eslint-config-presets

    1.0.1 • Public • Published

    JavaScript Style Guide() {

    This style guide is derivation from eslint-config-airbnb-base

    Usage

    npm info "eslint-config-presets@latest" peerDependencies

    If using npm 5+, use this shortcut

    npx install-peerdeps --dev eslint-config-presets

    If using npm < 5, Linux/OSX users can run

    (
      export PKG=eslint-config-airbnb-base;
      npm info "$PKG@latest" peerDependencies --json | command sed 's/[\{\},]//g ; s/: /@/g' | xargs npm install --save-dev "$PKG@latest"
    )

    Which produces and runs a command like:

      npm install --save-dev eslint-config-presets eslint@^#.#.# eslint-plugin-import@^#.#.#

    If using npm < 5, Windows users can either install all the peer dependencies manually, or use the install-peerdeps cli tool.

    npm install -g install-peerdeps
    install-peerdeps --dev eslint-config-presets

    The cli will produce and run a command like:

    npm install --save-dev eslint-config-presets eslint@^#.#.# eslint-plugin-import@^#.#.#
    1. Add "extends": "presets" to your .eslintrc.

    Documentations

    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. Control Statements
    18. Comments
    19. Whitespace
    20. Commas
    21. Semicolons
    22. Type Casting & Coercion
    23. Naming Conventions
    24. Accessors
    25. Events
    26. jQuery
    27. ECMAScript 5 Compatibility
    28. ECMAScript 6+ (ES 2015+) Styles
    29. Standard Library
    30. Testing
    31. Performance
    32. Resources
    33. In the Wild
    34. Translation
    35. The JavaScript Style Guide Guide
    36. Chat With Us About JavaScript
    37. Contributors
    38. License
    39. Amendments

    Types

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

      • string
      • number
      • boolean
      • null
      • undefined
      • symbol
      const foo = 1;
      let bar = foo;
      
      bar = 9;
      
      console.log(foo, bar); // => 1, 9
      • Symbols cannot be faithfully polyfilled, so they should not be used when targeting browsers/environments that don't support them natively.

    • 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

      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 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
      };

    • 3.3 Use object method shorthand. eslint: object-shorthand

      // bad
      const atom = {
        value: 1,
      
        addValue: function (value) {
          return atom.value + value;
        },
      };
      
      // good
      const atom = {
        value: 1,
      
        addValue(value) {
          return atom.value + value;
        }
      };

    • 3.4 Use property value shorthand. eslint: object-shorthand

      Why? It is shorter to write and descriptive.

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

    • 3.5 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
      };

    • 3.6 Only quote properties that are invalid identifiers. eslint: quote-props

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

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

    • 3.7 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 */
      import has from 'has'; // https://www.npmjs.com/package/has
      // ...
      console.log(has.call(object, key));

    • 3.8 Prefer the object spread operator over Object.assign to shallow-copy objects. Use the object rest operator to get a new object with certain properties omitted.

      // very bad
      const original = { a: 1, b: 2 };
      const copy = Object.assign(original, { c: 3 }); // this mutates `original` ಠ_ಠ
      delete copy.a; // so does this
      
      // bad
      const original = { a: 1, b: 2 };
      const copy = Object.assign({}, original, { c: 3 }); // copy => { a: 1, b: 2, c: 3 }
      
      // good
      const original = { a: 1, b: 2 };
      const copy = { ...original, c: 3 }; // copy => { a: 1, b: 2, c: 3 }
      
      const { a, ...noA } = copy; // noA => { b: 2, c: 3 }

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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 += 1) {
        itemsCopy[i] = items[i];
      }
      
      // good
      const itemsCopy = [...items];

    • 4.4 To convert an array-like object to an array, use spreads ... instead of Array.from.

      const foo = document.querySelectorAll('.foo');
      
      // good
      const nodes = Array.from(foo);
      
      // best
      const nodes = [...foo];

    • 4.5 Use Array.from instead of spread ... for mapping over iterables, because it avoids creating an intermediate array.

      // bad
      const baz = [...foo].map(bar);
      
      // good
      const baz = Array.from(foo, bar);

    • 4.6 Use return statements in array method callbacks. It’s ok to omit the return if the function body consists of a single statement returning an expression without side effects, 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 - no returned value means `acc` becomes undefined after the first iteration
      [[0, 1], [2, 3], [4, 5]].reduce((acc, item, index) => {
        const flatten = acc.concat(item);
        acc[index] = flatten;
      });
      
      // good
      [[0, 1], [2, 3], [4, 5]].reduce((acc, item, index) => {
        const flatten = acc.concat(item);
        acc[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;
      });

    • 4.7 Use line breaks after open and before close array brackets if an array has multiple lines

      // bad
      const arr = [
        [0, 1], [2, 3], [4, 5]
      ];
      
      const objectInArray = [{
        id: 1
      }, {
        id: 2
      }];
      
      const numberInArray = [
        1, 2
      ];
      
      // good
      const arr = [[0, 1], [2, 3], [4, 5]];
      
      const objectInArray = [
        {
          id: 1
        },
        {
          id: 2
        },
      ];
      
      const numberInArray = [
        1,
        2
      ];

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    Destructuring

    • 5.1 Use object destructuring when accessing and using multiple properties of an object. eslint: prefer-destructuring

      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. eslint: prefer-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.

      // 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

      // bad
      const name = "Capt. Janeway";
      
      // bad - template literals should contain interpolation or newlines
      const name = `Capt. Janeway`;
      
      // good
      const name = 'Capt. Janeway';

    • 6.2 Strings that cause the line to go over 100 characters should not be written across multiple lines using string concatenation.

      Why? Broken strings are painful to work with and make code less searchable.

      // 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.';

    • 6.3 When programmatically building up strings, use template strings instead of concatenation. eslint: prefer-template template-curly-spacing

      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}?`;
      }

    • 6.4 Never use eval() on a string, it opens too many vulnerabilities. eslint: no-eval

    • 6.5 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 = `my name is '${name}'`;

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    Functions

    • 7.1 Use named function expressions instead of function declarations. eslint: func-style

      Why? Function declarations are hoisted, which means that it’s easy - too easy - to reference the function before it is defined in the file. This harms readability and maintainability. If you find that a function’s definition is large or complex enough that it is interfering with understanding the rest of the file, then perhaps it’s time to extract it to its own module! Don’t forget to explicitly name the expression, regardless of whether or not the name is inferred from the containing variable (which is often the case in modern browsers or when using compilers such as Babel). This eliminates any assumptions made about the Error's call stack. (Discussion)

      // bad
      function foo() {
        // ...
      }
      
      // bad
      const foo = function () {
        // ...
      };
      
      // good
      // lexical name distinguished from the variable-referenced invocation(s)
      const short = function longUniqueMoreDescriptiveLexicalFoo() {
        // ...
      };

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

      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.

      // 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 foo(name, options, arguments) {
        // ...
      }
      
      // good
      function foo(name, options, args) {
        // ...
      }

    • 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. eslint: no-new-func

      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. eslint: space-before-function-paren space-before-blocks

      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) {
        // ...
      }

    • 7.14 Prefer the use of the spread operator ... to call variadic functions. eslint: prefer-spread

      Why? It’s cleaner, you don’t need to supply a context, and you can not easily compose new with apply.

      // bad
      const x = [1, 2, 3, 4, 5];
      console.log.apply(console, x);
      
      // good
      const x = [1, 2, 3, 4, 5];
      console.log(...x);
      
      // bad
      new (Function.prototype.bind.apply(Date, [null, 2016, 8, 5]));
      
      // good
      new Date(...[2016, 8, 5]);

    • 7.15 Functions with multiline signatures, or invocations, should be indented just like every other multiline list in this guide: with each item on a line by itself, with a trailing comma on the last item. eslint: function-paren-newline

      // bad
      function foo(bar,
                   baz,
                   quux) {
        // ...
      }
      
      // good
      function foo(
        bar,
        baz,
        quux
      ) {
        // ...
      }
      
      // bad
      console.log(foo,
        bar,
        baz);
      
      // good
      console.log(
        foo,
        bar,
        baz
      );

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    Arrow Functions

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

      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 named function expression.

      // 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 statement returning an expression without side effects, omit the braces and use the implicit return. Otherwise, keep the braces and use a return statement. eslint: arrow-parens, arrow-body-style

      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,
      }));
      
      // No implicit return with side effects
      function foo(callback) {
        const val = callback();
        if (val === true) {
          // Do something if callback returns true
        }
      }
      
      let bool = false;
      
      // bad
      foo(() => bool = true);
      
      // good
      foo(() => {
        bool = true;
      });

    • 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
      ['get', 'post', 'put'].map(httpMethod => Object.prototype.hasOwnProperty.call(
          httpMagicObjectWithAVeryLongName,
          httpMethod
        )
      );
      
      // good
      ['get', 'post', 'put'].map(httpMethod => (
        Object.prototype.hasOwnProperty.call(
          httpMagicObjectWithAVeryLongName,
          httpMethod
        )
      ));

    • 8.4 If your function takes a single argument and doesn’t use braces, omit the parentheses. Otherwise, always include parentheses around arguments for clarity and consistency. Note: it is also acceptable to always use parentheses, in which case use the “always” option for eslint. eslint: arrow-parens

      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 don’t want it to take up space on the .map line!`
      ));
      
      // 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 => (item.height > 256 ? item.largeSize : item.smallSize);
      
      // good
      const itemHeight = (item) => {
        const { height, largeSize, smallSize } = item;
        return height > 256 ? largeSize : smallSize;
      };

    • 8.6 Enforce the location of arrow function bodies with implicit returns. eslint: implicit-arrow-linebreak

      // bad
      (foo) =>
        bar;
      
      (foo) =>
        (bar);
      
      // good
      (foo) => bar;
      (foo) => (bar);
      (foo) => (
         bar
      )

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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; }
      }

    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.

      // 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

      Why? To encourage more files that only ever export one thing, which is better for readability and maintainability.

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

    • 10.7 Put all imports above non-import statements. eslint: import/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();

    • 10.8 Multiline imports should be indented just like multiline array and object literals.

      Why? The curly braces follow the same indentation rules as every other curly brace block in the style guide, as do the trailing commas.

      // bad
      import {longNameA, longNameB, longNameC, longNameD, longNameE} from 'path';
      
      // good
      import {
        longNameA,
        longNameB,
        longNameC,
        longNameD,
        longNameE
      } from 'path';

    • 10.9 Disallow Webpack loader syntax in module import statements. eslint: import/no-webpack-loader-syntax

      Why? Since using Webpack syntax in the imports couples the code to a module bundler. Prefer using the loader syntax in webpack.config.js.

      // bad
      import fooSass from 'css!sass!foo.scss';
      import barCss from 'style!css!bar.css';
      
      // good
      import fooSass from 'foo.scss';
      import barCss from 'bar.css';

    back to top

    Iterators and Generators

    • 11.1 Don’t use iterators. Prefer JavaScript’s higher-order functions instead of loops like for-in or for-of. eslint: no-iterator no-restricted-syntax

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

      Use map() / every() / filter() / find() / findIndex() / reduce() / some() / ... to iterate over arrays, and Object.keys() / Object.values() / Object.entries() to produce arrays so you can iterate over objects.

      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;
      
      // bad
      const increasedByOne = [];
      for (let i = 0; i < numbers.length; i++) {
        increasedByOne.push(numbers[i] + 1);
      }
      
      // good
      const increasedByOne = [];
      numbers.forEach((num) => {
        increasedByOne.push(num + 1);
      });
      
      // best (keeping it functional)
      const increasedByOne = numbers.map(num => num + 1);

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

    back to top

    Properties

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

      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');

    • 12.3 Use exponentiation operator ** when calculating exponentiations. eslint: no-restricted-properties.

      // bad
      const binary = Math.pow(2, 10);
      
      // good
      const binary = 2 ** 10;

    back to top

    Variables

    • 13.1 Always use const or let 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. eslint: no-undef prefer-const

      // bad
      superPower = new SuperPower();
      
      // good
      const superPower = new SuperPower();

    • 13.2 Use one const or let declaration per variable. eslint: one-var

      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;
      }

    • 13.5 Don’t chain variable assignments. eslint: no-multi-assign

      Why? Chaining variable assignments creates implicit global variables.

      // bad
      (function example() {
        // JavaScript interprets this as
        // let a = ( b = ( c = 1 ) );
        // The let keyword only applies to variable a; variables b and c become
        // global variables.
        let a = b = c = 1;
      }());
      
      console.log(a); // throws ReferenceError
      console.log(b); // 1
      console.log(c); // 1
      
      // good
      (function example() {
        let a = 1;
        let b = a;
        let c = a;
      }());
      
      console.log(a); // throws ReferenceError
      console.log(b); // throws ReferenceError
      console.log(c); // throws ReferenceError
      
      // the same applies for `const`

    • 13.6 Avoid using unary increments and decrements (++, --). eslint no-plusplus

      Why? Per the eslint documentation, unary increment and decrement statements are subject to automatic semicolon insertion and can cause silent errors with incrementing or decrementing values within an application. It is also more expressive to mutate your values with statements like num += 1 instead of num++ or num ++. Disallowing unary increment and decrement statements also prevents you from pre-incrementing/pre-decrementing values unintentionally which can also cause unexpected behavior in your programs.

      // bad
      
      const array = [1, 2, 3];
      let num = 1;
      num++;
      --num;
      
      let sum = 0;
      let truthyCount = 0;
      for (let i = 0; i < array.length; i++) {
        let value = array[i];
        sum += value;
        if (value) {
          truthyCount++;
        }
      }
      
      // good
      
      const array = [1, 2, 3];
      let num = 1;
      num += 1;
      num -= 1;
      
      const sum = array.reduce((a, b) => a + b, 0);
      const truthyCount = array.filter(Boolean).length;

    • 13.7 Avoid linebreaks before or after = in an assignment. If your assignment violates max-len, surround the value in parens. eslint operator-linebreak.

      Why? Linebreaks surrounding = can obfuscate the value of an assignment.

      // bad
      const foo =
        superLongLongLongLongLongLongLongLongFunctionName();
      
      // bad
      const foo
        = 'superLongLongLongLongLongLongLongLongString';
      
      // good
      const foo = (
        superLongLongLongLongLongLongLongLongFunctionName()
      );
      
      // good
      const foo = 'superLongLongLongLongLongLongLongLongString';

    • 13.8 Disallow unused variables. eslint: no-unused-vars

      Why? Variables that are declared and not used anywhere in the code are most likely an error due to incomplete refactoring. Such variables take up space in the code and can lead to confusion by readers.

      // bad
      
      var some_unused_var = 42;
      
      // Write-only variables are not considered as used.
      var y = 10;
      y = 5;
      
      // A read for a modification of itself is not considered as used.
      var z = 0;
      z = z + 1;
      
      // Unused function arguments.
      function getX(x, y) {
          return x;
      }
      
      // good
      
      function getXPlusY(x, y) {
        return x + y;
      }
      
      var x = 1;
      var y = a + 2;
      
      alert(getXPlusY(x, y));
      
      // 'type' is ignored even if unused because it has a rest property sibling.
      // This is a form of extracting an object that omits the specified keys.
      var { type, ...coords } = data;
      // 'coords' is now the 'data' object without its 'type' property.

    back to top

    Hoisting

    • 14.1 var declarations get hoisted to the top of their closest enclosing function 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 !=. 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 for booleans, but explicit comparisons for strings and numbers.

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

    • 15.5 Use braces to create blocks in case and default clauses that contain lexical declarations (e.g. let, const, function, and class). eslint: no-case-declarations

      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.

      // 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: no-nested-ternary

      // bad
      const foo = maybe1 > maybe2
        ? "bar"
        : value1 > value2 ? "baz" : null;
      
      // split into 2 separated ternary expressions
      const maybeNull = value1 > value2 ? 'baz' : null;
      
      // better
      const foo = maybe1 > maybe2
        ? 'bar'
        : maybeNull;
      
      // best
      const foo = maybe1 > maybe2 ? 'bar' : maybeNull;

    • 15.7 Avoid unneeded ternary statements. eslint: 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;

    • 15.8 When mixing operators, enclose them in parentheses. The only exception is the standard arithmetic operators (+, -, *, & /) since their precedence is broadly understood. eslint: no-mixed-operators

      Why? This improves readability and clarifies the developer’s intention.

      // bad
      const foo = a && b < 0 || c > 0 || d + 1 === 0;
      
      // bad
      const bar = a ** b - 5 % d;
      
      // bad
      // one may be confused into thinking (a || b) && c
      if (a || b && c) {
        return d;
      }
      
      // good
      const foo = (a && b < 0) || c > 0 || (d + 1 === 0);
      
      // good
      const bar = (a ** b) - (5 % d);
      
      // good
      if (a || (b && c)) {
        return d;
      }
      
      // good
      const bar = a + b / c * d;

    back to top

    Blocks

    • 16.1 Use braces with all multi-line blocks. eslint: nonblock-statement-body-position

      // 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 same line as your if block’s closing brace. eslint: brace-style

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

    • 16.3 If an if block always executes a return statement, the subsequent else block is unnecessary. A return in an else if block following an if block that contains a return can be separated into multiple if blocks. eslint: no-else-return

      // bad
      function foo() {
        if (x) {
          return x;
        } else {
          return y;
        }
      }
      
      // bad
      function cats() {
        if (x) {
          return x;
        } else if (y) {
          return y;
        }
      }
      
      // bad
      function dogs() {
        if (x) {
          return x;
        } else {
          if (y) {
            return y;
          }
        }
      }
      
      // good
      function foo() {
        if (x) {
          return x;
        }
      
        return y;
      }
      
      // good
      function cats() {
        if (x) {
          return x;
        }
      
        if (y) {
          return y;
        }
      }
      
      // good
      function dogs(x) {
        if (x) {
          if (z) {
            return y;
          }
        } else {
          return z;
        }
      }

    back to top

    Control Statements

    • 17.1 In case your control statement (if, while etc.) gets too long or exceeds the maximum line length, each (grouped) condition could be put into a new line. The logical operator should begin the line.

      Why? Requiring operators at the beginning of the line keeps the operators aligned and follows a pattern similar to method chaining. This also improves readability by making it easier to visually follow complex logic.

      // bad
      if ((foo === 123 || bar === 'abc') && doesItLookGoodWhenItBecomesThatLong() && isThisReallyHappening()) {
        thing1();
      }
      
      // bad
      if (foo === 123 &&
        bar === 'abc') {
        thing1();
      }
      
      // bad
      if (foo === 123
        && bar === 'abc') {
        thing1();
      }
      
      // bad
      if (
        foo === 123 &&
        bar === 'abc'
      ) {
        thing1();
      }
      
      // good
      if (
        foo === 123
        && bar === 'abc'
      ) {
        thing1();
      }
      
      // good
      if (
        (foo === 123 || bar === 'abc')
        && doesItLookGoodWhenItBecomesThatLong()
        && isThisReallyHappening()
      ) {
        thing1();
      }
      
      // good
      if (foo === 123 && bar === 'abc') {
        thing1();
      }

    • 17.2 Don't use selection operators in place of control statements.

      // bad
      !isRunning && startRunning();
      
      // good
      if (!isRunning) {
        startRunning();
      }

    back to top

    Comments

    • 18.1 Use /** ... */ for multi-line comments.

      // bad
      // make() returns a new element
      // based on the passed in tag name
      //
      // @param {String} tag
      // @return {Element} element
      function make(tag) {
      
        // ...
      
        return element;
      }
      
      // good
      /**
       * make() returns a new element
       * based on the passed-in tag name
       */
      function make(tag) {
      
        // ...
      
        return element;
      }

    • 18.2 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;
      }

    • 18.3 Start all comments with a space to make it easier to read. eslint: spaced-comment

      // bad
      //is current tab
      const active = true;
      
      // good
      // is current tab
      const active = true;
      
      // bad
      /**
       *make() returns a new element
       *based on the passed-in tag name
       */
      function make(tag) {
      
        // ...
      
        return element;
      }
      
      // good
      /**
       * make() returns a new element
       * based on the passed-in tag name
       */
      function make(tag) {
      
        // ...
      
        return element;
      }

    • 18.4 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.

    • 18.5 Use // FIXME: to annotate problems.

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

    • 18.6 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;
        }
      }

    back to top

    Whitespace

    • 19.1 Use soft tabs (space character) set to 2 spaces. eslint: indent

      // bad
      function foo() {
      ∙∙∙∙let name;
      }
      
      // bad
      function bar() {
      ∙let name;
      }
      
      // good
      function baz() {
      ∙∙let name;
      }

    • 19.2 Place 1 space before the leading brace. eslint: space-before-blocks

      // 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',
      });

    • 19.3 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

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

    • 19.4 Set off operators with spaces. eslint: space-infix-ops

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

    • 19.5 End files with a single newline character. eslint: eol-last

      // bad
      import { es6 } from './AirbnbStyleGuide';
        // ...
      export default es6;
      // bad
      import { es6 } from './AirbnbStyleGuide';
        // ...
      export default es6;
      
      // good
      import { es6 } from './AirbnbStyleGuide';
        // ...
      export default es6;

    • 19.6 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);

    • 19.7 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;
      
      // bad
      const arr = [
        function foo() {
        },
        function bar() {
        },
      ];
      return arr;
      
      // good
      const arr = [
        function foo() {
        },
      
        function bar() {
        },
      ];
      
      return arr;

    • 19.8 Do not pad your blocks with blank lines. eslint: padded-blocks

      // bad
      function bar() {
      
        console.log(foo);
      
      }
      
      // bad
      if (baz) {
      
        console.log(qux);
      } else {
        console.log(foo);
      
      }
      
      // bad
      class Foo {
      
        constructor(bar) {
          this.bar = bar;
        }
      }
      
      // good
      function bar() {
        console.log(foo);
      }
      
      // good
      if (baz) {
        console.log(qux);
      } else {
        console.log(foo);
      }

    • 19.9 Do not add spaces inside parentheses. eslint: space-in-parens

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

    • 19.10 Do not add spaces inside brackets. eslint: array-bracket-spacing

      // bad
      const foo = [ 1, 2, 3 ];
      console.log(foo[ 0 ]);
      
      // good
      const foo = [1, 2, 3];
      console.log(foo[0]);

    • 19.11 Add spaces inside curly braces. eslint: object-curly-spacing

      // bad
      const foo = {clark: 'kent'};
      
      // good
      const foo = { clark: 'kent' };

    • 19.12 Avoid having lines of code that are longer than 100 characters (including whitespace). Note: per above, long strings are exempt from this rule, and should not be broken up. eslint: max-len

      Why? This ensures readability and maintainability.

      // bad
      const foo = jsonData && jsonData.foo && jsonData.foo.bar && jsonData.foo.bar.baz && jsonData.foo.bar.baz.quux && jsonData.foo.bar.baz.quux.xyzzy;
      
      // 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 = jsonData
        && jsonData.foo
        && jsonData.foo.bar
        && jsonData.foo.bar.baz
        && jsonData.foo.bar.baz.quux
        && jsonData.foo.bar.baz.quux.xyzzy;
      
      // good
      $.ajax({
        method: 'POST',
        url: 'https://airbnb.com/',
        data: { name: 'John' },
      })
        .done(() => console.log('Congratulations!'))
        .fail(() => console.log('You have failed this city.'));

    • 19.13 Require consistent spacing inside an open block token and the next token on the same line. This rule also enforces consistent spacing inside a close block token and previous token on the same line. eslint: block-spacing

      // bad
      function foo() {return true;}
      if (foo) { bar = 0;}
      
      // good
      function foo() { return true; }
      if (foo) { bar = 0; }

    • 19.14 Avoid spaces before commas and require a space after commas. eslint: comma-spacing

      // bad
      var foo = 1,bar = 2;
      var arr = [1 , 2];
      
      // good
      var foo = 1, bar = 2;
      var arr = [1, 2];

    • 19.15 Enforce spacing inside of computed properties. eslint: computed-property-spacing

      // bad
      obj[foo ]
      obj[ 'foo']
      var x = {[ b ]: a}
      obj[foo[ bar ]]
      
      // good
      obj[foo]
      obj['foo']
      var x = { [b]: a }
      obj[foo[bar]]

    • 19.16 Enforce spacing between functions and their invocations. eslint: func-call-spacing

      // bad
      func ();
      
      func
      ();
      
      // good
      func();

    • 19.17 Enforce spacing between keys and values in object literal properties. eslint: key-spacing

      // bad
      var obj = { "foo" : 42 };
      var obj2 = { "foo":42 };
      
      // good
      var obj = { "foo": 42 };

    • 19.19 Avoid multiple empty lines and only allow one newline at the end of files. eslint: no-multiple-empty-lines

      // bad
      var x = 1;
      
      
      
      var y = 2;
      
      // good
      var x = 1;
      
      var y = 2;

    back to top

    Commas

    • 20.1 Leading commas: Nope. eslint: comma-style

      // 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'
      };

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    Semicolons

    • 21.1 Yup. eslint: semi

      Why? When JavaScript encounters a line break without a semicolon, it uses a set of rules called Automatic Semicolon Insertion to determine whether or not it should regard that line break as the end of a statement, and (as the name implies) place a semicolon into your code before the line break if it thinks so. ASI contains a few eccentric behaviors, though, and your code will break if JavaScript misinterprets your line break. These rules will become more complicated as new features become a part of JavaScript. Explicitly terminating your statements and configuring your linter to catch missing semicolons will help prevent you from encountering issues.

      // bad - raises exception
      const luke = {}
      const leia = {}
      [luke, leia].forEach(jedi => jedi.father = 'vader')
      
      // bad - raises exception
      const reaction = "No! That's impossible!"
      (async function meanwhileOnTheFalcon() {
        // handle `leia`, `lando`, `chewie`, `r2`, `c3p0`
        // ...
      }())
      
      // bad - returns `undefined` instead of the value on the next line - always happens when `return` is on a line by itself because of ASI!
      function foo() {
        return
          'search your feelings, you know it to be foo'
      }
      
      // good
      const luke = {};
      const leia = {};
      [luke, leia].forEach((jedi) => {
        jedi.father = 'vader';
      });
      
      // good
      const reaction = "No! That's impossible!";
      (async function meanwhileOnTheFalcon() {
        // handle `leia`, `lando`, `chewie`, `r2`, `c3p0`
        // ...
      }());
      
      // good
      function foo() {
        return 'search your feelings, you know it to be foo';
      }

      Read more.

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    Type Casting & Coercion

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

    • 22.2 Strings: eslint: no-new-wrappers

      // => this.reviewScore = 9;
      
      // bad
      const totalScore = new String(this.reviewScore); // typeof totalScore is "object" not "string"
      
      // 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);

    • 22.3 Numbers: Use Number for type casting and parseInt always with a radix for parsing strings. eslint: radix no-new-wrappers

      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);

    • 22.4 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.

      // 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;

    • 22.5 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

    • 22.6 Booleans: eslint: no-new-wrappers

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

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    Naming Conventions

    • 23.1 Avoid single letter names. Be descriptive with your naming. eslint: id-length

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

    • 23.2 Use camelCase when naming objects, functions, and instances. eslint: camelcase

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

    • 23.3 Use PascalCase only when naming constructors or classes. eslint: new-cap

      // 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'
      });

    • 23.4 Do not use trailing or leading underscores. eslint: no-underscore-dangle

      Why? JavaScript does not have the concept of privacy in terms of properties or methods. Although a leading underscore is a common convention to mean “private”, in fact, these properties are fully public, and as such, are part of your public API contract. This convention might lead developers to wrongly think that a change won’t count as breaking, or that tests aren’t needed. tl;dr: if you want something to be “private”, it must not be observably present.

      // bad
      this.__firstName__ = 'Panda';
      this.firstName_ = 'Panda';
      this._firstName = 'Panda';
      
      // good
      this.firstName = 'Panda';
      
      // good, in environments where WeakMaps are available
      // see https://kangax.github.io/compat-table/es6/#test-WeakMap
      const firstNames = new WeakMap();
      firstNames.set(this, 'Panda');

    • 23.5 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);
        };
      }

    • 23.6 A base filename should exactly match the name of its default export.

      // file 1 contents
      class CheckBox {
        // ...
      }
      export default CheckBox;
      
      // file 2 contents
      export default function fortyTwo() { return 42; }
      
      // file 3 contents
      export default function insideDirectory() {}
      
      // in some other file
      // bad
      import CheckBox from './checkBox'; // PascalCase import/export, camelCase filename
      import FortyTwo from './FortyTwo'; // PascalCase import/filename, camelCase export
      import InsideDirectory from './InsideDirectory'; // PascalCase import/filename, camelCase export
      
      // bad
      import CheckBox from './check_box'; // PascalCase import/export, snake_case filename
      import forty_two from './forty_two'; // snake_case import/filename, camelCase export
      import inside_directory from './inside_directory'; // snake_case import, camelCase export
      import index from './inside_directory/index'; // requiring the index file explicitly
      import insideDirectory from './insideDirectory/index'; // requiring the index file explicitly
      
      // good
      import CheckBox from './CheckBox'; // PascalCase export/import/filename
      import fortyTwo from './fortyTwo'; // camelCase export/import/filename
      import insideDirectory from './insideDirectory'; // camelCase export/import/directory name/implicit "index"
      // ^ supports both insideDirectory.js and insideDirectory/index.js

    • 23.7 Use camelCase when you export-default a function. Your filename should be identical to your function’s name.

      function makeStyleGuide() {
        // ...
      }
      
      export default makeStyleGuide;

    • 23.8 Use PascalCase when you export a constructor / class / singleton / function library / bare object.

      const AirbnbStyleGuide = {
        es6: {
        },
      };
      
      export default AirbnbStyleGuide;

    • 23.9 Acronyms and initialisms should always be all capitalized, or all lowercased.

      Why? Names are for readability, not to appease a computer algorithm.

      // bad
      import SmsContainer from './containers/SmsContainer';
      
      // bad
      const HttpRequests = [
        // ...
      ];
      
      // good
      import SMSContainer from './containers/SMSContainer';
      
      // good
      const HTTPRequests = [
        // ...
      ];
      
      // also good
      const httpRequests = [
        // ...
      ];
      
      // best
      import TextMessageContainer from './containers/TextMessageContainer';
      
      // best
      const requests = [
        // ...
      ];

    • 23.10 You may optionally uppercase a constant only if it (1) is exported, (2) is a const (it can not be reassigned), and (3) the programmer can trust it (and its nested properties) to never change.

      Why? This is an additional tool to assist in situations where the programmer would be unsure if a variable might ever change. UPPERCASE_VARIABLES are letting the programmer know that they can trust the variable (and its properties) not to change.

      • What about all const variables? - This is unnecessary, so uppercasing should not be used for constants within a file. It should be used for exported constants however.
      • What about exported objects? - Uppercase at the top level of export (e.g. EXPORTED_OBJECT.key) and maintain that all nested properties do not change.
      // bad
      const PRIVATE_VARIABLE = 'should not be unnecessarily uppercased within a file';
      
      // bad
      export const THING_TO_BE_CHANGED = 'should obviously not be uppercased';
      
      // bad
      export let REASSIGNABLE_VARIABLE = 'do not use let with uppercase variables';
      
      // ---
      
      // allowed but does not supply semantic value
      export const apiKey = 'SOMEKEY';
      
      // better in most cases
      export const API_KEY = 'SOMEKEY';
      
      // ---
      
      // bad - unnecessarily uppercases key while adding no semantic value
      export const MAPPING = {
        KEY: 'value'
      };
      
      // good
      export const MAPPING = {
        key: 'value'
      };

    back to top

    Accessors

    • 24.1 Accessor functions for properties are not required.

    • 24.2 Do not use JavaScript getters/setters as they cause unexpected side effects and are harder to test, maintain, and reason about. Instead, if you do make accessor functions, use getVal() and setVal('hello').

      // bad
      class Dragon {
        get age() {
          // ...
        }
      
        set age(value) {
          // ...
        }
      }
      
      // good
      class Dragon {
        getAge() {
          // ...
        }
      
        setAge(value) {
          // ...
        }
      }

    • 24.3 If the property/method is a boolean, use isVal() or hasVal().

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

    • 24.4 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

    • 25.1 When attaching data payloads to events (whether DOM events or something more proprietary like Backbone events), pass an object literal (also known as 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', (e, listingID) => {
        // do something with listingID
      });

      prefer:

      // good
      $(this).trigger('listingUpdated', { listingID: listing.id });
      
      // ...
      
      $(this).on('listingUpdated', (e, data) => {
        // do something with data.listingID
      });

    back to top

    jQuery

    • 26.1 Prefix jQuery object variables with a $.

      // bad
      const sidebar = $('.sidebar');
      
      // good
      const $sidebar = $('.sidebar');
      
      // good
      const $sidebarBtn = $('.sidebar-btn');

    • 26.2 Cache jQuery lookups.

      // bad
      function setSidebar() {
        $('.sidebar').hide();
      
        // ...
      
        $('.sidebar').css({
          'background-color': 'pink',
        });
      }
      
      // good
      function setSidebar() {
        const $sidebar = $('.sidebar');
        $sidebar.hide();
      
        // ...
      
        $sidebar.css({
          'background-color': 'pink',
        });
      }

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

    • 26.4 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

    back to top

    ECMAScript 6+ (ES 2015+) Styles

    • 28.1 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. Exponentiation Operator
    13. Iterators and Generators
    14. Modules

    • 28.2 Do not use TC39 proposals that have not reached stage 3.

      Why? They are not finalized, and they are subject to change or to be withdrawn entirely. We want to use JavaScript, and proposals are not JavaScript yet.

    back to top

    Standard Library

    The Standard Library contains utilities that are functionally broken but remain for legacy reasons.

    • 29.1 Use Number.isNaN instead of global isNaN. eslint: no-restricted-globals

      Why? The global isNaN coerces non-numbers to numbers, returning true for anything that coerces to NaN. If this behavior is desired, make it explicit.

      // bad
      isNaN('1.2'); // false
      isNaN('1.2.3'); // true
      
      // good
      Number.isNaN('1.2.3'); // false
      Number.isNaN(Number('1.2.3')); // true

    • 29.2 Use Number.isFinite instead of global isFinite. eslint: no-restricted-globals

      Why? The global isFinite coerces non-numbers to numbers, returning true for anything that coerces to a finite number. If this behavior is desired, make it explicit.

      // bad
      isFinite('2e3'); // true
      
      // good
      Number.isFinite('2e3'); // false
      Number.isFinite(parseInt('2e3', 10)); // true

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    Testing

    • 30.1 Yup.

      function foo() {
        return true;
      }

    • 30.2 No, but seriously:
      • Whichever testing framework you use, you should be writing tests!
      • Strive to write many small pure functions, and minimize where mutations occur.
      • Be cautious about stubs and mocks - they can make your tests more brittle.
      • We primarily use mocha and jest at Airbnb. tape is also used occasionally for small, separate modules.
      • 100% test coverage is a good goal to strive for, even if it’s not always practical to reach it.
      • Whenever you fix a bug, write a regression test. A bug fixed without a regression test is almost certainly going to break again in the future.

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    Performance

    back to top

    Improving this config

    Consider adding test cases if you're making complicated rules changes, like anything involving regexes. Perhaps in a distant future, we could use literate programming to structure our README as test cases for our .eslintrc?

    You can run tests with npm test.

    You can make sure this module lints with itself using npm run lint.

    License

    (The MIT License)

    Copyright (c) 2018 Suhendra Ahmad

    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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    Amendments

    We encourage you to fork this guide and change the rules to fit your team’s style guide. Below, you may list some amendments to the style guide. This allows you to periodically update your style guide without having to deal with merge conflicts.

    };

    Install

    npm i eslint-config-presets

    DownloadsWeekly Downloads

    294

    Version

    1.0.1

    License

    MIT

    Unpacked Size

    162 kB

    Total Files

    20

    Last publish

    Collaborators

    • npm