
 * exactly that.
 *
 *     var myObj = {val: 1}
 *     , noop = function () {};
 *
 *     expect(noop).to.not.change(myObj, 'val'); // Recommended
 *     expect(noop).to.not.increase(myObj, 'val'); // Not recommended
 *
 * `.increase` accepts an optional `msg` argument which is a custom error
 * message to show when the assertion fails. The message can also be given as
 * the second argument to `expect`. When not providing two arguments, always
 * use the second form.
 *
 *     var myObj = {val: 1}
 *     , noop = function () {};
 *
 *     expect(noop).to.increase(myObj, 'val', 'nooo why fail??');
 *
 *     var val = 1
 *     , noop = function () {}
 *     , getVal = function () { return val; };
 *
 *     expect(noop, 'nooo why fail??').to.increase(getVal);
 *
 * The alias `.increases` can be used interchangeably with `.increase`.
 *
 * @name increase
 * @alias increases
 * @param {string | Function} subject
 * @param {string} prop name _optional_
 * @param {string} msg _optional_
 * @namespace BDD
 * @public
 */
function assertIncreases (subject, prop, msg) {
  if (msg) flag(this, 'message', msg);
  var fn = flag(this, 'object')
    , flagMsg = flag(this, 'message')
    , ssfi = flag(this, 'ssfi');
  new Assertion(fn, flagMsg, ssfi, true).is.a('function');

  var initial;
  if (!prop) {
    new Assertion(subject, flagMsg, ssfi, true).is.a('function');
    initial = subject();
  } else {
    new Assertion(subject, flagMsg, ssfi, true).to.have.property(prop);
    initial = subject[prop];
  }

  // Make sure that the target is a number
  new Assertion(initial, flagMsg, ssfi, true).is.a('number');

  fn();

  var final = prop === undefined || prop === null ? subject() : subject[prop];
  var msgObj = prop === undefined || prop === null ? initial : '.' + prop;

  flag(this, 'deltaMsgObj', msgObj);
  flag(this, 'initialDeltaValue', initial);
  flag(this, 'finalDeltaValue', final);
  flag(this, 'deltaBehavior', 'increase');
  flag(this, 'realDelta', final - initial);

  this.assert(
    final - initial > 0
    , 'expected ' + msgObj + ' to increase'
    , 'expected ' + msgObj + ' to not increase'
  );
}

Assertion.addMethod('increase', assertIncreases);
Assertion.addMethod('increases', assertIncreases);

/**
 * ### .decrease(subject[, prop[, msg]])
 *
 * When one argument is provided, `.decrease` asserts that the given function
 * `subject` returns a lesser number when it's invoked after invoking the
 * target function compared to when it's invoked beforehand. `.decrease` also
 * causes all `.by` assertions that follow in the chain to assert how much
 * lesser of a number is returned. It's often best to assert that the return
 * value decreased by the expected amount, rather than asserting it decreased
 * by any amount.
 *
 *     var val = 1
 *       , subtractTwo = function () { val -= 2; }
 *       , getVal = function () { return val; };
 *
 *     expect(subtractTwo).to.decrease(getVal).by(2); // Recommended
 *     expect(subtractTwo).to.decrease(getVal); // Not recommended
 *
 * When two arguments are provided, `.decrease` asserts that the value of the
 * given object `subject`'s `prop` property is lesser after invoking the
 * target function compared to beforehand.
 *
 *     var myObj = {val: 1}
 *       , subtractTwo = function () { myObj.val -= 2; };
 *
 *     expect(subtractTwo).to.decrease(myObj, 'val').by(2); // Recommended
 *     expect(subtractTwo).to.decrease(myObj, 'val'); // Not recommended
 *
 * Add `.not` earlier in the chain to negate `.decrease`. However, it's
 * dangerous to do so. The problem is that it creates uncertain expectations
 * by asserting that the subject either increases, or that it stays the same.
 * It's often best to identify the exact output that's expected, and then
 * write an assertion that only accepts that exact output.
 *
 * When the subject is expected to increase, it's often best to assert that it
 * increased by the expected amount.
 *
 *     var myObj = {val: 1}
 *       , addTwo = function () { myObj.val += 2; };
 *
 *     expect(addTwo).to.increase(myObj, 'val').by(2); // Recommended
 *     expect(addTwo).to.not.decrease(myObj, 'val'); // Not recommended
 *
 * When the subject is expected to stay the same, it's often best to assert
 * exactly that.
 *
 *     var myObj = {val: 1}
 *       , noop = function () {};
 *
 *     expect(noop).to.not.change(myObj, 'val'); // Recommended
 *     expect(noop).to.not.decrease(myObj, 'val'); // Not recommended
 *
 * `.decrease` accepts an optional `msg` argument which is a custom error
 * message to show when the assertion fails. The message can also be given as
 * the second argument to `expect`. When not providing two arguments, always
 * use the second form.
 *
 *     var myObj = {val: 1}
 *       , noop = function () {};
 *
 *     expect(noop).to.decrease(myObj, 'val', 'nooo why fail??');
 *
 *     var val = 1
 *       , noop = function () {}
 *       , getVal = function () { return val; };
 *
 *     expect(noop, 'nooo why fail??').to.decrease(getVal);
 *
 * The alias `.decreases` can be used interchangeably with `.decrease`.
 *
 * @name decrease
 * @alias decreases
 * @param {string | Function} subject
 * @param {string} prop name _optional_
 * @param {string} msg _optional_
 * @namespace BDD
 * @public
 */
function assertDecreases (subject, prop, msg) {
  if (msg) flag(this, 'message', msg);
  var fn = flag(this, 'object')
    , flagMsg = flag(this, 'message')
    , ssfi = flag(this, 'ssfi');
  new Assertion(fn, flagMsg, ssfi, true).is.a('function');

  var initial;
  if (!prop) {
    new Assertion(subject, flagMsg, ssfi, true).is.a('function');
    initial = subject();
  } else {
    new Assertion(subject, flagMsg, ssfi, true).to.have.property(prop);
    initial = subject[prop];
  }

  // Make sure that the target is a number
  new Assertion(initial, flagMsg, ssfi, true).is.a('number');

  fn();

  var final = prop === undefined || prop === null ? subject() : subject[prop];
  var msgObj = prop === undefined || prop === null ? initial : '.' + prop;

  flag(this, 'deltaMsgObj', msgObj);
  flag(this, 'initialDeltaValue', initial);
  flag(this, 'finalDeltaValue', final);
  flag(this, 'deltaBehavior', 'decrease');
  flag(this, 'realDelta', initial - final);

  this.assert(
    final - initial < 0
    , 'expected ' + msgObj + ' to decrease'
    , 'expected ' + msgObj + ' to not decrease'
  );
}

Assertion.addMethod('decrease', assertDecreases);
Assertion.addMethod('decreases', assertDecreases);

/**
 * ### .by(delta[, msg])
 *
 * When following an `.increase` assertion in the chain, `.by` asserts that
 * the subject of the `.increase` assertion increased by the given `delta`.
 *
 *     var myObj = {val: 1}
 *       , addTwo = function () { myObj.val += 2; };
 *
 *     expect(addTwo).to.increase(myObj, 'val').by(2);
 *
 * When following a `.decrease` assertion in the chain, `.by` asserts that the
 * subject of the `.decrease` assertion decreased by the given `delta`.
 *
 *     var myObj = {val: 1}
 *       , subtractTwo = function () { myObj.val -= 2; };
 *
 *     expect(subtractTwo).to.decrease(myObj, 'val').by(2);
 *
 * When following a `.change` assertion in the chain, `.by` asserts that the
 * subject of the `.change` assertion either increased or decreased by the
 * given `delta`. However, it's dangerous to use `.change.by`. The problem is
 * that it creates uncertain expectations. It's often best to identify the
 * exact output that's expected, and then write an assertion that only accepts
 * that exact output.
 *
 *     var myObj = {val: 1}
 *       , addTwo = function () { myObj.val += 2; }
 *       , subtractTwo = function () { myObj.val -= 2; };
 *
 *     expect(addTwo).to.increase(myObj, 'val').by(2); // Recommended
 *     expect(addTwo).to.change(myObj, 'val').by(2); // Not recommended
 *
 *     expect(subtractTwo).to.decrease(myObj, 'val').by(2); // Recommended
 *     expect(subtractTwo).to.change(myObj, 'val').by(2); // Not recommended
 *
 * Add `.not` earlier in the chain to negate `.by`. However, it's often best
 * to assert that the subject changed by its expected delta, rather than
 * asserting that it didn't change by one of countless unexpected deltas.
 *
 *     var myObj = {val: 1}
 *       , addTwo = function () { myObj.val += 2; };
 *
 *     // Recommended
 *     expect(addTwo).to.increase(myObj, 'val').by(2);
 *
 *     // Not recommended
 *     expect(addTwo).to.increase(myObj, 'val').but.not.by(3);
 *
 * `.by` accepts an optional `msg` argument which is a custom error message to
 * show when the assertion fails. The message can also be given as the second
 * argument to `expect`.
 *
 *     var myObj = {val: 1}
 *       , addTwo = function () { myObj.val += 2; };
 *
 *     expect(addTwo).to.increase(myObj, 'val').by(3, 'nooo why fail??');
 *     expect(addTwo, 'nooo why fail??').to.increase(myObj, 'val').by(3);
 *
 * @name by
 * @param {number} delta
 * @param {string} msg _optional_
 * @namespace BDD
 * @public
 */
function assertDelta(delta, msg) {
  if (msg) flag(this, 'message', msg);

  var msgObj = flag(this, 'deltaMsgObj');
  var initial = flag(this, 'initialDeltaValue');
  var final = flag(this, 'finalDeltaValue');
  var behavior = flag(this, 'deltaBehavior');
  var realDelta = flag(this, 'realDelta');

  var expression;
  if (behavior === 'change') {
    expression = Math.abs(final - initial) === Math.abs(delta);
  } else {
    expression = realDelta === Math.abs(delta);
  }

  this.assert(
    expression
    , 'expected ' + msgObj + ' to ' + behavior + ' by ' + delta
    , 'expected ' + msgObj + ' to not ' + behavior + ' by ' + delta
  );
}

Assertion.addMethod('by', assertDelta);

/**
 * ### .extensible
 *
 * Asserts that the target is extensible, which means that new properties can
 * be added to it. Primitives are never extensible.
 *
 *     expect({a: 1}).to.be.extensible;
 *
 * Add `.not` earlier in the chain to negate `.extensible`.
 *
 *     var nonExtensibleObject = Object.preventExtensions({})
 *     , sealedObject = Object.seal({})
 *     , frozenObject = Object.freeze({});
 *
 *     expect(nonExtensibleObject).to.not.be.extensible;
 *     expect(sealedObject).to.not.be.extensible;
 *     expect(frozenObject).to.not.be.extensible;
 *     expect(1).to.not.be.extensible;
 *
 * A custom error message can be given as the second argument to `expect`.
 *
 *     expect(1, 'nooo why fail??').to.be.extensible;
 *
 * @name extensible
 * @namespace BDD
 * @public
 */
Assertion.addProperty('extensible', function() {
  var obj = flag(this, 'object');

  // In ES5, if the argument to this method is a primitive, then it will cause a TypeError.
  // In ES6, a non-object argument will be treated as if it was a non-extensible ordinary object, simply return false.
  // https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/isExtensible
  // The following provides ES6 behavior for ES5 environments.

  var isExtensible = obj === Object(obj) && Object.isExtensible(obj);

  this.assert(
    isExtensible
    , 'expected #{this} to be extensible'
    , 'expected #{this} to not be extensible'
  );
});

/**
 * ### .sealed
 *
 * Asserts that the target is sealed, which means that new properties can't be
 * added to it, and its existing properties can't be reconfigured or deleted.
 * However, it's possible that its existing properties can still be reassigned
 * to different values. Primitives are always sealed.
 *
 *     var sealedObject = Object.seal({});
 *     var frozenObject = Object.freeze({});
 *
 *     expect(sealedObject).to.be.sealed;
 *     expect(frozenObject).to.be.sealed;
 *     expect(1).to.be.sealed;
 *
 * Add `.not` earlier in the chain to negate `.sealed`.
 *
 *     expect({a: 1}).to.not.be.sealed;
 *
 * A custom error message can be given as the second argument to `expect`.
 *
 *     expect({a: 1}, 'nooo why fail??').to.be.sealed;
 *
 * @name sealed
 * @namespace BDD
 * @public
 */
Assertion.addProperty('sealed', function() {
  var obj = flag(this, 'object');

  // In ES5, if the argument to this method is a primitive, then it will cause a TypeError.
  // In ES6, a non-object argument will be treated as if it was a sealed ordinary object, simply return true.
  // See https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/isSealed
  // The following provides ES6 behavior for ES5 environments.

  var isSealed = obj === Object(obj) ? Object.isSealed(obj) : true;

  this.assert(
    isSealed
    , 'expected #{this} to be sealed'
    , 'expected #{this} to not be sealed'
  );
});

/**
 * ### .frozen
 *
 * Asserts that the target is frozen, which means that new properties can't be
 * added to it, and its existing properties can't be reassigned to different
 * values, reconfigured, or deleted. Primitives are always frozen.
 *
 *     var frozenObject = Object.freeze({});
 *
 *     expect(frozenObject).to.be.frozen;
 *     expect(1).to.be.frozen;
 *
 * Add `.not` earlier in the chain to negate `.frozen`.
 *
 *     expect({a: 1}).to.not.be.frozen;
 *
 * A custom error message can be given as the second argument to `expect`.
 *
 *     expect({a: 1}, 'nooo why fail??').to.be.frozen;
 *
 * @name frozen
 * @namespace BDD
 * @public
 */
Assertion.addProperty('frozen', function() {
  var obj = flag(this, 'object');

  // In ES5, if the argument to this method is a primitive, then it will cause a TypeError.
  // In ES6, a non-object argument will be treated as if it was a frozen ordinary object, simply return true.
  // See https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/isFrozen
  // The following provides ES6 behavior for ES5 environments.

  var isFrozen = obj === Object(obj) ? Object.isFrozen(obj) : true;

  this.assert(
    isFrozen
    , 'expected #{this} to be frozen'
    , 'expected #{this} to not be frozen'
  );
});

/**
 * ### .finite
 *
 * Asserts that the target is a number, and isn't `NaN` or positive/negative
 * `Infinity`.
 *
 *     expect(1).to.be.finite;
 *
 * Add `.not` earlier in the chain to negate `.finite`. However, it's
 * dangerous to do so. The problem is that it creates uncertain expectations
 * by asserting that the subject either isn't a number, or that it's `NaN`, or
 * that it's positive `Infinity`, or that it's negative `Infinity`. It's often
 * best to identify the exact output that's expected, and then write an
 * assertion that only accepts that exact output.
 *
 * When the target isn't expected to be a number, it's often best to assert
 * that it's the expected type, rather than asserting that it isn't one of
 * many unexpected types.
 *
 *     expect('foo').to.be.a('string'); // Recommended
 *     expect('foo').to.not.be.finite; // Not recommended
 *
 * When the target is expected to be `NaN`, it's often best to assert exactly
 * that.
 *
 *     expect(NaN).to.be.NaN; // Recommended
 *     expect(NaN).to.not.be.finite; // Not recommended
 *
 * When the target is expected to be positive infinity, it's often best to
 * assert exactly that.
 *
 *     expect(Infinity).to.equal(Infinity); // Recommended
 *     expect(Infinity).to.not.be.finite; // Not recommended
 *
 * When the target is expected to be negative infinity, it's often best to
 * assert exactly that.
 *
 *     expect(-Infinity).to.equal(-Infinity); // Recommended
 *     expect(-Infinity).to.not.be.finite; // Not recommended
 *
 * A custom error message can be given as the second argument to `expect`.
 *
 *     expect('foo', 'nooo why fail??').to.be.finite;
 *
 * @name finite
 * @namespace BDD
 * @public
 */
Assertion.addProperty('finite', function(msg) {
  var obj = flag(this, 'object');

  this.assert(
      typeof obj === 'number' && isFinite(obj)
    , 'expected #{this} to be a finite number'
    , 'expected #{this} to not be a finite number'
  );
});