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Expect-like assertions that works seamlessly in node and browsers


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Assume is an expect inspired assertion library who's sole purpose is to create a working and human readable assert library for browsers and node. The library is designed to work with different assertion styles.

I've been trying out a lot of libraries over the last couple of years and none of the assertion libraries that I found "tickled my fancy". They either only worked in node or had really bad browser support. I wanted something that I can use starting from Internet Explorer 5 to the latest version while maintaining the expect like API that we all know and love. Writing tests should be dead simple and not cause any annoyances. This library attempts to achieve all of this.


Assume is written with client and server-side JavaScript in mind and uses the commonjs module system to export it self. The library is released in the public npm registry and can be installed using:

npm install --save-dev assume

The --save-dev flag tells npm to automatically add this package.json and it's installed version to the devDependencies of your module.

As code is written against the commonjs module system we also ship a standalone version in the module which introduces an assume global. The standalone version can be found in the dist folder after installation. The dist file is not commited to GitHub.

Table of Contents


We support a lot of different syntaxes and assertion styles. The only thing we will no (read never) support is the should syntax as we will never extend build-in objects/primitives of JavaScript.

The default syntax that we support is modeled after expect so you can replace any assertion library which implements this API by simply changing the require to:

var expect = require('assume');


As you can see in the example above the is property is used to make the assertion more readable. We support the following aliases which allow these kind of chains:

  • to
  • be
  • been
  • is
  • was
  • and
  • has
  • have
  • had
  • with
  • that
  • at
  • of
  • some
  • does
  • did
  • itself
  • which

So you can just write:


But do note that these aliases are optionally so the above example can also be written as:



The module can be configured globally be changing the properties on the config object:

var assume = require('assume');
assume.config.includeStack = false;

Or locally for each assertions by supplying the assume function with an optional configuration object:

assume('foo', { includeStack: false }).is.a('buffer');

The following options can be configured:

  • includeStack Should we output a stack trace. Defaults to true.
  • showDIff Show difference between the given and expected values. Defaults to true.

Feature Detection

Certain assertions only work in certain JavaScript/EcmaScript environments. Things like the generator assertions only work in ES6 as the function * is invalid syntax. The results of the feature detection is publicly stored in the assume.supports object. You can use this object to add some conditional tests to your test suite. The following features are currently detected:

  • generators Are generators supported in the host environment.
  • native Is V8 native syntax supported.
if (assume.supports.native) {
  it('does things', function () {

If you are a plugin author, feel free to add your own feature detections to this object (as long as you do not override any pre-existing values).

Performance Testing

The performance testing is only available for environments that use V8 and more specifically the --allow-natives-syntax flags. These flags can be supplied in chrome before you start browser. These flags are necessary to get access to the V8 internals which expose optimization and de-optimization information.

If you are running iojs or node on the server, you can pass in these flags directly:

iojs --allow-natives-syntax


If you are using mocha as test runner you usually add mocha as executable. But unfortunately, the mocha binary doesn't allow you to pass V8 flags. So instead of using the mocha binary directly you can use the node and call the _mocha binary instead:

node --allow-natives-syntax --harmony ./node_modules/mocha/bin/_mocha test/test.js

You can check if your host environment supports these performance tests by checking the assume.supports.native variable.


There are various of assertions available in assume. If you want the failed assertion to include a custom message or reason you can always add this as last argument of the assertion function.

assume(true).is.false('how odd, true is not a false');

The behaviours of the assertions can be chained using special "flags" or "prefixes". We currently support the following prefixes.

  • .not, .doesnt, .dont Instead of assuming that your assertions assert to true they will now assert for false.
  • .deep, .deeply, .strict .strictly Instructs the assertions to do a deep equal, so it checks if the contents match instead of an object it self matches.

For example:



Now, a special word of caution for those of you who are using this library to write cross browser tests. Internet Explorer has issues when you use keywords as functions. Using the true(), instanceof() etc. functions to assert you will run in to issues. So the rule of thumb here is that if you need to do cross browser support do not assert with the keyword based names.


Let's take a closer look to all assertions that we're supporting:

a, an

Asserts if the given value is the correct type. We need to use Object.toString here because there are some implementation bugs the typeof operator:

  • Chrome <= 9: /Regular Expressions/ are evaluated to function

As well as all common flaws like Arrays being seen as Objects etc. This eliminates all these edge cases.


instanceof is a keyword and might cause cross browser issues

eitherOfType, oneOfType

Asserts if the given value is one of the acceptable types.

The same caveats regarding typeof apply as described in a, an.

assume([]).is.oneOfType(['array', 'string']);

instanceOf, instanceof, inherits, inherit

Asserts that the value is instanceof the given constructor.

function Classy() {}

var classes = new Classy();


include, includes, contain, contains

Assert that value includes a given value. I know this sounds vague but an example might be more useful here. It can check this for strings, objects and arrays.

assume({foo: 'bar'}).contains('foo');
assume('hello world').includes('world');

ok, okay, truthy, truely

Assert that the value is truthy.


falsely, falsey

Assert that the value is falsey.



Explicitly check that the value is the boolean true.


true is a keyword and might cause cross browser issues


Explicitly check that the value is the boolean false.


false is a keyword and might cause cross browser issues

exists, exist

Check if the value not not null.

assume(undefined).exists(); // throws

length, lengthOf, size

Assert if the given value has the given length. It accepts arrays, strings, functions, object and anything else that has a .length property.

assume({ foo: 'bar' }).has.length(1);


Short hand function for assume(val).length(0) so it can check if objects, arrays, strings are empty.


// Also works against everything that has a `.length` property

above, gt, greater, greaterThan

Assert if the value is above the given value. If you need greater or equal check out the least method. If value to assert is not a number we automatically extract the length out of it so you can use it check the length of arrays etc.


least, gte, atleast

Assert if the value is above or equal to the given value. If you just need greater check out the above method. If value to assert is not a number we automatically extract the length out of it so you can use it check the length of arrays etc.


below, lt, less, lessThan

Assert if the value is less than the given value. If you need less or equal check out the most method. If value to assert is not a number we automatically extract the length out of it so you can use it check the length of arrays etc.


most, lte, atmost

Assert if the value is less or equal to the given value. If you just need less, check out the less method. If value to assert is not a number we automatically extract the length out of it so you can use it check the length of arrays etc.


within, between

Check if the value is between or equal to a given range. If value to assert is not a number we automatically extract the length out of it so you can use it check the length of arrays etc.

assume(100).is.between(90, 100);
assume([1, 213, 13, 94, 5, 6, 7]).is.between(2, 10);

hasOwn, own, ownProperty, haveOwnProperty, property, owns, hasown

Assert that the value has the specified property and optionally deeply check its value.

assume({foo: 'bar'}).owns('foo');
assume({foo: 'bar'}).owns('foo', 'bar');

match, matches

Matches the value against a given Regular Expression. If a string is given instead of an actual Regular Expression we automatically transform it to an new RegExp.

assume('hello world').matches(/world$/);

equal, equals, eq, eqs, exactly

Assert that given value is strictly (===) equal to the supplied value.


eql, eqls

Assert that the given value deeply equals the supplied value.



Assert that the value is either one of the values of the given array. It can be prefixed with .deep for deep assertions.

assume('foo').is.either(['bar', 'banana', 'foo']);
assume({ foo: 'bar' }).is.either(['bar', 'banana', { foo: 'bar' }]);

throw, throws, fail, fails

Assert that the given function throws an error. The error can match a string, regexp or function instance.

function arrow() { throw new Error('you have failed this city'); }

assume(arrow).throws(/failed this city/);
assume(arrow).throws('failed this city');
assume(arrow).does.not.throw('your mom');

throw is a keyword and might cause cross browser issues

finite, isFinite, finiteness

Assert that the given value is finite.


If deep assertion style is used it will use the much stricter Number.isFinite instead of the regular isFinite functionality.


Assert that the given value is an EcmaScript 6 based generator.

assume(function *() {}).is.generator();

Please note that this will only work if Generators are enabled in the host environment or you might end up with false positives

optimisation, optimization

Please see the Performance Testing section information to enable these assertions as they require specific V8 flags to be enabled.

optimised, optimized

Please see the Performance Testing section information to enable these assertions as they require specific V8 flags to be enabled.

start, starts, startsWith, startWith

Assert that the value starts with the given string.


end, ends, endsWith, endWith

Assert that the value ends with the given string.

assume('foor bar, banana').endsWith('ana');

closeTo, close, approximately, near

Assert a float point number is near a given value within a delta margin.

assume(1.5).is.approximately(1.4, 0.2);


The asserts we write are assumptions that we receive a given value. While you're writing tests you hope that they all pass. We could write these tests using an i.hope.that(value) syntax:

var i = require('assume');


rejected, rejects, throwAsync, throwsAsync, failAsync, failsAsync

Assert that the thenable results in a rejected state.

await assume(thisFunctionAsyncThrows()).to.throwAsync();

resolveSync, resolvesSync, resolvedSync, completeSync, completesSync, completedSync

Assert that the thenable completed and the result was filled synchronously.

await assume(thisFuncCompletesSynchronously()).to.completeSync();

Note that Promise always complete asynchronously even if it's already in a resolved or rejected state.


The assume.plan method allows you to plan the amount of assertions that should be executed by your test suite. This method accepts 2 arguments:

  1. The amount of assertions you expect to run. This should always be an exact number.
  2. An optional async callback that should be called with error as first argument on failure instead of throwing an error. This makes it ideal for async testing as you can just pass your continuation function.

The method will return a function that should be called at the end of your tests. This method will still allow you to pass in an error as first argument so the supplied callback in second argument will be called directly with it.

When the method is called we will count the amount of assertions that we're executed. If it's less or more than the supplied amount we will throw an error.

var end = assume.plan(10);

end(); // This throws an error as we only executed 1 out of the 10 asserts.

And with optional async callback:

next = assume.plan(7, next);

for (var i = 0; i < 10; i++) {

next(); // Also throws an error as we've executed 10 assertions instead of 7.


Writing async tests can be hard, especially if you have to juggle with callbacks and wait untill 2 callbacks are completed before you can continue with the test suite. The assume.wait function helps you with orchestration of tests and callbacks. The method accepts 3 arguments:

  1. The amount of times the returned callback should be called before calling the supplied callback.
  2. Optionally, the amount of assertions you expect to run. We will wrap the returned callback with assume.plan this way.
  3. Completion callback which is called after the callbacks have been called.

The method will return a function that should be used as callback for your async tests. It follows an error first callback pattern and instantly calls the supplied callback once an error has be passed in as error argument.

 it('does async things', function (next) {
   next = assume.wait(2, 4, next);

   asynctask(function (err, data) {


   asynctaskfail(function (err, data) {



We've done our best to include a bunch of assertions that should make it easier to test your code but it's always possible that we're missing assertions or you just want to eliminate repetition in your code. So we've got a plugin interface which allows you to extend the assume instance with even more assertions.


For the sake of discoverablity ability of your plugins on npm we suggest to either suffix or prefix your module with assume and adding the assume keyword in to your keywords list.


Let's assume that we've want to extend the library with a method for checking the headers of a passed in HTTP request object. If it was released in npm we could add it as following:


The use method returns assume so it can be used to chain multiple plugin calls together:


The assume-headers plugin/module should export a function which receives the assume instance to extend as illustrated by the example below:

module.exports = function plugin(assume, util) {
   * Assert that the received HTTP request contains a given header.
   * @param {String} name Name of the header that we should have received.
   * @param {String} ms Reason of failure.
   * @returns {Assume}
   * @api public
  assume.add('header', function header(name, msg) {
    var expect = '`'+ util.string(this.value.header)'` to @ have header '+ util.string(name);

    return this.test(name in this.value.headers, msg, expect);

The plugin receives 2 arguments:

  1. A reference to the assume instance so it can be extended.
  2. Small assertion helper library which contains all the internals we're using.

The helper library contains the following methods:

  • name, Reference to the module so you extract names from functions.
  • get, Reference to the get method of the pathval module.
  • string, Inspection function which safely transforms objects, numbers, dates etc. to a string.
  • deep, A deep assertion if the two argument deeply equal to each other.
  • type, Extract the type of an object to an lowercase string. Useful for detecting the difference between object, array, arguments, date, buffer etc.
  • size, A function which returns the size of given object or array.
  • each, Simple iterator which accepts an array/object and calls the supplied callback with the value and key/index.
  • nodejs, Boolean indicating if we are running on nodejs.

New flags can be introduced by adding properties to the flags object. The flags object has the following structure:

  • key This is the name of the property which will be added to the assume instance. The property is set to false by default and will be to true once once of the flags is accessed.
  • value These are the aliases that can be used to the set the property to true.

For our .not flags we've set the following key/value's:

Assert.flags.untrue = 'doesnt, not, dont';

Please do note that you should try to limit the amount of flags that you add as they are quite expensive to process every single time.

Adding new assertions to assume can be done using the following methods:


The assume.add method is a convince method for adding new methods to the assume prototype. It was created using the assign method so it can automatically add aliases/shorthand's of the method to the prototype in one go. The method requires 2 arguments:

  1. A string which is comma or space separated or an array which contains the names of the methods that should be added to the prototype.
  2. The function or value that is assigned for all these properties.
module.exports = function (assume, util) {
  util.each(['GET', 'POST', 'PUT', 'DELETE'], function each(method) {
    assume.add(method, function () {
      var expect = '`'+ util.string(this.value.method)+'` to @ be '+ method;

      return this.test(this.value.method === method, msg, expect);

If you want to add more aliases for the .function() method you can simply do a:

assume.add('execute, executes, exec', function () {
  return this.clone().is.a('function');

The value to assert is stored in the value property of the instance. If the deep flag is set, the deeply property is set to true.


This is the method that handles all the assertion passing and failing. It's the most important method of all. It accepts the following arguments:

  • passed, a boolean which indicates if the assertion failed or passed.
  • msg, a string which is the reason or message provided by the users.
  • expectation, a compiled template which explains what the assertion expected.
  • slice, a number which slices of stacks from the stack trace. This is keeps the stack trace clear of all references to our own assertion library and only contains the part of the test/suite where the assertion was initiated. This value is optional and defaults to 2 so it removes the test and the assertion from the stack.

If the assertion passes the method will return it self, or it will throw.

assume.add('true', function (msg) {
  var expectation = format('value to @ be true');
  return this.test(this.value === true, msg, expectation);

In example above you might have noticed the odd @ in the expection value. This is a special character and will be replaced with not if the .not flag was used or completely removed (including an extra whitespace at the end).


Assign multiple values to a given thing. This method accepts one argument which is an object or prototype on where we should assign things. It will return a function that is responsible for the assignment on that given thing. The return function will require 2 arguments:

  1. A string which is comma or space separated or an array which contains the names of the methods that should be added to the prototype.
  2. The function or value that is assigned for all these properties.

To create your own custom add method you could simply do:

var add = assume.assign(assign.prototype);

And the add function would now do exactly the same as the assume.add method.


Create an exact clone of the assume instance so it all flags and options are identical to the current assume instance. The method accepts one optional argument which is the value it should assert. If nothing is given it uses the current value.

This can be helpful if you want to run assertions in your assertions so you can assert while you assert.

// Yo dawg, I herd you like assertions so I put assertions in the assertions so
// you can assert while you assert.

assume.add('something', function somethign(value, msg) {

  return this.test(this.value ==== 'something', msg, 'the value should be something');




Expect-like assertions that works seamlessly in node and browsers