smartprix-js-conventions

Base JS ESLint config used by Smartprix

Usage no npm install needed!

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  import smartprixJsConventions from 'https://cdn.skypack.dev/smartprix-js-conventions';
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README

Smartprix JavaScript Style Guide() {

A mostly reasonable approach to JavaScript

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Table of Contents

  1. Types
  2. References
  3. Objects
  4. Arrays
  5. Destructuring
  6. Strings
  7. Functions
  8. Arrow Functions
  9. Classes & 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. 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
    

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References

  • 2.1 Use const for all of your references; avoid using var. eslint: prefer-const, no-const-assign

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

    // bad
    var a = 1;
    var b = 2;
    
    // good
    const a = 1;
    const b = 2;
    

  • 2.2 If you must reassign references, use let instead of var. eslint: no-var jscs: disallowVar

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

    // 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
    

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Objects

  • 3.1 Use the literal syntax for object creation. eslint: no-new-object

    // bad
    const item = new Object();
    
    // good
    const item = {};
    

  • 3.2 If your code will be executed in browsers in script context, don't use reserved words as keys. It won't work in IE8. More info. It’s OK to use them in ES6 modules and server-side code. jscs: disallowIdentifierNames

    // 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. jscs: disallowIdentifierNames

    // 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.

    
    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. eslint: object-shorthand jscs: requireEnhancedObjectLiterals

    // 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. eslint: object-shorthand jscs: requireEnhancedObjectLiterals

    Why? It is shorter to write and descriptive.

    const lukeSkywalker = 'Luke Skywalker';
    
    // bad
    const obj = {
      lukeSkywalker: lukeSkywalker,
    };
    
    // good
    const obj = {
        lukeSkywalker,
    };
    

  • 3.7 Group your shorthand properties at the beginning of your object declaration.

    Why? It's easier to tell which properties are using the shorthand.

    const anakinSkywalker = 'Anakin Skywalker';
    const lukeSkywalker = 'Luke Skywalker';
    
    // bad
    const obj = {
        episodeOne: 1,
        twoJediWalkIntoACantina: 2,
        lukeSkywalker,
        episodeThree: 3,
        mayTheFourth: 4,
        anakinSkywalker,
    };
    
    // good
    const obj = {
        lukeSkywalker,
        anakinSkywalker,
        episodeOne: 1,
        twoJediWalkIntoACantina: 2,
        episodeThree: 3,
        mayTheFourth: 4,
    };
    

Why? In general we consider it subjectively easier to read. It improves syntax highlighting, and is also more easily optimized by many JS engines.

```javascript
// bad
const bad = {
    'foo': 3,
    'bar': 4,
    'data-blah': 5,
};

// good
const good = {
    foo: 3,
    bar: 4,
   'data-blah': 5,
};
```

  • 3.9 Do not call Object.prototype methods directly, such as hasOwnProperty, propertyIsEnumerable, and isPrototypeOf.

Why? These methods may be shadowed by properties on the object in question - consider { hasOwnProperty: false } - or, the object may be a null object (Object.create(null)).

// bad
console.log(object.hasOwnProperty(key));

// good
console.log(Object.prototype.hasOwnProperty.call(object, key));

// best
const has = Object.prototype.hasOwnProperty; // cache the lookup once, in module scope.
/* or */
const has = require('has');
…
console.log(has.call(object, key));

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Arrays

  • 4.1 Use the literal syntax for array creation. eslint: no-array-constructor

    // 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);
    

  • 4.5 Use return statements in array method callbacks. It's ok to omit the return if the function body consists of a single statement following 8.2. eslint: array-callback-return

    // good
    [1, 2, 3].map((x) => {
        const y = x + 1;
        return x * y;
    });
    
    // good
    [1, 2, 3].map(x => x + 1);
    
    // bad
    const flat = {};
    [[0, 1], [2, 3], [4, 5]].reduce((memo, item, index) => {
        const flatten = memo.concat(item);
        flat[index] = flatten;
    });
    
    // good
    const flat = {};
    [[0, 1], [2, 3], [4, 5]].reduce((memo, item, index) => {
        const flatten = memo.concat(item);
        flat[index] = flatten;
        return flatten;
    });
    
    // bad
    inbox.filter((msg) => {
        const { subject, author } = msg;
        if (subject === 'Mockingbird') {
            return author === 'Harper Lee';
        }
        else {
            return false;
        }
    });
    
    // good
    inbox.filter((msg) => {
        const { subject, author } = msg;
        if (subject === 'Mockingbird') {
            return author === 'Harper Lee';
        }
    
        return false;
    });
    

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Destructuring

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

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

    // bad
    function getFullName(user) {
        const firstName = user.firstName;
        const lastName = user.lastName;
    
        return `${firstName} ${lastName}`;
    }
    
    // good
    function getFullName(user) {
        const { firstName, lastName } = user;
        return `${firstName} ${lastName}`;
    }
    
    // best
    function getFullName({ firstName, lastName }) {
        return `${firstName} ${lastName}`;
    }
    

  • 5.2 Use array destructuring. jscs: requireArrayDestructuring

    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. jscs: disallowArrayDestructuringReturn

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

    // 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, top } = processInput(input);
    

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Strings

  • 6.1 Use single quotes '' for strings. eslint: quotes jscs: validateQuoteMarks

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

  • 6.2 Strings that cause the line to go over 100 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 template strings instead of concatenation. eslint: prefer-template template-curly-spacing jscs: requireTemplateStrings

    Why? Template strings give you a readable, concise syntax with proper newlines and string interpolation features.

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

  • 6.5 Never use eval() on a string, it opens too many vulnerabilities.

  • 6.6 Do not unnecessarily escape characters in strings. eslint: no-useless-escape

    Why? Backslashes harm readability, thus they should only be present when necessary.

    // bad
    const foo = '\'this\' \i\s \"quoted\"';
    
    // good
    const foo = '\'this\' is "quoted"';
    const foo = `'this' is "quoted"`;
    

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Functions

  • 7.1 Use function declarations instead of function expressions. jscs: requireFunctionDeclarations

    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.

    // bad
    const foo = function () {
    };
    
    // good
    function foo() {
    }
    

  • 7.2 Wrap immediately invoked function expressions in parentheses. eslint: wrap-iife jscs: requireParenthesesAroundIIFE

    Why? An immediately invoked function expression is a single unit - wrapping both it, and its invocation parens, in parens, cleanly expresses this. Note that in a world with modules everywhere, you almost never need an IIFE.

    // immediately-invoked function expression (IIFE)
    (function () {
        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. eslint: no-loop-func

  • 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. eslint: prefer-rest-params

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

    // 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.

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

  • 7.9 Always put default parameters last.

    // bad
    function handleThings(opts = {}, name) {
        // ...
    }
    
    // good
    function handleThings(name, opts = {}) {
        // ...
    }
    

  • 7.10 Never use the Function constructor to create a new function.

    Why? Creating a function in this way evaluates a string similarly to eval(), which opens vulnerabilities.

    // bad
    var add = new Function('a', 'b', 'return a + b');
    
    // still bad
    var subtract = Function('a', 'b', 'return a - b');
    

  • 7.11 Spacing in a function signature.

    Why? Consistency is good, and you shouldn’t have to add or remove a space when adding or removing a name.

    // bad
    const f = function(){};
    const g = function (){};
    const h = function() {};
    
    // good
    const x = function () {};
    const y = function a() {};
    

  • 7.12 Never mutate parameters. eslint: no-param-reassign

    Why? Manipulating objects passed in as parameters can cause unwanted variable side effects in the original caller.

    // bad
    function f1(obj) {
        obj.key = 1;
    };
    
    // good
    function f2(obj) {
        const key = Object.prototype.hasOwnProperty.call(obj, 'key') ? obj.key : 1;
    };
    

  • 7.13 Never reassign parameters. eslint: no-param-reassign

    Why? Reassigning parameters can lead to unexpected behavior, especially when accessing the arguments object. It can also cause optimization issues, especially in V8.

    // bad
    function f1(a) {
        a = 1;
    }
    
    function f2(a) {
        if (!a) { a = 1; }
    }
    
    // good
    function f3(a) {
        const b = a || 1;
    }
    
    function f4(a = 1) {
    }
    

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Arrow Functions

  • 8.1 When you must use function expressions (as when passing an anonymous function), use arrow function notation. eslint: prefer-arrow-callback, arrow-spacing jscs: requireArrowFunctions

    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.

    // bad
    [1, 2, 3].map(function (x) {
        const y = x + 1;
        return x * y;
    });
    
    // good
    [1, 2, 3].map((x) => {
        const y = x + 1;
        return x * y;
    });
    

  • 8.2 If the function body consists of a single expression, omit the braces and use the implicit return. Otherwise, keep the braces and use a return statement. eslint: arrow-parens, arrow-body-style jscs: disallowParenthesesAroundArrowParam, requireShorthandArrowFunctions

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

    // bad
    [1, 2, 3].map(number => {
        const nextNumber = number + 1;
        `A string containing the ${nextNumber}.`;
    });
    
    // good
    [1, 2, 3].map(number => `A string containing the ${number}.`);
    
    // good
    [1, 2, 3].map((number) => {
        const nextNumber = number + 1;
        return `A string containing the ${nextNumber}.`;
    });
    
    // good
    [1, 2, 3].map((number, index) => ({
        index: number
    }));
    

  • 8.3 In case the expression spans over multiple lines, wrap it in parentheses for better readability.

    Why? It shows clearly where the function starts and ends.

    // bad
    [1, 2, 3].map(number => 'As time went by, the string containing the ' +
        `${number} became much longer. So we needed to break it over multiple ` +
        'lines.'
    );
    
    // good
    [1, 2, 3].map(number => (
        `As time went by, the string containing the ${number} became much ` +
        'longer. So we needed to break it over multiple lines.'
    ));
    

  • 8.4 If your function takes a single argument and doesn’t use braces, omit the parentheses. Otherwise, always include parentheses around arguments. eslint: arrow-parens jscs: disallowParenthesesAroundArrowParam

    Why? Less visual clutter.

    // bad
    [1, 2, 3].map((x) => x * x);
    
    // good
    [1, 2, 3].map(x => x * x);
    
    // good
    [1, 2, 3].map(number => (
        `A long string with the ${number}. It’s so long that we’ve broken it ` +
        'over multiple lines!'
    ));
    
    // bad
    [1, 2, 3].map(x => {
        const y = x + 1;
        return x * y;
    });
    
    // good
    [1, 2, 3].map((x) => {
        const y = x + 1;
        return x * y;
    });
    

  • 8.5 Avoid confusing arrow function syntax (=>) with comparison operators (<=, >=). eslint: no-confusing-arrow

    // bad
    const itemHeight = item => item.height > 256 ? item.largeSize : item.smallSize;
    
    // bad
    const itemHeight = (item) => item.height > 256 ? item.largeSize : item.smallSize;
    
    // good
    const itemHeight = (item) => { return item.height > 256 ? item.largeSize : item.smallSize; };
    

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Classes & Constructors

  • 9.1 Always use class. Avoid manipulating prototype directly.

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

    // 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.

    // 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 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;
    };
    
    const luke = new Jedi();
    luke.jump(); // => true
    luke.setHeight(20); // => undefined
    
    // good
    class Jedi {
        jump() {
            this.jumping = true;
            return this;
        }
    
        setHeight(height) {
            this.height = height;
            return this;
        }
    }
    
    const luke = new Jedi();
    
    luke.jump()
      .setHeight(20);
    

  • 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()}`;
        }
    }
    

  • 9.5 Classes have a default constructor if one is not specified. An empty constructor function or one that just delegates to a parent class is unnecessary. eslint: no-useless-constructor

    // bad
    class Jedi {
        constructor() {}
    
        getName() {
            return this.name;
        }
    }
    
    // bad
    class Rey extends Jedi {
        constructor(...args) {
            super(...args);
        }
    }
    
    // good
    class Rey extends Jedi {
        constructor(...args) {
            super(...args);
            this.name = 'Rey';
        }
    }
    

  • 9.6 Avoid duplicate class members. eslint: no-dupe-class-members

    Why? Duplicate class member declarations will silently prefer the last one - having duplicates is almost certainly a bug.

    // bad
    class Foo {
        bar() { return 1; }
        bar() { return 2; }
    }
    
    // good
    class Foo {
        bar() { return 1; }
    }
    
    // good
    class Foo {
        bar() { return 2; }
    }
    

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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.

    // 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 Do not use wildcard imports.

    Why? This makes sure you have a single default export.

    // bad
    import * as AirbnbStyleGuide from './AirbnbStyleGuide';
    
    // good
    import AirbnbStyleGuide from './AirbnbStyleGuide';
    

  • 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.

    // bad
    // filename es6.js
    export { es6 as default } from './airbnbStyleGuide';
    
    // good
    // filename es6.js
    import { es6 } from './AirbnbStyleGuide';
    export default es6;
    

  • 10.4 Only import from a path in one place. eslint: no-duplicate-imports

    Why? Having multiple lines that import from the same path can make code harder to maintain.

    // bad
    import foo from 'foo';
    // … some other imports … //
    import { named1, named2 } from 'foo';
    
    // good
    import foo, { named1, named2 } from 'foo';
    
    // good
    import foo, {
        named1,
        named2,
    } from 'foo';
    

  • 10.5 Do not export mutable bindings. eslint: import/no-mutable-exports

    Why? Mutation should be avoided in general, but in particular when exporting mutable bindings. While this technique may be needed for some special cases, in general, only constant references should be exported.

    // bad
    let foo = 3;
    export { foo }
    
    // good
    const foo = 3;
    export { foo }
    

  • 10.6 In modules with a single export, prefer default export over named export. eslint: import/prefer-default-export

    // bad
    export function foo() {}
    
    // good
    export default function foo() {}
    

  • 10.7 Put all imports above non-import statements. eslint: import/imports-first

    Why? Since imports are hoisted, keeping them all at the top prevents surprising behavior.

    // bad
    import foo from 'foo';
    foo.init();
    
    import bar from 'bar';
    
    // good
    import foo from 'foo';
    import bar from 'bar';
    
    foo.init();
    

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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. eslint: no-iterator

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

    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.

  • 11.3 If you must use generators, or if you disregard our advice, make sure their function signature is spaced properly. eslint: generator-star-spacing

    Why? function and * are part of the same conceptual keyword - * is not a modifier for function, function* is a unique construct, different from function.

    // bad
    function * foo() {
    }
    
    const bar = function * () {
    }
    
    const baz = function *() {
    }
    
    const quux = function*() {
    }
    
    function*foo() {
    }
    
    function *foo() {
    }
    
    // very bad
    function
    *
    foo() {
    }
    
    const wat = function
    *
    () {
    }
    
    // good
    function* foo() {
    }
    
    const foo = function* () {
    }
    

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Properties

  • 12.1 Use dot notation when accessing properties. eslint: dot-notation jscs: requireDotNotation

    const luke = {
        jedi: true,
        age: 28,
    };
    
    // bad
    const isJedi = luke['jedi'];
    
    // good
    const isJedi = luke.jedi;
    

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

    const luke = {
      jedi: true,
      age: 28,
    };
    
    function getProp(prop) {
      return luke[prop];
    }
    
    const isJedi = getProp('jedi');
    

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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. eslint: one-var jscs: disallowMultipleVarDecl

    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. You can also step through each declaration with the debugger, instead of jumping through all of them at once.

    // 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 Group all your consts and then group all your lets.

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

    // bad
    let i, len, dragonball,
        items = getItems(),
        goSportsTeam = true;
    
    // bad
    let i;
    const items = getItems();
    let dragonball;
    const goSportsTeam = true;
    let len;
    
    // good
    const goSportsTeam = true;
    const items = getItems();
    let dragonball;
    let i;
    let length;
    

  • 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.

    // bad - unnecessary function call
    function checkName(hasName) {
        const name = getName();
    
        if (hasName === 'test') {
            return false;
        }
    
        if (name === 'test') {
            this.setName('');
            return false;
        }
    
        return name;
    }
    
    // good
    function checkName(hasName) {
        if (hasName === 'test') {
            return false;
        }
    
        const name = getName();
    
        if (name === 'test') {
            this.setName('');
            return false;
        }
    
        return name;
    }
    

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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.

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Comparison Operators & Equality

  • 15.1 Use === and !== over == and !=. eslint: eqeqeq

  • 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 (even an empty one) is an object, objects will evaluate to true
    }
    

  • 15.3 Use shortcuts.

    // bad
    if (name !== '') {
        // ...stuff...
    }
    
    // good
    if (name) {
        // ...stuff...
    }
    
    // bad
    if (collection.length > 0) {
        // ...stuff...
    }
    
    // good
    if (collection.length) {
        // ...stuff...
    }
    

  • 15.5 Use braces to create blocks in case and default clauses that contain lexical declarations (e.g. let, const, function, and class).

Why? Lexical declarations are visible in the entire switch block but only get initialized when assigned, which only happens when its case is reached. This causes problems when multiple case clauses attempt to define the same thing.

eslint rules: no-case-declarations.

```javascript
// bad
switch (foo) {
    case 1:
        let x = 1;
        break;
    case 2:
        const y = 2;
        break;
    case 3:
        function f() {}
        break;
    default:
        class C {}
}

// good
switch (foo) {
    case 1: {
        let x = 1;
        break;
    }
    case 2: {
        const y = 2;
        break;
    }
    case 3: {
        function f() {}
        break;
    }
    case 4:
        bar();
        break;
    default: {
        class C {}
    }
}
```

  • 15.6 Ternaries should not be nested and generally be single line expressions.

    eslint rules: no-nested-ternary.

    // bad
    const foo = maybe1 > maybe2
        ? "bar"
        : value1 > value2 ? "baz" : null;
    
    // better
    const maybeNull = value1 > value2 ? 'baz' : null;
    
    const foo = maybe1 > maybe2
        ? 'bar'
        : maybeNull;
    
    // best
    const maybeNull = value1 > value2 ? 'baz' : null;
    
    const foo = maybe1 > maybe2 ? 'bar' : maybeNull;
    

  • 15.7 Avoid unneeded ternary statements.

    eslint rules: no-unneeded-ternary.

    // bad
    const foo = a ? a : b;
    const bar = c ? true : false;
    const baz = c ? false : true;
    
    // good
    const foo = a || b;
    const bar = !!c;
    const baz = !c;
    

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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 foo() { return false; }
    
    // good
    function bar() {
        return false;
    }
    

  • 16.2 If you're using multi-line blocks with if and else, put else on the next line after your if block's closing brace. eslint: brace-style jscs: disallowNewlineBeforeBlockStatements

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

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Comments

  • 17.1 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;
    }
    

  • 17.2 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 unless it's on the first line of a block.

    // bad
    const active = true;  // is current tab
    
    // good
    // is current tab
    const active = true;
    
    // bad
    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;
    }
    
    // also good
    function getType() {
        // set the default type to 'no type'
        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 extends Abacus {
        constructor() {
            super();
    
            // FIXME: shouldn't use a global here
            total = 0;
        }
    }
    

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

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

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Whitespace

  • 18.1 Use hard tabs set to 4 spaces. eslint: indent jscs: validateIndentation

    // bad
    function foo() {
    ∙∙const name;
    }
    
    // bad
    function bar() {
    ∙∙∙∙const name;
    }
    
    // good
    function baz() {
    --->const name;
    }
    

  • 18.2 Place 1 space before the leading brace. eslint: space-before-blocks jscs: requireSpaceBeforeBlockStatements

    // 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 between the argument list and the function name in function calls and declarations. eslint: keyword-spacing jscs: requireSpaceAfterKeywords

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

  • 18.5 End files with a single newline character.

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

  • 18.6 Use indentation when making long method chains (more than 2 method chains). Use a leading dot, which emphasizes that the line is a method call, not a new statement. eslint: newline-per-chained-call no-whitespace-before-property

    // 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
    const 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
    const 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
    const leds = stage.selectAll('.led').data(data);
    

  • 18.7 Leave a blank line after blocks and before the next statement. jscs: requirePaddingNewLinesAfterBlocks

    // 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;
    
    // bad
    const arr = [
        function foo() {
        },
        function bar() {
        },
    ];
    return arr;
    
    // good
    const arr = [
        function foo() {
        },
    
        function bar() {
        },
    ];
    
    return arr;
    

  • 18.8 Do not pad your blocks with blank lines. eslint: padded-blocks jscs: disallowPaddingNewlinesInBlocks

    // bad
    function bar() {
    
        console.log(foo);
    
    }
    
    // also bad
    if (baz) {
    
        console.log(qux);
    }
    else {
        console.log(foo);
    
    }
    
    // good
    function bar() {
        console.log(foo);
    }
    
    // good
    if (baz) {
        console.log(qux);
    }
    else {
        console.log(foo);
    }
    

  • 18.9 Do not add spaces inside parentheses. eslint: space-in-parens jscs: disallowSpacesInsideParentheses

    // bad
    function bar( foo ) {
        return foo;
    }
    
    // good
    function bar(foo) {
        return foo;
    }
    
    // bad
    if ( foo ) {
        console.log(foo);
    }
    
    // good
    if (foo) {
        console.log(foo);
    }
    

  • 18.12 Avoid having lines of code that are longer than 100 characters (including whitespace). eslint: max-len jscs: maximumLineLength

    Why? This ensures readability and maintainability.

    // bad
    const foo = 'Whatever national crop flips the window. The cartoon reverts within the screw. Whatever wizard constrains a helpful ally. The counterpart ascends!';
    
    // bad
    $.ajax({ method: 'POST', url: 'https://airbnb.com/', data: { name: 'John' } }).done(() => console.log('Congratulations!')).fail(() => console.log('You have failed this city.'));
    
    // good
    const foo = 'Whatever national crop flips the window. The cartoon reverts within the screw. ' +
      'Whatever wizard constrains a helpful ally. The counterpart ascends!';
    
    // good
    $.ajax({
        method: 'POST',
        url: 'https://airbnb.com/',
        data: { name: 'John' },
    })
    .done(() => console.log('Congratulations!'))
    .fail(() => console.log('You have failed this city.'));
    

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Commas

  • 19.1 Leading commas: Nope. eslint: comma-style jscs: requireCommaBeforeLineBreak

    // 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. eslint: comma-dangle jscs: requireTrailingComma

    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.

    // 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. eslint: semi jscs: requireSemicolons

    // bad
    (function () {
        const name = 'Skywalker'
        return name
    })()
    
    // good
    (function () {
        const name = 'Skywalker';
        return name;
    }());
    
    // good, but legacy (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 + ''; // invokes this.reviewScore.valueOf()
    
    // bad
    const totalScore = this.reviewScore.toString(); // isn't guaranteed to return a string
    
    // good
    const totalScore = String(this.reviewScore);