Javascript coding conventions

##Introduction This set of advices is not intended to cover all coding situations. Its main goal is to give us common guide lines to follow in order to keep our code clean, readable, scalable and maintainable. Keep in mind that your code should not be marked of your style, at least in its form, and a person who would read a class implementation should not think many developers have been working on. Clean code also facilitate debugging as well as making pull requests easier to read.

##Tools From a simple text editor to a real IDE, Javascript doesn't need complex tools to work on and you can use the one you prefer. Despite everything, Webstorm, the web IDE from Jetbrains, has a good support for both Node.js and browser Javascript development. It also provides support for the syntax of several frameworks and templating engines, such as Handlebars, EJS, Angular.js...

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

Objects

Use the literal syntax for object creation.

// bad
var item = new Object();

// good
var item = {};

Don't use as keys. It won't work in IE8.

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

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

Use readable synonyms in place of reserved words.

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

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

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

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.

var len = items.length;
var itemsCopy = [];
var i;

// bad
for(i = 0; i < len; i++) {
  itemsCopy[i] = items[i];
}

// 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);
  ...
}

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. You should configure your IDE for that.

Note: If overused, long strings with concatenation could impact performance.

// 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.';

When programmatically building up a string, use Array#join instead of string concatenation. Mostly for IE.

var items;
var messages;
var length;
var i;

messages = [{
  state: 'success',
  message: 'This one worked.'
}, {
  state: 'success',
  message: 'This one worked as well.'
}, {
  state: 'error',
  message: 'This one did not work.'
}];

length = messages.length;

// bad
function inbox(messages) {
  items = '<ul>';

  for (i = 0; i < length; i++) {
    items += '<li>' + messages[i].message + '</li>';
  }

  return items + '</ul>';
}

// good
function inbox(messages) {
  items = [];

  for(i = 0; i < length; i++) {
    items[i] = messages[i].message;
  }

  return '<ul><li>' + items.join('</li><li>') + '</li></ul>';
}

Functions

Function expressions:

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

// named function expression
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). Assign the function to a variable instead. Browsers will allow you to do it, but they all interpret it differently, which is bad news bears.

Note: ECMA-262 defines a block as a list of statements. A function declaration is not a statement.

// 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) {
  // ...stuff...
}

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

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');

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 each variables and declare each variable on a newline.

// bad
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 i, len, dragonball,
    items = getItems(),
    goSportsTeam = true;

// bad
var i, items = getItems(),
    dragonball,
    goSportsTeam = true,
    len;

// good
var items = getItems();
var goSportsTeam = true;
var dragonball;
var length;
var i;

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

// bad
function() {
  test();
  console.log('doing stuff..');

  //..other stuff..

  var name = getName();

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

  return name;
}

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

  test();
  console.log('doing stuff..');

  //..other stuff..

  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;
}

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 an empty string '', otherwise true
```
if ([0]) {
  // true
  // An array is an object, objects evaluate to true
}
```

Use shortcuts.

// bad
if(name !== '') {
  // ...stuff...
}

// good
if(name) {
  // ...stuff...
}

// bad
if(collection.length > 0) {
  // ...stuff...
}

// good
if(collection.length) {
  // ...stuff...
}

Blocks

Use braces with all multi-line blocks.

// bad
if(test)
  return false;

// good
if(test) return false;

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

Comments

Use /** ... */ for multiline comments. Include a description, specify types and values for all parameters and return values.

// bad
// make() returns a new element
// based on the passed in tag name
//
// @param <String> tag
// @return <Element> element
function make(tag) {

  // ...stuff...

  return element;
}

// good
/**
 * make() returns a new element
 * based on the passed in tag name
 *
 * @param <String> tag
 * @return <Element> element
 */
function make(tag) {

  // ...stuff...

  return element;
}

Use // for single line comments. Place single line comments on a newline above the subject of the comment. Put an empty line before the comment.

// bad
var active = true;  // is current tab

// good
// is current tab
var active = true;

// bad
function getType() {
  console.log('fetching type...');
  // set the default type to 'no type'
  var type = this._type || 'no type';

  return type;
}

// good
function getType() {
  console.log('fetching type...');

  // set the default type to 'no type'
  var type = this._type || 'no type';

  return type;
}

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;
}



## Whitespace

- Use soft tabs set to 2 spaces

  ```javascript
  // bad
  function() {
  ····var name;
  }

  // bad
  function() {
  ·var name;
  }

  // good
  function() {
  ··var name;
  }
  ```

- Place 1 space before the leading brace.

  ```javascript
  // 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.

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

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

- Use indentation when making long method chains.

  ```javascript
  // 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);
  ```

## Commas

- Leading commas: **Nope.**

  // 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 :

> 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.

  ```javascript
  // bad
  var hero = {
    firstName: 'Kevin',
    lastName: 'Flynn',
  };

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

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

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


## Semicolons

- **Yup.**

  ```javascript
  // bad
  (function() {
    var name = 'Skywalker'
    return name
  })()

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

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


## Type Casting & Coercion

- Perform type coercion at the beginning of the statement.
- Strings:

  ```javascript
  //  => 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` for Numbers and always with a radix for type casting.

  ```javascript
  var inputValue = '4';

  // bad
  var val = new Number(inputValue);

  // bad
  var val = +inputValue;

  // bad
  var val = inputValue >> 0;

  // bad
  var val = parseInt(inputValue);

  // good
  var val = Number(inputValue);

  // good
  var val = parseInt(inputValue, 10);
  ```
- Booleans:

  ```javascript
  var age = 0;

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

  // good
  var hasAge = Boolean(age);

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


## Naming Conventions

- Avoid single letter names. Be descriptive with your naming.

  ```javascript
  // bad
  function q() {
    // ...stuff...
  }

  // good
  function query() {
    // ..stuff..
  }
  ```

- Use camelCase when naming objects, functions, and instances

  ```javascript
  // 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

  ```javascript
  // bad
  function user(options) {
    this.name = options.name;
  }

  var bad = new user({
    name: 'nope'
  });

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

  var good = new User({
    name: 'yup'
  });
  ```

- Use a leading underscore `_` when naming private properties

  ```javascript
  // bad
  this.__firstName__ = 'Panda';
  this.firstName_ = 'Panda';

  // good
  this._firstName = 'Panda';
  ```

- When saving a reference to `this` use `self`.

  ```javascript
  // 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);
    };
  }
  ```


## Accessors

- Accessor functions for properties are not required
- If you do make accessor functions use getVal() and setVal('hello')

  ```javascript
  // bad
  dragon.age();

  // good
  dragon.getAge();

  // bad
  dragon.age(25);

  // good
  dragon.setAge(25);
  ```

- If the property is a boolean, use isVal() or hasVal()

  ```javascript
  // bad
  if(!dragon.age()) {
    return false;
  }

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

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

  ```javascript
  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];
  };
  ```


## Constructors

- 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!

  ```javascript
  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.

  ```javascript
  // 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.

  ```javascript
  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();
  };
  ```


## jQuery

- Prefix jQuery object variables with a `$`.

  ```javascript
  // bad
  var sidebar = $('.sidebar');

  // good
  var $sidebar = $('.sidebar');
  ```

- Cache jQuery lookups.

  ```javascript
  // 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'
    });
  }
  ```

- For DOM queries use Cascading `$('.sidebar ul')` or parent > child `$('.sidebar > ul')`.
- Use `find` with scoped jQuery object queries.

  ```javascript
  // bad
  $('ul', '.sidebar').hide();

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

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

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

  // good
  $sidebar.find('ul').hide();
  ```

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Javascript coding conventions

Types

Objects

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Functions

Properties

Variables

Conditional Expressions & Equality

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Javascript coding conventions

Types

Objects

Arrays

Strings

Functions

Properties

Variables

Conditional Expressions & Equality

Blocks

Comments