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DRAFT IRESS ECMAScript 6 Style Guide() {

A mostly reasonable approach to JavaScript

With thanks to Airbnb.

Table of Contents

  1. Types
  2. References
  3. Objects
  4. Arrays
  5. Destructuring
  6. Strings
  7. Functions
  8. Arrow Functions
  9. Constructors
  10. Modules
  11. Iterators and Generators
  12. Properties
  13. Variables
  14. Hoisting
  15. Comparison Operators & Equality
  16. Blocks
  17. Comments
  18. Whitespace
  19. Commas
  20. Semicolons
  21. Type Casting & Coercion
  22. Naming Conventions
  23. Accessors
  24. Events
  25. jQuery
  26. ECMAScript 5 Compatibility
  27. ECMAScript 6 Styles
  28. Testing
  29. Performance
  30. Resources
  31. In the Wild
  32. Translation
  33. The JavaScript Style Guide Guide
  34. Contributors
  35. IRESS Amendments
  36. License

Types

  • 1.1 Primitives: When you access a primitive type you work directly on its value.

    • string
    • number
    • boolean
    • null
    • undefined
    const foo = 1;
let bar = foo;

bar = 9;

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

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

    • object
    • array
    • function
    const foo = [1, 2];
const bar = foo;

bar[0] = 9;

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

⬆ back to top

References

  • 2.1 Use const for all of your references; never use var.

Why? This ensures that you can't reassign your references (mutation), which can lead to bugs and difficult to comprehend code.

```javascript
// bad
var a = 1;
var b = 2;

// good
const a = 1;
const b = 2;
```
  • 2.2 If you must mutate references, use let instead of var.

Why? let is block-scoped rather than function-scoped like var.

```javascript
// bad
var count = 1;
if (true) {
    count += 1;
}

// good, use the let.
let count = 1;
if (true) {
    count += 1;
}
```

  • 2.3 Note that both let and const are block-scoped.

    // const and let only exist in the blocks they are defined in.
{
    let a = 1;
    const b = 1;
}
console.log(a); // ReferenceError
console.log(b); // ReferenceError

⬆ back to top

Objects

  • 3.1 Use the literal syntax for object creation.

    // bad
const item = new Object();

// good
const item = {};

  • 3.2 Avoid using reserved words as keys, where that is within your control.

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

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

  • 3.3 Use readable synonyms in place of reserved words.

    // bad
const superman = {
    class: 'alien',
};

// bad
const superman = {
    klass: 'alien',
};

// good
const superman = {
    type: 'alien',
};

  • 3.4 Use computed property names when creating objects with dynamic property names.

Why? They allow you to define all the properties of an object in one place.

```javascript

function getKey(k) {
    return `a key named ${k}`;
}

// bad
const obj = {
    id: 5,
    name: 'San Francisco',
};
obj[getKey('enabled')] = true;

// good
const obj = {
    id: 5,
    name: 'San Francisco',
    [getKey('enabled')]: true,
};
```

  • 3.5 Use object method shorthand.

    // bad
const atom = {
    value: 1,

    addValue: function (value) {
        return atom.value + value;
    },
};

// good
const atom = {
    value: 1,

    addValue(value) {
        return atom.value + value;
    },
};

  • 3.6 Use property value shorthand.

Why? It is shorter to write and descriptive.

```javascript
const lukeSkywalker = 'Luke Skywalker';

// bad
const obj = {
    lukeSkywalker: lukeSkywalker,
};

// good
const obj = {
    lukeSkywalker,
};
```
  • 3.7 Rule 3.7 has been removed

⬆ back to top

Arrays

  • 4.1 Use the literal syntax for array creation.

    // bad
const items = new Array();

// good
const items = [];

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

    const someStack = [];


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

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

  • 4.3 Use array spreads ... to copy arrays.

    // bad
const len = items.length;
const itemsCopy = [];
let i;

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

// good
const itemsCopy = [...items];

  • 4.4 To convert an array-like object to an array, use Array#from.

    const foo = document.querySelectorAll('.foo');
const nodes = Array.from(foo);

⬆ back to top

Destructuring

  • 5.1 Use object destructuring when accessing and using multiple properties of an object.

Why? Destructuring saves you from creating temporary references for those properties.

```javascript
// bad
function getFullName(user) {
    const firstName = user.firstName;
    const lastName = user.lastName;

    return `${firstName} ${lastName}`;
}

// good
function getFullName(obj) {
    const { firstName, lastName } = obj;
    return `${firstName} ${lastName}`;
}

// best
function getFullName({ firstName, lastName }) {
    return `${firstName} ${lastName}`;
}
```

  • 5.2 Use array destructuring.

    const arr = [1, 2, 3, 4];

// bad
const first = arr[0];
const second = arr[1];

// good
const [first, second] = arr;

  • 5.3 Use object destructuring for multiple return values, not array destructuring.

Why? You can add new properties over time or change the order of things without breaking call sites.

```javascript
// bad
function processInput(input) {
    // then a miracle occurs
    return [left, right, top, bottom];
}

// the caller needs to think about the order of return data
const [left, __, top] = processInput(input);

// good
function processInput(input) {
    // then a miracle occurs
    return { left, right, top, bottom };
}

// the caller selects only the data they need
const { left, right } = processInput(input);
```

⬆ back to top

Strings

  • 6.1 Use single quotes '' for strings.

    // bad
const name = "Capt. Janeway";

// good
const name = 'Capt. Janeway';

  • 6.2 Strings longer than 80 characters should be written across multiple lines using string concatenation.

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

    // bad
const 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
const 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
const 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.';

  • 6.4 When programmatically building up strings, use _.sprintf instead of template strings or concatenation.

Why? printf formatting gives you a readable, concise syntax with proper newlines and string interpolation features, and is compatible with gettext-style translation libraries.

```javascript
// bad
function sayHi(name) {
    return 'How are you, ' + name + '?';
}

// bad
function sayHi(name) {
    return ['How are you, ', name, '?'].join();
}

// okay
function sayHi(name) {
    // The string literal is not passed to the translation engine, so a
    // template string is acceptable.
    window.console.log(`How are you, ${name}?`);
}

// good
function sayHi(name) {
    return _.sprintf(gettext(`How are you, %s`), name);
}
```

⬆ back to top

Functions

  • 7.1 Use function declarations instead of function expressions.

Why? Function declarations are named, so they're easier to identify in call stacks. Also, the whole body of a function declaration is hoisted, whereas only the reference of a function expression is hoisted. This rule makes it possible to always use Arrow Functions in place of function expressions.

```javascript
// bad
const foo = function () {
};

// good
function foo() {
}
```

  • 7.2 Function expressions:

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

  • 7.3 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.

  • 7.4 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
let test;
if (currentUser) {
    test = () => {
        console.log('Yup.');
    };
}

  • 7.5 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...
}

  • 7.6 Never use arguments, opt to use rest syntax ... instead.

Why? ... is explicit about which arguments you want pulled. Plus rest arguments are a real Array and not Array-like like arguments.

```javascript
// bad
function concatenateAll() {
    const args = Array.prototype.slice.call(arguments);
    return args.join('');
}

// good
function concatenateAll(...args) {
    return args.join('');
}
```

  • 7.7 Use default parameter syntax rather than mutating function arguments.

    // really bad
function handleThings(opts) {
    // No! We shouldn't mutate function arguments.
    // Double bad: if opts is falsy it'll be set to an object which may
    // be what you want but it can introduce subtle bugs.
    opts = opts || {};
    // ...
}

// still bad
function handleThings(opts) {
    if (opts === void 0) {
        opts = {};
    }
    // ...
}

// good
function handleThings(opts = {}) {
    // ...
}

  • 7.8 Avoid side effects with default parameters

Why? They are confusing to reason about.

```javascript
var b = 1;
// bad
function count(a = b++) {
    console.log(a);
}
count();  // 1
count();  // 2
count(3); // 3
count();  // 3
```

  • 7.9 Be conservative with function chaining.

    Why? Chains can be hard to read, and fluent-interface-style chains are unnecessary.

    Guidelines for chaining:

    • Chains should be used to form a pipeline linking output of one function to the input of another. Chains should not be used as a fluent interface to aggregate loosely-coupled calls on the same object.
    • Limit chains to a maximum of 4 function calls.
    • Do not pass complex parameters (such as anonymous functions).
    • Use the formatting rules in section 18.5. ie. Use a new line for each function call in the chain, indent subsequent lines, and start each subsequent line with the "." character.
    // bad - too many links in the chain
session
    .getUser()
    .getLocale()
    .getDefaultTimeZone()
    .getCurrentTime()
    .asUTC();

// bad - poor formatting
session.getUser().getLocale().
    formatDate(dt);

// good
session
    .getUser()
    .getLocale()
    .formatDate(dt);

// bad - use as a fluent interface
rectangle
    .setWidth(10)
    .setHeight(5)
    .setColour("red");

// good - multiple independent calls instead of fluent interface
rectangle.setWidth(10);
rectangle.setHeight(5);
rectangle.setColour("red");

⬆ back to top

Arrow Functions

  • 8.1 When you must use function expressions (as when passing an anonymous function), use arrow function notation.

Why? It creates a version of the function that executes in the context of this, which is usually what you want, and is a more concise syntax.

Why not? If you have a fairly complicated function, you might move that logic out into its own function declaration.

```javascript
// bad
[1, 2, 3].map(function (x) {
    return x * x;
});

// good
[1, 2, 3].map((x) => {
    return x * x;
});
```
  • 8.2 If the function body fits on one line and there is only a single argument, feel free to omit the braces and parentheses, and use the implicit return. Otherwise, add the parentheses, braces, and use a return statement.

Why? Syntactic sugar. It reads well when multiple functions are chained together.

Why not? If you plan on returning an object.

```javascript
// good
[1, 2, 3].map(x => x * x);

// good
[1, 2, 3].reduce((total, n) => {
    return total + n;
}, 0);
```

⬆ back to top

Constructors

  • 9.1 Always use class. Avoid manipulating prototype directly.

Why? class syntax is more concise and easier to reason about.

```javascript
// bad
function Queue(contents = []) {
    this._queue = [...contents];
}
Queue.prototype.pop = function() {
    const value = this._queue[0];
    this._queue.splice(0, 1);
    return value;
}


// good
class Queue {
    constructor(contents = []) {
        this._queue = [...contents];
    }
    pop() {
        const value = this._queue[0];
        this._queue.splice(0, 1);
        return value;
    }
}
```
  • 9.2 Use extends for inheritance.

Why? It is a built-in way to inherit prototype functionality without breaking instanceof.

```javascript
// bad
const inherits = require('inherits');
function PeekableQueue(contents) {
    Queue.apply(this, contents);
}
inherits(PeekableQueue, Queue);
PeekableQueue.prototype.peek = function() {
    return this._queue[0];
}

// good
class PeekableQueue extends Queue {
    peek() {
        return this._queue[0];
    }
}
```

  • 9.3 Rule 9.3 has been removed.

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

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

    getName() {
        return this.name;
    }

    toString() {
        return `Jedi - ${this.getName()}`;
    }
}

⬆ back to top

Modules

  • 10.1 Always use modules (import/export) over a non-standard module system. You can always transpile to your preferred module system.

Why? Modules are the future, let's start using the future now.

```javascript
// bad
const AirbnbStyleGuide = require('./AirbnbStyleGuide');
module.exports = AirbnbStyleGuide.es6;

// ok
import AirbnbStyleGuide from './AirbnbStyleGuide';
export default AirbnbStyleGuide.es6;

// best
import { es6 } from './AirbnbStyleGuide';
export default es6;
```
  • 10.2 Avoid wildcard imports, but if they are necessary put them in their own namespace.

Why? This enforces a single default export in modules you write.

```javascript
// bad
import * as AirbnbStyleGuide from './AirbnbStyleGuide';

// good
import AirbnbStyleGuide from './AirbnbStyleGuide';

// bad - imports multiple functions
import * from ‘ui_toolkit’;

// okay - we can't modify the external module `ui_toolkit` to have a
// single export, but we can change our import to bind them to a
// single name in the global namespace.
import * as UI from ‘ui_toolkit’;
new UI.button(...);
```
  • 10.3 And do not export directly from an import.

Why? Although the one-liner is concise, having one clear way to import and one clear way to export makes things consistent.

```javascript
// bad
// filename es6.js
export { es6 as default } from './airbnbStyleGuide';

// good
// filename es6.js
import { es6 } from './AirbnbStyleGuide';
export default es6;
```

⬆ back to top

Iterators and Generators

  • 11.1 Don't use iterators. Prefer JavaScript's higher-order functions like map() and reduce() instead of loops like for-of.

Why? This enforces our immutable rule. Dealing with pure functions that return values is easier to reason about than side-effects.

```javascript
const numbers = [1, 2, 3, 4, 5];

// bad
let sum = 0;
for (let num of numbers) {
    sum += num;
}

sum === 15;

// good
let sum = 0;
numbers.forEach((num) => sum += num);
sum === 15;

// best (use the functional force)
const sum = numbers.reduce((total, num) => total + num, 0);
sum === 15;
```
  • 11.2 Don't use generators for now.

Why? They don't transpile well to ES5.

⬆ back to top

Properties

  • 12.1 Use dot notation when accessing properties.

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

// bad
const isJedi = luke['jedi'];

// good
const isJedi = luke.jedi;

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

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

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

const isJedi = getProp('jedi');

⬆ back to top

Variables

  • 13.1 Always use const 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
const superPower = new SuperPower();

  • 13.2 Use one const declaration per variable.

    Why? 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
const items = getItems(),
    goSportsTeam = true,
    dragonball = 'z';

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

// good
const items = getItems();
const goSportsTeam = true;
const dragonball = 'z';

  • 13.3 Rule 13.3 has been removed.

  • 13.4 Assign variables where you need them, but place them in a reasonable place.

Why? let and const are block scoped and not function scoped.

```javascript
// good
function() {
    test();
    console.log('doing stuff..');

    //..other stuff..

    const name = getName();

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

    return name;
}

// bad - unnecessary function call
function(hasName) {
    const name = getName();

    if (!hasName) {
        return false;
    }

    this.setFirstName(name);

    return true;
}

// good
function(hasName) {
    if (!hasName) {
        return false;
    }

    const name = getName();
    this.setFirstName(name);

    return true;
}
```

⬆ back to top

Hoisting

  • 14.1 var declarations get hoisted to the top of their scope, their assignment does not. const and let declarations are blessed with a new concept called Temporal Dead Zones (TDZ). It's important to know why typeof is no longer safe.

    // 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() {
    let declaredButNotAssigned;
    console.log(declaredButNotAssigned); // => undefined
    declaredButNotAssigned = true;
}

// using const and let
function example() {
    console.log(declaredButNotAssigned); // => throws a ReferenceError
    console.log(typeof declaredButNotAssigned); // => throws a ReferenceError
    const declaredButNotAssigned = true;
}

  • 14.2 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');
    };
}

  • 14.3 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');
    }
}

  • 14.4 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.

⬆ back to top

Comparison Operators & Equality

  • 15.1 Use === and !== over == and !=.

    In particular, if you want to check for null or undefined (eg. as commonly done for function parameters), then either explicitly check for both with ===, or create a helper function that does the same thing.

    Why? Using === and !== will avoid unexpected results from coercion. It is easier to make a strict rule than to allow exceptions for the use cases that are safe.

    // bad
if (param != null)
    doComplexCalculation(param);
}

// good
if (param !== undefined && param !== null) {
    doComplexCalculation(param);
}

// good
function isParameterDefined(param) {
    return param !== undefined && param !== null;
}

if isParameterDefined(param) {
    doComplexCalculation(param);
}

  • 15.2 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
}

  • 15.3 Do not use shortcuts, we prefer explicit tests.

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

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

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

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

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

⬆ back to top

Blocks

  • 16.1 Use braces with all multi-line blocks.

    // bad
if (test)
    return false;

// good
if (test) return false;

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

// bad
function() { return false; }

// good
function() {
    return false;
}

  • 16.2 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();
}

  • 16.3 Blocks should always be across multiple lines.

    // bad - no braces (rule 16.1)
if (test) return false;

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

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

// bad
function() { return false; }

// good
function() {
    return false;
}

⬆ back to top

Comments

  • 17.1 Use multi-line /** ... */ comments for documenting functions and classes. 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;
}

  • 17.2 Where necessary, write comments that elaborate on the purpose or structure of a specific section of code.

    • You may use either // (single-line comments only) or /* .. */.
    • Place comments on a newline above the subject of the comment, with an empty line before the comment. Avoid inline comments (those placed on the end of the line as the code they describe), but where they exist use //.
    • Do not use mid-line comments (those placed in between two pieces of code on the same line, using /* .. */).
    // bad
result = foo(bar, /* 42, */ 3.1416);

// bad
const active = true;  /* is current tab */

// okay, but use with restraint
const active = true;  // is current tab

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

// good
/*  is current tab */
const active = true;

// bad - no introductory blank line
function getType() {
    console.log('fetching type...');
    // set the default type to 'no type'
    const type = this._type || 'no type';

    return type;
}

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

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

    return type;
}

// bad - double-slash comment on multiple lines.
function getType() {
    console.log('fetching type...');

    // set the default type to 'no type', which is the most
    // reasonable way to handle a partially initialised object.
    const type = this._type || 'no type';

    return type;
}

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

    /*
     * set the default type to 'no type', which is the most
     * reasonable way to handle a partially initialised object.
     */
    const type = this._type || 'no type';

    return type;
}

  • 17.3 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.

  • 17.4 Use // FIXME: to annotate problems.

    class Calculator {
    constructor() {
        // FIXME: shouldn't use a global here
        total = 0;
    }
}

  • 17.5 Use // TODO: to annotate solutions to problems.

    class Calculator {
    constructor() {
        // TODO: total should be configurable by an options param
        this.total = 0;
    }
}

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Whitespace

  • 18.1 Use soft tabs set to 4 spaces.

    // bad
function() {
∙∙const name;
}

// bad
function() {
∙const name;
}

// good
function() {
∙∙∙∙const name;
}

  • 18.2 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',
});

  • 18.3 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!');
}

  • 18.4 Set off operators with spaces.

    // bad
const x=y+5;

// good
const x = y + 5;

  • 18.5 End files with a single newline character.

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

  • 18.5 Use indentation when making long method chains. Use a leading dot, which emphasizes that the line is a method call, not a new statement. Remember to be conservative with your chaining, keeping chains short and pipeline-oriented.

    // bad
$('#items').find('.selected').highlight().end();

// bad
$('#items').
    find('.selected').
        highlight().
        end();

// good
$('#items')
    .find('.selected')
        .highlight()
        .end();

  • 18.6 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
const obj = {
    foo() {
    },
    bar() {
    },
};
return obj;

// good
const obj = {
    foo() {
    },

    bar() {
    },
};

return obj;

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Commas

  • 19.1 Leading commas: Nope.

    // bad
const story = [
      once
    , upon
    , aTime
];

// good
const story = [
    once,
    upon,
    aTime,
];

// bad
const hero = {
      firstName: 'Ada'
    , lastName: 'Lovelace'
    , birthYear: 1815
    , superPower: 'computers'
};

// good
const hero = {
    firstName: 'Ada',
    lastName: 'Lovelace',
    birthYear: 1815,
    superPower: 'computers',
};

  • 19.2 Additional trailing comma: Yup.

Why? This leads to cleaner git diffs. Also, transpilers like Babel will remove the additional trailing comma in the transpiled code which means you don't have to worry about the trailing comma problem in legacy browsers.

```javascript
// bad - git diff without trailing comma
const hero = {
       firstName: 'Florence',
-      lastName: 'Nightingale'
+      lastName: 'Nightingale',
+      inventorOf: ['coxcomb graph', 'modern nursing']
}

// good - git diff with trailing comma
const hero = {
       firstName: 'Florence',
       lastName: 'Nightingale',
+      inventorOf: ['coxcomb chart', 'modern nursing'],
}

// bad
const hero = {
    firstName: 'Dana',
    lastName: 'Scully'
};

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

// good
const hero = {
    firstName: 'Dana',
    lastName: 'Scully',
};

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

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Semicolons

  • 20.1 Yup.

    // bad
(function() {
    const name = 'Skywalker'
    return name
})()

// good
(() => {
    const name = 'Skywalker';
    return name;
})();

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

    Read more.

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Type Casting & Coercion

  • 21.1 Perform type coercion at the beginning of the statement.

  • 21.2 Strings:

    //  => this.reviewScore = 9;

// bad
const totalScore = this.reviewScore + '';

// good
const totalScore = String(this.reviewScore);

  • 21.3 Use parseInt for Numbers and always with a radix for type casting.

    const inputValue = '4';

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

// bad
const val = +inputValue;

// bad
const val = inputValue >> 0;

// bad
const val = parseInt(inputValue);

// good
const val = Number(inputValue);

// good
const val = parseInt(inputValue, 10);

  • 21.4 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. You will also need to get approval from your team lead and the architecture team.

    Why? Bitshifting is an opaque way to perform type conversion, which we want to strongly discourage.

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

  • 21.5 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

  • 21.6 Booleans:

    const age = 0;

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

// good
const hasAge = Boolean(age);

// bad
const hasAge = !!age;

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Naming Conventions

  • 22.1 Avoid single letter names. Be descriptive with your naming.

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

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

  • 22.2 Use camelCase when naming objects, functions, and instances.

    // bad
const OBJEcttsssss = {};
const this_is_my_object = {};
function c() {}

// good
const thisIsMyObject = {};
function thisIsMyFunction() {}

  • 22.3 Use PascalCase when naming constructors or classes.

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

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

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

const good = new User({
    name: 'yup',
});

  • 22.4 Use a single leading underscore _ when naming private properties.

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

// good
this._firstName = 'Panda';

  • 22.5 Don't save references to this. Use arrow functions or Function#bind.

    // bad
function foo() {
    const self = this;
    return function() {
        console.log(self);
    };
}

// bad
function foo() {
    const that = this;
    return function() {
        console.log(that);
    };
}

// good
function foo() {
    return () => {
        console.log(this);
    };
}

  • 22.6 If your file exports a single class, function, library or other object, your filename should reflect the name of that exported object.

The filename should be a snake_cased version of the exported object's name. If the filename has a prefix corresponding to the most local part of the folder hierarchy, that prefix may be removed from the filename.

```javascript
// file contents
class PortfolioSecuritySelector {
    // ...
}
export default PortfolioSecuritySelector;

// in some other file
// bad
import PortfolioSecuritySelector from './portfolioSecuritySelector';

// bad
import PortfolioSecuritySelector from './PortfolioSecuritySelector';

// good
import PortfolioSecuritySelector from './portfolio_security_selector';

// good
import PortfolioSecuritySelector from './portfolio/security_selector';
```

  • 22.7 Rule 22.7 has been combined with rule 22.6.

  • 22.8 Rule 22.8 has been combined with rule 22.6.

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Accessors

  • 23.1 Accessor functions for properties are not required.

  • 23.2 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);

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

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

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

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

    class Jedi {
    constructor(options = {}) {
        const lightsaber = options.lightsaber || 'blue';
        this.set('lightsaber', lightsaber);
    }

    set(key, val) {
        this[key] = val;
    }

    get(key) {
        return this[key];
    }
}

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Events

  • 24.1 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
});

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jQuery

  • 25.1 Do not prefix jQuery object variables with a $.

    Why? Hungarian notation has limited value, and applying it to only some types of variable limits its usefulness even further.

    // bad
const $sidebar = $('.sidebar');

// good
const sidebar = $('.sidebar');

  • 25.2 Cache jQuery lookups.

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

    // ...stuff...

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

// good
function setSidebar() {
    const sidebar = $('.sidebar');
    sidebar.hide();

    // ...stuff...

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

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

  • 25.4 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();

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ECMAScript 5 Compatibility

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ECMAScript 6 Styles

  • 27.1 This is a collection of links to the various es6 features.
  1. Arrow Functions
  2. Classes
  3. Object Shorthand
  4. Object Concise
  5. Object Computed Properties
  6. Template Strings
  7. Destructuring
  8. Default Parameters
  9. Rest
  10. Array Spreads
  11. Let and Const
  12. Iterators and Generators
  13. Modules

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Testing

  • 28.1 Yup.

    function() {
    return true;
}

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Performance

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Resources

Learning ES6

Read This

Tools

Other Style Guides

Other Styles

Further Reading

Books

Blogs

Podcasts

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In the Wild

This is a list of organizations that are using this style guide. Send us a pull request or open an issue and we'll add you to the list.

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Translation

This style guide is also available in other languages:

The JavaScript Style Guide Guide

Contributors

IRESS Amendments

In IRESS we have made the following amendments to the base Airbnb style guide. These changes are already incorporated into this document.

Section Amendment Reason
2.1 Tighten restriction on var. We don't want var under any circumstances, so be clear on this.
3.2 Relax restriction on (formerly) reserved words. We don't support IE8, and the transpiler converts reserved words anyway. A motivating example is that some 3rd party libraries use them as function names, which we overwrite.
3.7 Remove rule. Developers should arrange property introductions in whatever way provides best code clarity. Sometimes this will be achieved by using logical groups.
6.4 Use _.sprintf() instead of template strings in most situations. Because the ES6 template string syntax converted by the transpiler before being passed to gettext, template strings break translation.
7.9, 9.3, 18.5 Restrict our usage of chaining. Even when formatted well, long chains can be hard to read. Additionally, the fluent interface seems unnecessary and can be confused with pipelining.
10.2 Allow wildcard imports if necessary. We can;t always control the content of 3rd party libraries we use.
13.3 Remove rule. Developers should arrange variable declarations in whatever way provides best code clarity, independent of whether the variable is const or let.
15.1 Reiterate this rule. There is often a desire to allow != null, which we want to explicitly reject.
15.3 Reverse rule. We prefer code to be explicit, which means avoiding the implicit tests of shortcuts.
16.3 Forbid single-line blocks. This makes all blocks consistent, and there is no obvious benefit to single-line blocks.
17.1, 17.2 Clarify commenting guidelines. The existing guidelines were vague about multi-line explanatory comments.
18.1 Use 4 space soft tabs for indenting. Consistency with our other languages, and we just prefer it that way.
21.4 Explicitly restrict bitshift string-to-int coercion even further. This practice is evil, but has occasional valid uses. Since we don't want to forbid it entirely, we want to make every effort to ensure it is used only when absolutely necessary.
21.6 Forbid the !!value int-to-bool shortcut. Prefer explicit, and this example has multiple ambiguities and pitfalls.
22.4 Emphasise single underscore only (not double underscore). Python developers draw a distinction between single and double underscore. We want to be clear that this distinction does not apply in Javascript.
22.6, 22.7, 22.8 Combine related rules, and use a camelcase rule for filenames. The existing rules were unnecessarily repetitive. We prefer camelcased filenames rather than case-sensitive ones.
25.1 Don't use $ prefix for jQuery object variables. We don't see the point of Hungarian notation.
26.2 Forbid ES6-specific functionality with poor support. If it's not supported well in our target browsers, we shouldn't use it.

Any changes to this style guide (whether new amendments, or pulling in updates from the base style guide) will need to be approved by all of the following:

  • XPLAN architecture team.

License

(The MIT License)

Copyright (c) 2015 Airbnb, IRESS

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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};

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