Mirego JavaScript Style Guide() {

A mostly reasonable approach to JavaScript

Types

Primitives: When you access a primitive type you work directly on its value.
- string
- number
- boolean
- null
- undefined
```
var foo = 1;
var bar = foo;

bar = 9;

console.log(foo, bar); // => 1, 9
```

Complex: When you access a complex type you work on a reference to its value.

object
array
function

var foo = [1, 2];
var bar = foo;

bar[0] = 9;

console.log(foo[0], bar[0]); // => 9, 9

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Objects

Use the literal syntax for object creation.

// Bad
var item = new Object();

// Good
var item = {};

Don’t use reserved words as keys. It won’t work in IE8.

// Bad
var superman = {
  default: { clark: 'kent' },
  private: true
};

// Good
var superman = {
  defaults: { clark: 'kent' },
  hidden: true
};

If you really must use a reserved word, use the subscript notation:

// Bad
require('some-module').default;

// Good
require('some-module')['default'];

Use readable synonyms in place of reserved words.

// Bad
var superman = {
  class: 'alien'
};

// Bad
var superman = {
  klass: 'alien'
};

// Good
var superman = {
  type: 'alien'
};

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Arrays

Use the literal syntax for array creation.

// Bad
var items = new Array();

// Good
var items = [];

If you don’t know array length use Array#push.

var someStack = [];

// Bad
someStack[someStack.length] = 'abracadabra';

// Good
someStack.push('abracadabra');

When you need to copy an array use Array#slice. jsPerf

var length = items.length;
var itemsCopy = [];
var index;

// Bad
for (index = 0; index < length; index++) {
  itemsCopy[index] = items[index];
}

// Good
itemsCopy = items.slice();

To convert an array-like object to an array, use Array#slice.

function trigger() {
  var args = Array.prototype.slice.call(arguments);
  // ...
}

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Strings

Use single quotes '' for strings.

// Bad
var name = "Bob Parr";

// Good
var name = 'Bob Parr';

// Bad
var fullName = "Bob " + this.lastName;

// Good
var fullName = 'Bob ' + this.lastName;

Strings longer than 80 characters should be written across multiple lines using string concatenation. Strings on the following lines should be indented to indicate they’re part of the previous expression.

// Bad
var errorMessage = 'This is a super long error that was thrown because of Batman. When you stop to think about how Batman had anything to do with this, you would get nowhere fast.';

// Bad
var errorMessage = 'This is a super long error that was thrown because \
of Batman. When you stop to think about how Batman had anything to do \
with this, you would get nowhere \
fast.';

// Good
var errorMessage = 'This is a super long error that was thrown because ' +
  'of Batman. When you stop to think about how Batman had anything to do ' +
  'with this, you would get nowhere fast.';

String interpolation should be done with backticks when using an ECMAScript 6 to ECMAScript 5 transpiler.

var message = `Hello ${firstName} ${lastName}`;

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Functions

Knowing the difference between a function declaration and a function expression

// Function declaration
function foo() {
  console.log('I’m a declaration');
}

// Function expression
var bar = function() {
  console.log('I’m an expression');
};

Function expressions:

// Anonymous function expression
var anonymous = function() {
  return true;
};

// Named function expression
// Name function expression are convenient when debugging because
// the DevTools will show their name instead of "anonymous function"
var named = function named() {
  return true;
};

// Immediately-invoked function expression (IIFE)
(function() {
  console.log('Welcome to the Internet. Please follow me.');
})();

Never declare a function in a non-function block (if, while, etc). Browsers will allow you to do it, but they all interpret it differently. Assign the function to a variable instead.

Note: ECMA-262 defines a "block" as a list of statements. A function declaration is not a statement. Read ECMA-262's note on this issue.

// Bad
if (currentUser) {
  function test() {
    console.log('Nope.');
  }
}

// Good
var test;
if (currentUser) {
  test = function test() {
    console.log('Yup.');
  };
}

Never name a parameter arguments, this will take precedence over the arguments object that is given to every function scope.

// Bad
function nope(name, options, arguments) {
  // ...
}

// Good
function yup(name, options, args) {
  // ...
}

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Properties

Use dot notation when accessing properties.

var luke = {
  jedi: true,
  age: 28
};

// Bad
var isJedi = luke['jedi'];

// Good
var isJedi = luke.jedi;

Use subscript notation [] when accessing properties with a variable.

var luke = {
  jedi: true,
  age: 28
};

function getProp(prop) {
  return luke[prop];
}

var isJedi = getProp('jedi');

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Variables

Always use var to declare variables. Not doing so will result in global variables. We want to avoid polluting the global namespace.
```
// Bad
superPower = new SuperPower();

// Good
var superPower = new SuperPower();
```

Use one var declaration per variable. It’s easier to add new variable declarations this way, and you never have to worry about swapping out a ; for a , or introducing punctuation-only diffs.

// Bad
var items = getItems(),
    goSportsTeam = true,
    dragonball = 'z';

// Bad
// (compare to above, and try to spot the mistake)
var items = getItems(),
    goSportsTeam = true;
    dragonball = 'z';

// Good
var items = getItems();
var goSportsTeam = true;
var dragonball = 'z';

Declare unassigned variables last. This is helpful when later on you might need to assign a variable depending on one of the previous assigned variables.

// Bad
var index, length, dragonball,
    items = getItems(),
    goSportsTeam = true;

// Bad
var index;
var items = getItems();
var dragonball;
var goSportsTeam = true;
var length;

// Good
var items = getItems();
var goSportsTeam = true;
var dragonball;
var length;
var index;

Assign variables at the top of their scope. This helps avoid issues with variable declaration and assignment hoisting related issues.

// Bad
function() {
  test();

  //...

  var name = getName();

  if (name === 'test') return false;

  return name;
}

// Good
function() {
  var name = getName();

  test();

  //...

  if (name === 'test') {
    return false;
  }

  return name;
}

// Bad
function() {
  var name = getName();

  if (!arguments.length) {
    return false;
  }

  return true;
}

// Good
function() {
  if (!arguments.length) {
    return false;
  }

  var name = getName();

  return true;
}

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Hoisting

Variable declarations get hoisted to the top of their scope, their assignment does not.

// We know this wouldn’t work
// (assuming there is no notDefined global variable)
function example() {
  console.log(notDefined); // => throws a ReferenceError
}

// Creating a variable declaration after you reference the variable will work due to
// variable hoisting. Note: the assignment value of `true` is not hoisted.
function example() {
  console.log(declaredButNotAssigned); // => undefined
  var declaredButNotAssigned = true;
}

// The interpreter is hoisting the variable declaration to the top of the scope,
// which means our example could be rewritten as:
function example() {
  var declaredButNotAssigned;
  console.log(declaredButNotAssigned); // => undefined
  declaredButNotAssigned = true;
}

Anonymous function expressions hoist their variable name, but not the function assignment.

function example() {
  console.log(anonymous); // => undefined

  anonymous(); // => TypeError anonymous is not a function

  var anonymous = function() {
    console.log('anonymous function expression');
  };
}

Named function expressions hoist the variable name, not the function name or the function body.

function example() {
  console.log(named); // => undefined

  named(); // => TypeError named is not a function

  superPower(); // => ReferenceError superPower is not defined

  var named = function superPower() {
    console.log('Flying');
  };
}

// The same is true when the function name is the same as the variable name.
function example() {
  console.log(named); // => undefined

  named(); // => TypeError named is not a function

  var named = function named() {
    console.log('named');
  }
}

Function declarations hoist their name and the function body.

function example() {
  superPower(); // => Logs 'Flying'

  function superPower() {
    console.log('Flying');
  }
}

For more information refer to JavaScript Scoping & Hoisting by Ben Cherry.

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Conditional Expressions & Equality

Use === and !== over == and !=.
Conditional expressions are evaluated using coercion with the ToBoolean method and always follow these simple rules:
- Objects evaluate to true
- undefined evaluates to false
- null evaluates to false
- Booleans evaluate to the value of the boolean
- Numbers evaluate to false if +0, -0, or NaN, otherwise true
- Strings evaluate to false if empty, otherwise true
```
if ([0]) {
  // This is executed
  // An array is an object, objects evaluate to `true`
}
```

Use shortcuts.

// Bad
if (name !== '') {
  // ...
}

// Good
if (name) {
  // ...
}

// Bad
if (collection.length > 0) {
  // ...
}

// Good
if (collection.length) {
  // ...
}

For more information see Truth Equality and JavaScript by Angus Croll.

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Blocks

Use braces with all multi-line blocks. You can omit braces when a block contains only one statement but still fits in a 80-characters line.

// Bad
if (test)
  return false;

// Good
if (test) return false;

// Good
if (test) {
  return false;
}

// Bad
function() { return false; }

// Good
function() {
  return false;
}

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Comments

Comments should not be common place in your code, we aim to write code that is simple and understandable at a glance. When we do write comments, it is to:
- put emphasis on a hack, explain why it is there and explain how it works;
- explain complex logic;
- explain domain logic which might not be obvious;
- separate code blocks (mainly configuration blocks);
Prefixing your comments with FIXME or TODO helps other developers quickly understand if you’re pointing out a problem that needs to be revisited, or if you’re suggesting a solution to the problem that needs to be implemented. These are different than regular comments because they are actionable. The actions are FIXME: need to figure this out or TODO: need to implement.

Use // FIXME to annotate problems.

function Calculator() {
  // FIXME shouldn’t use a global here
  total = 0;

  return this;
}

Use // TODO to annotate solutions to problems.

function Calculator() {
  // TODO total should be configurable by an options param
  this.total = 0;

  return this;
}


**[⬆ back to top](#table-of-contents)**

## Whitespace

- Use soft tabs set to 2 spaces.

  ```js
  // Bad
  function() {
  ∙∙∙∙var name;
  }

  // Bad
  function() {
  ∙var name;
  }

  // Good
  function() {
  ∙∙var name;
  }
  ```

- Place 1 space before the leading brace.

  ```js
  // Bad
  function test(){
    console.log('test');
  }

  // Good
  function test() {
    console.log('test');
  }

  // Bad
  dog.set('attr',{
    age: '1 year',
    breed: 'Bernese Mountain Dog'
  });

  // Good
  dog.set('attr', {
    age: '1 year',
    breed: 'Bernese Mountain Dog'
  });
  ```

- Set off operators with spaces.

  ```js
  // bad
  var x=y+5;

  // good
  var x = y + 5;
  ```

- End files with a single newline character.

  ```js
  // Bad
  (function(global) {
    // ...
  })(this);
  ```

  ```js
  // Bad
  (function(global) {
    // ...
  })(this);↵
  ↵
  ```

  ```js
  // Good
  (function(global) {
    // ...
  })(this);↵
  ```

- Use indentation when making long method chains. Use a leading dot, which
  emphasizes that the line is a method call, not a new statement.

  ```js
  // Bad
  $('#items').find('.selected').highlight().end().find('.open').updateCount();

  // Bad
  $('#items').
    find('selected').
      highlight().
      end().
    find('.open').
      updateCount();

  // Good
  $('#items')
    .find('.selected')
      .highlight()
      .end()
    .find('.open')
      .updateCount();

  // Bad
  var leds = stage.selectAll('.led').data(data).enter().append('svg:svg').class('led', true)
      .attr('width',  (radius + margin) * 2).append('svg:g')
      .attr('transform', 'translate(' + (radius + margin) + ',' + (radius + margin) + ')')
      .call(tron.led);

  // good
  var leds = stage.selectAll('.led')
      .data(data)
    .enter().append('svg:svg')
      .class('led', true)
      .attr('width',  (radius + margin) * 2)
    .append('svg:g')
      .attr('transform', 'translate(' + (radius + margin) + ',' + (radius + margin) + ')')
      .call(tron.led);
  ```

**[⬆ back to top](#table-of-contents)**

## Commas

- Leading commas: **Nope.**

  ```js
  // Bad
  var story = [
      once
    , upon
    , aTime
  ];

  // Good
  var story = [
    once,
    upon,
    aTime
  ];

  // Bad
  var hero = {
      firstName: 'Bob'
    , lastName: 'Parr'
    , heroName: 'Mr. Incredible'
    , superPower: 'strength'
  };

  // Good
  var hero = {
    firstName: 'Bob',
    lastName: 'Parr',
    heroName: 'Mr. Incredible',
    superPower: 'strength'
  };
  ```

- Additional trailing comma: **Nope.** This can cause problems with IE6/7 and IE9 if it’s in quirksmode. Also, in some implementations of ES3 would add length to an array if it had an additional trailing comma. This was clarified in ES5 ([source](http://es5.github.io/#D)):

> Edition 5 clarifies the fact that a trailing comma at the end of an ArrayInitialiser does not add to the length of the array. This is not a semantic change from Edition 3 but some implementations may have previously misinterpreted this.

  ```js
  // Bad
  var hero = {
    firstName: 'Kevin',
    lastName: 'Flynn',
  };

  var heroes = [
    'Batman',
    'Superman',
  ];

  // Good
  var hero = {
    firstName: 'Kevin',
    lastName: 'Flynn'
  };

  var heroes = [
    'Batman',
    'Superman'
  ];
  ```

**[⬆ back to top](#table-of-contents)**

## Semicolons

- **Always.**

  ```js
  // Bad
  (function() {
    var name = 'Skywalker'
    return name
  })()

  // Good
  (function() {
    var name = 'Skywalker';
    return name;
  })();

  // Good (guards against the function becoming an argument when two files with IIFEs are concatenated)
  ;(function() {
    var name = 'Skywalker';
    return name;
  })();
  ```

  [Read more](http://stackoverflow.com/a/7365214/1712802).

**[⬆ back to top](#table-of-contents)**

## Type Casting & Coercion

- Perform type coercion at the beginning of the statement.

- Strings:

  ```js
  //  => this.reviewScore = 9;

  // Bad
  var totalScore = this.reviewScore + '';

  // Good
  var totalScore = '' + this.reviewScore;

  // Bad
  var totalScore = '' + this.reviewScore + ' total score';

  // Good
  var totalScore = this.reviewScore + ' total score';
  ```

- Use `parseInt` and `parseFloat` for Numbers. When using `parseInt` always specify a radix.

  ```js
  var integerValue = '4';
  var floatValue = '4.5';

  // Bad
  var value = new Number(integerValue);

  // Bad
  var value = + integerValue;

  // Bad
  var value = integerValue >> 0;

  // Bad
  var value = parseInt(integerValue);

  // Good
  var value = Number(integerValue);

  // Good
  var value = parseInt(integerValue, 10);

  // Good
  var value = parseFloat(floatValue);
  ```

- If for whatever reason you are doing something wild and `parseInt` is your bottleneck and need to use Bitshift for [performance reasons](http://jsperf.com/coercion-vs-casting/3), leave a comment explaining why and what you're doing.

  ```js
  // Good

  // `parseInt` was the reason my code was slow. Bitshifting the String
  // to coerce it to a Number made it a lot faster.
  var value = inputValue >> 0;
  ```

- **Note:** Be careful when using bitshift operations. Numbers are represented as [64-bit values](http://es5.github.io/#x4.3.19), but Bitshift operations always return a 32-bit integer ([source](http://es5.github.io/#x11.7)). Bitshift can lead to unexpected behavior for integer values larger than 32 bits. Largest signed 32-bit Int is 2,147,483,647:

  ```js
  2147483647 >> 0 // => 2147483647
  2147483648 >> 0 // => -2147483648
  2147483649 >> 0 // => -2147483647
  ```

- Booleans:

  ```js
  var age = 0;

  // Bad
  var hasAge = new Boolean(age);

  // Good
  var hasAge = Boolean(age);

  // Good
  var hasAge = !!age;
  ```

**[⬆ back to top](#table-of-contents)**


## Naming Conventions

- Avoid single letter names. Be descriptive with your naming. We prefer readable and understandable code to hand-minified code. Don’t worry about the few extra bytes, minifiers and gzip take care of that for us.

  ```js
  // Bad
  function q() {
    // ...
  }

  // Good
  function query() {
    // ...
  }
  ```

- Use camelCase when naming objects, functions, and instances.

  ```js
  // Bad
  var OBJEcttsssss = {};

  var this_is_my_object = {};

  function c() {}

  var u = new user({
    name: 'Bob Parr'
  });

  // Good
  var thisIsMyObject = {};

  function thisIsMyFunction() {}

  var user = new User({
    name: 'Bob Parr'
  });
  ```

- Use PascalCase when naming constructors or classes.

  ```js
  // Bad
  function user(options) {
    this.name = options.name;
  }

  class user {
    constructor(options) {
      this.name = options.name;
    }
  }

  // Good
  function User(options) {
    this.name = options.name;
  }

  class User {
    constructor(options) {
      this.name = options.name;
    }
  }
  ```

- Use UPPER\_CASE\_SNAKE\_CASE when naming constants.

```js
var API_URL = '...';

Use a leading underscore _ when naming private properties.

// Bad
this.__firstName__ = 'Panda';
this.firstName_ = 'Panda';

// Good
this._firstName = 'Panda';

When saving a reference to this use self.

// Bad
function() {
  var _this = this;
  return function() {
    console.log(_this);
  };
}

// Bad
function() {
  var that = this;
  return function() {
    console.log(that);
  };
}

// Good
function() {
  var self = this;
  return function() {
    console.log(self);
  };
}

Name your functions expressions. This is helpful for stack traces.

// Bad
var log = function(msg) {
  console.log(msg);
};

// Good
var log = function log(msg) {
  console.log(msg);
};

Note: IE8 and below exhibit some quirks with named function expressions. See http://kangax.github.io/nfe/ for more info.

⬆ back to top

Accessors

Accessor functions for properties are not required.

If you do make accessor functions use getValue() and setValue('hello').

// Bad
dragon.age();

// Good
dragon.getAge();

// Bad
dragon.age(25);

// Good
dragon.setAge(25);

If the property is a boolean, use isValue() or hasValue().

// Bad
if (!dragon.age()) {
  return false;
}

// Good
if (!dragon.hasAge()) {
  return false;
}

It’s okay to create get() and set() functions, but be consistent.

function Jedi(options) {
  options || (options = {});
  var lightsaber = options.lightsaber || 'blue';
  this.set('lightsaber', lightsaber);
}

Jedi.prototype.set = function(key, val) {
  this[key] = val;
};

Jedi.prototype.get = function(key) {
  return this[key];
};

⬆ back to top

Classes

When possible use an ECMAScript 6 to ECMAScript 5 transpiler and write classes.

class Jedi {
  constructor() {
    console.log('new jedi');
  }
}

If you must write classes the old way, assign methods to the prototype object instead of overwriting the prototype with a new object. Overwriting the prototype makes inheritance impossible: by resetting the prototype you’ll overwrite the base!

function Jedi() {
  console.log('new jedi');
}

// Bad
Jedi.prototype = {
  fight: function fight() {
    console.log('fighting');
  },

  block: function block() {
    console.log('blocking');
  }
};

// Good
Jedi.prototype.fight = function fight() {
  console.log('fighting');
};

Jedi.prototype.block = function block() {
  console.log('blocking');
};

Methods can return this to help with method chaining.

// Bad
Jedi.prototype.jump = function() {
  this.jumping = true;
  return true;
};

Jedi.prototype.setHeight = function(height) {
  this.height = height;
};

var luke = new Jedi();
luke.jump(); // => true
luke.setHeight(20); // => undefined

// Good
Jedi.prototype.jump = function() {
  this.jumping = true;
  return this;
};

Jedi.prototype.setHeight = function(height) {
  this.height = height;
  return this;
};

var luke = new Jedi();

luke.jump()
  .setHeight(20);

It’s okay to write a custom toString() method, just make sure it works successfully and causes no side effects.

function Jedi(options) {
  options || (options = {});
  this.name = options.name || 'no name';
}

Jedi.prototype.getName = function getName() {
  return this.name;
};

Jedi.prototype.toString = function toString() {
  return 'Jedi - ' + this.getName();
};

⬆ back to top

Events

When attaching data payloads to events (whether DOM events or custom events), pass a hash instead of a raw value. This allows a subsequent contributor to add more data to the event payload without finding and updating every handler for the event. For example, instead of:

// Bad
$(this).trigger('listingUpdated', listing.id);
...

$(this).on('listingUpdated', function(event, listingId) {
  // do something with listingId
});

prefer:

// Good
$(this).trigger('listingUpdated', { listingId : listing.id });
...

$(this).on('listingUpdated', function(event, data) {
  // do something with data.listingId
});

⬆ back to top

Modules

Use ECMAScript 6 module syntax when possible.

import Ember from 'ember';

export default Ember.Component.extend();

⬆ back to top

jQuery

Prefix jQuery object variables with a $.

// Bad
var sidebar = $('.sidebar');

// Good
var $sidebar = $('.sidebar');

Cache DOM lookups.

// Bad
function setSidebar() {
  $('.sidebar').hide();

  // ...stuff...

  $('.sidebar').css({
    'background-color': 'pink'
  });
}

// Good
function setSidebar() {
  var $sidebar = $('.sidebar');
  $sidebar.hide();

  // ...stuff...

  $sidebar.css({
    'background-color': 'pink'
  });
}

Use find with scoped jQuery object queries.

// Bad
$('ul', '.sidebar').hide();

// Bad
$('.sidebar').find('ul').hide();

// Good
$('.sidebar ul').hide();

// Good
$('.sidebar > ul').hide();

// Best
$sidebar.find('ul').hide();

⬆ back to top

ECMAScript versions compatibility

Refer to Kangax’s compatibility tables
- ES5: http://kangax.github.io/compat-table/es5/.
- ES6: http://kangax.github.io/compat-table/es6/.

⬆ back to top

Testing

Yup. You should setup your project with:
- testem
- Mocha
- Chai
If possible, setup your project to run your specs on TravisCI along with style validations.

⬆ back to top

Resources

Read This

Annotated ECMAScript 5.1

Tools

Code Style Linters

⬆ back to top

License

(The MIT License)

Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the 'Software'), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED 'AS IS', WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.

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Mirego JavaScript Style Guide() {

Table of Contents

Types

Objects

Arrays

Strings

Functions

Properties

Variables

Hoisting

Conditional Expressions & Equality

Blocks

Comments

Accessors

Classes

Events

Modules

jQuery

ECMAScript versions compatibility

Testing

Resources

License

};

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README.md

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Mirego JavaScript Style Guide() {

Table of Contents

Types

Objects

Arrays

Strings

Functions

Properties

Variables

Hoisting

Conditional Expressions & Equality

Blocks

Comments

Accessors

Classes

Events

Modules

jQuery

ECMAScript versions compatibility

Testing

Resources

License

};

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