JS Styleguide

Types
Objects
Arrays
Strings
Functions
Properties
Variables
Try / Catch
Hoisting
Comparison Operators & Equality
Blocks
Comments
Whitespace
Commas
Semicolons
Type Casting & Coercion
Naming Conventions
Accessors
Constructors
Events
Modules
jQuery
Testing
Performance
Resources

All code in any code-base should look like a single person typed it, no matter how many people contributed.

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. More info.

// 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'
};

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Arrays

Use the literal syntax for array creation.

// bad 
var items = new Array();
 
// good 
var items = [];

Use Array#push instead of direct assignment to add items to an array.

var someStack = [];
 
// bad 
someStack[someStack.length] = 'abracadabra';
// only to be used when performance is the main goal, as it's faster that push() method.(for example, populating an array with 1000 elements) 
 
// good 
someStack.push('abracadabra');

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

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);
  ... 
}

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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 100 characters should be written across multiple lines using string concatenation.

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

// 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: jsPerf.

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++) {
    // use direct assignment in this case because we're micro-optimizing. 
    items[i] = '<li>' + messages[i].message + '</li>';
  }
 
  return '<ul>' + items.join('') + '</ul>';
}

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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.

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

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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. Captain Planet warned us of that.
```
// bad 
superPower = new SuperPower();
 
// good 
var superPower = new SuperPower();
```

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;
var items = getItems();
var dragonball;
var goSportsTeam = true;
var 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 - unnecessary function call 
function() {
  var name = getName();
 
  if (!arguments.length) {
    return false;
  }
 
  this.setFirstName(name);
 
  return true;
}
 
// good 
function() {
  var name;
 
  if (!arguments.length) {
    return false;
  }
 
  name = getName();
  this.setFirstName(name);
 
  return true;
}

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Try/Catch

You should always use try…catch statements in your code. It will prevent that the whole website stops working because os an error in one part of the logic. The show must go on, don't let an error to ruin the UX.

// bad 
function updateProfile(data) {
    var user = new User();
    var userData;
 
    // If the data comes not well formed, so the WHOLE app will stop running 
    userData = JSON.parse(data);
    user.setData = userData;
 
}
 
// good 
function updateProfile(data) {
    var user = new User();
    var output = false;
    var userData;
 
    try {
        userData = JSON.parse(data);
        user.setData = userData;
        output = true;
    } catch (err) {
        // Now the app continues working and we can let the user know that there was an error and the cause. 
        UI.showAlert("Data is not well formed");
    } finally {
        return output;
    }    
}

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;
var items = getItems();
var dragonball;
var goSportsTeam = true;
var 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 - unnecessary function call 
function() {
  var name = getName();
 
  if (!arguments.length) {
    return false;
  }
 
  this.setFirstName(name);
 
  return true;
}
 
// good 
function() {
  var name;
 
  if (!arguments.length) {
    return false;
  }
 
  name = getName();
  this.setFirstName(name);
 
  return true;
}

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Hoisting

Variable declarations get hoisted to the top of their scope, but 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(); // => Flying 
 
  function superPower() {
    console.log('Flying');
  }
}

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

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Comparison Operators & Equality

Use === and !== over == and !=.
Conditional statements such as the if statement evaluate their expression using coercion with the ToBoolean abstract 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... 
}

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

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Blocks

Use braces with all multi-line blocks.

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

If you're using multi-line blocks with if and else, put else on the same line as your if block's closing brace.

// bad 
if (test) {
  thing1();
  thing2();
}
else {
  thing3();
}
 
// good 
if (test) {
  thing1();
  thing2();
} else {
  thing3();
}

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Comments

Use /** ... */ for multi-line 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;
}

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Whitespace

Use soft tabs set to 4 spaces.

// bad 
function() {
var name;
}
 
// bad 
function() {
∙var name;
}
 
// good 
function() {
∙∙∙∙var name;
}

Place 1 space before the leading brace.

// 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'
});

Place 1 space before the opening parenthesis in control statements (if, while etc.). Place no space before the argument list in function calls and declarations.

// bad 
if(isJedi) {
  fight ();
}
 
// good 
if (isJedi) {
  fight();
}
 
// bad 
function fight () {
  console.log ('Swooosh!');
}
 
// good 
function fight() {
  console.log('Swooosh!');
}

Set off operators with spaces.

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

End files with a single newline character.

// bad 
(function(global) {
  // ...stuff... 
})(this);

// bad 
(function(global) {
  // ...stuff... 
})(this);↵
↵

// good 
(function(global) {
  // ...stuff... 
})(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.

// 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').classed('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')
    .classed('led', true)
    .attr('width', (radius + margin) * 2)
  .append('svg:g')
    .attr('transform', 'translate(' + (radius + margin) + ',' + (radius + margin) + ')')
    .call(tron.led);

Leave a blank line after blocks and before the next statement

// bad 
if (foo) {
  return bar;
}
return baz;
 
// good 
if (foo) {
  return bar;
}
 
return baz;
 
// bad 
var obj = {
  foo: function() {
  },
  bar: function() {
  }
};
return obj;
 
// good 
var obj = {
  foo: function() {
  },
 
  bar: function() {
  }
};
 
return obj;

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Commas

Leading commas: Nope.

// 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):

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.
```
// bad 
var hero = {
  firstName: 'Kevin',
  lastName: 'Flynn',
};
 
var heroes = [
  'Batman',
  'Superman',
];
 
// good 
var hero = {
  firstName: 'Kevin',
  lastName: 'Flynn'
};
 
var heroes = [
  'Batman',
  'Superman'
];
```

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Semicolons

Yup.

// 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;
})();

Type Casting & Coercion

Perform type coercion at the beginning of the statement.

Strings:

//  => 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.

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);

If for whatever reason you are doing something wild and parseInt is your bottleneck and need to use Bitshift for performance reasons, leave a comment explaining why and what you're doing.

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

Note: Be careful when using bitshift operations. Numbers are represented as 64-bit values, but Bitshift operations always return a 32-bit integer (source). Bitshift can lead to unexpected behavior for integer values larger than 32 bits. Discussion. Largest signed 32-bit Int is 2,147,483,647:
```
2147483647 >> 0 //=> 2147483647 
2147483648 >> 0 //=> -2147483648 
2147483649 >> 0 //=> -2147483647 
```

Booleans:

var age = 0;
 
// bad 
var hasAge = new Boolean(age);
 
// good 
var hasAge = Boolean(age);
 
// good 
var hasAge = !!age;

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Naming Conventions

Avoid single letter names. Be descriptive with your naming.

// bad 
function q() {
  // ...stuff... 
}
 
// good 
function query() {
  // ..stuff.. 
}

Use camelCase when naming objects, functions, and instances.

// bad 
var OBJEcttsssss = {};
var this_is_my_object = {};
var o = {};
function c() {}
 
// good 
var thisIsMyObject = {};
function thisIsMyFunction() {}

Use PascalCase when naming constructors or classes.

// 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.

// bad 
this.__firstName__ = 'Panda';
this.firstName_ = 'Panda';
 
// good 
this._firstName = 'Panda';

When saving a reference to this use _this.

// bad 
function() {
  var self = this;
  return function() {
    console.log(self);
  };
}
 
// bad 
function() {
  var that = this;
  return function() {
    console.log(that);
  };
}
 
// good 
function() {
  var _this = this;
  return function() {
    console.log(_this);
  };
}

Name your functions. 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.

If your file exports a single class, your filename should be exactly the name of the class.

// file contents 
class CheckBox {
  // ... 
}
module.exports = CheckBox;
 
// in some other file 
// bad 
var CheckBox = require('./checkBox');
 
// bad 
var CheckBox = require('./check_box');
 
// good 
var CheckBox = require('./CheckBox');

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Accessors

Accessor functions for properties are not required.

If you do make accessor functions use getVal() and setVal('hello').

// bad 
dragon.age();
 
// good 
dragon.getAge();
 
// bad 
dragon.age(25);
 
// good 
dragon.setAge(25);

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

// 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];
};

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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!

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();
};

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Events

When attaching data payloads to events (whether DOM events or something more proprietary like Backbone 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(e, listingId) {
  // do something with listingId 
});
```
prefer:
```
// good 
$(this).trigger('listingUpdated', { listingId : listing.id });
 
... 
 
$(this).on('listingUpdated', function(e, data) {
  // do something with data.listingId 
});
```
- Use event delegation ALWAYS. Event delegation allows us to attach a single event listener, to a parent element, that will fire for all descendants matching a selector, whether those descendants exist now or are added in the future. Understanding how events propagate is an important factor in being able to leverage Event Delegation. Any time one of our anchor tags is clicked, a click event is fired for that anchor, and then bubbles up the DOM tree, triggering each of its parent click event handlers. This means that anytime you click one of our bound anchor tags, you are effectively clicking the entire document body! This is called event bubbling or event propagation.
```
// bad 
$('.js-btn-delete-user').on('click', function(e) {
    ... // actions to delete user 
});
 
... 
// This affects the performance as it's listening for the all the events in all the DOM. Performance matters. 
$(document).on('click', '.js-btn-delete-user', function(e) {
    ... // actions to delete user 
});
```
  prefer:
```
// good 
$(".js-list-users").on('click', '.js-btn-delete-user', function(e) {
    ... // actions to delete user 
});
```
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jQuery

Prefix jQuery object variables with a $.

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

Cache jQuery 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'
  });
}

For DOM queries use Cascading $('.sidebar ul') or parent > child $('.sidebar > ul'). jsPerf

Use find with scoped jQuery object queries.

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