Rapha Javascript Styleguide

1. Overview
2. What is a style guide?
3. Why is consistent code style important?
4. Our Focus
5. Syntax
6. Types
7. References
8. Objects
9. Arrays
10. Destructuring
11. Strings
12. Functions
13. Arrow Functions
14. Modules
15. Iterators and Generators
16. Properties
17. Variables
18. Hoisting
19. Comparison Operators & Equality
20. Blocks
21. Control Statements
22. Comments
23. Whitespace
24. Commas
25. Semicolons
26. Type Casting & Coercion
27. Naming Conventions
28. Accessors
29. Events
30. jQuery
31. ECMAScript 5 Compatibility
32. ECMAScript 6+ (ES 2015+) Styles
33. Standard Library
34. Testing
35. Performance
36. Resources
37. In the Wild
38. Translation
39. The JavaScript Style Guide Guide
40. Chat With Us About JavaScript
41. Contributors
42. License
43. Amendments

Overview

This document serves as the complete definition of Rapha’s coding standards for source code in the JavaScript programming language. A JavaScript source file is described as being in Rapha Style if and only if it adheres to the rules herein.

Like other programming style guides, the issues covered span not only aesthetic issues of formatting, but other types of conventions or coding standards as well. However, this document focuses primarily on the hard-and-fast rules that we follow universally, and avoids giving advice that isn't clearly enforceable (whether by human or tool).

What is a style guide?

Before we begin, let us remind ourselves exactly what a code style guide is:

A style guide or style manual is a set of standards for the writing and design of code. The implementation of a style guide provides uniformity in code style and formatting, often covering guidelines regarding indentation (tabs vs. spaces), variable and function naming conventions, where best to apply whitespace and so on.

Why is consistent code style important?

The more readable and consistent our source code is, the easier it is to maintain. We have previously covered this in our Technical Sprawl & Standardisation documentation.

Following a consistent style guide both helps enforce this concept and improves the overall quality of the code we write. This facilitates other developers stepping in to assist with maintenance more easily and can certainly save time in the long haul.

Consistently styled code can:

• Reduce the lead time required to understand an implementation
• Make it easier to establish what code can be reused
• Clarify how updates to an implementation should be styled or structured (remember that consistent code, even when written by a team, should look like one person wrote it).
• Be predictable

To quote Robert C. Martin — author of the excellent “Clean Code” — “You know you are working on clean code when each routine turns out to be pretty much what you expected”.

Our focus

We strive for code quality in all the web applications we build. For us, code quality encompasses the following principles:

• Modern syntax
• Best practices
• Prefer open source
• Readability
• Testability
• Maintainability
• Scalability

Much of this is taken care by

• Eslint
• Prettier

Areas we need to improve in are

• CI/CD and how we automate this in a DRY (Don’t Repeat Yourself) fashion. This would look for common pitfalls like nested conditional statements, use of classes when we are writing functional code and other bad practises.

Syntax

We should be using ES10/ES2020 as much as possible. This means we reduce the need of third party libraries such as lodash and we are in total control of our code. Below are some of the core functionality we should be striving to use:

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

Types

Primitives:

https://developer.mozilla.org/en-US/docs/Glossary/Primitive

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures

In JavaScript, a primitive (primitive value, primitive data type) is data that is not an object and has no methods.

All primitives are immutable, i.e., they cannot be altered. It is important not to confuse a primitive itself with a variable assigned a primitive value. The variable may be reassigned a new value, but the existing value can not be changed in the ways that objects, arrays, and functions can be altered.

When you access a primitive type you work directly on its value. There are 7 primitive data types:

• string
• number
• boolean
• null
• undefined
• symbol
• bigint

const foo = 1;
let bar = foo;

bar = 9;

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

Avoid using symbol unless we are compiling to ES10. We are currently targeting ES6 in most projects and there is no way to get a description of the Symbol object before ES10.

Avoid using bigint unless we are compiling to ES10. It is not supported in earlier versions.

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

References

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/const

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/let

• 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, whereas var is function-scoped.

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

  In the above code, you can see that referencing a and b will produce a ReferenceError, while c contains the number. This is because a and b are block scoped, while c is scoped to the containing function.

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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 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. I don't believe there is a way to enforce this rule so this is more of a best practise.

  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. eslint: no-prototype-builtins

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

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

  // 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 iterable 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 for converting an array-like object to an array.

  const arrLike = { 0: 'foo', 1: 'bar', 2: 'baz', length: 3 };
  
  // bad
  const arr = Array.prototype.slice.call(arrLike);
  
  // good
  const arr = Array.from(arrLike);

• 4.6 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);

• 4.7 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);
  });
  
  // good
  [
    [0, 1],
    [2, 3],
    [4, 5],
  ].reduce((acc, item, index) => {
    const flatten = acc.concat(item);
    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;
  });

• 4.8 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, and from repetitive access of the object. Repeating object access creates more repetitive code, requires more reading, and creates more opportunities for mistakes. Destructuring objects also provides a single site of definition of the object structure that is used in the block, rather than requiring reading the entire block to determine what is used.

  // 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 }) => `${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);
  
  // best
  function processInput(input) => { left, right, top, bottom };

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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}?`;
  }
  
  // best
  function sayHi(name) => `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\' is "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. Function declarations are fine when you want to create a function on the global scope but it most cases this is a bad option for us.

  // bad
  function foo() {
    // ...
  }
  
  // good
  const foo = function () {
    // ...
  };
  
  // 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 but we would never do this
  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('');
  }
  
  // best
  const concatenateAll = (...args) => 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. eslint: default-param-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() {};
  const z = function b() => {};

• 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;
  }
  
  // best
  const f3 = (obj) => {
    const key = 'key' in obj;
    // ...
  };

  Why to use in vs hasOwnProperty. In will return true for inherited properties, whereas hasOwnProperty() will return false for inherited properties. http://adripofjavascript.com/blog/drips/the-uses-of-in-vs-hasownproperty.html

• 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 syntax ... 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 + 1}.`);
  
  // 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 Always include parentheses around arguments for clarity and consistency. eslint: arrow-parens

  Why? Minimizes diff churn when adding or removing arguments.

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

⬆ 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 RaphaStyleGuide = require('./RaphaStyleGuide');
  module.exports = RaphaStyleGuide.es6;
  
  // ok
  import RaphaStyleGuide from './RaphaStyleGuide';
  export default RaphaStyleGuide.es6;
  
  // best
  import { es6 } from './RaphaStyleGuide';
  export default es6;

• 10.2 Do not use wildcard imports.

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

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

• 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 './RaphaStyleGuide';
  
  // good
  // filename es6.js
  import { es6 } from './RaphaStyleGuide';
  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. eslint: object-curly-newline

  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';

• 10.10 Do not include JavaScript filename extensions eslint: import/extensions

  Why? Including extensions inhibits refactoring, and inappropriately hardcodes implementation details of the module you're importing in every consumer.

  // bad
  import foo from './foo.js';
  import bar from './bar.jsx';
  import baz from './baz/index.jsx';
  
  // good
  import foo from './foo';
  import bar from './bar';
  import baz from './baz';

⬆ 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 over square-bracket notation. eslint: dot-notation

  We of course should prefer-destructuring over dot notation but this is purely an example of what NOT to do

  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. eslint: no-undef prefer-const

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

• 13.2 Use one const or let declaration per variable or assignment. 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 previously 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) {
    // ...
  }

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

• 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: +, -, and ** since their precedence is broadly understood. We recommend enclosing / and * in parentheses because their precedence can be ambiguous when they are mixed. 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;
  }
  
  // bad
  const bar = a + (b / c) * 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 multiline 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 multiline 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 multiline 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;
    }
  }

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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');
  }

• 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();
  }

• 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 './RaphaStyleGuide';
  // ...
  export default es6;

  // bad
  import { es6 } from './RaphaStyleGuide';
    // ...
  export default es6;↵
  ↵

  // good
  import { es6 } from './RaphaStyleGuide';
    // ...
  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);
  const svg = leds.enter().append('svg:svg');
  svg.classed('led', true).attr('width', (radius + margin) * 2);
  const g = svg.append('svg:g');
  g.attr('transform', `translate(${radius + margin},${radius + margin})`).call(
    tron.led
  );

• 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 use multiple blank lines to pad your code. eslint: no-multiple-empty-lines

  // bad
  class Person {
    constructor(fullName, email, birthday) {
      this.fullName = fullName;
  
      this.email = email;
  
      this.setAge(birthday);
    }
  
    setAge(birthday) {
      const today = new Date();
  
      const age = this.getAge(today, birthday);
  
      this.age = age;
    }
  
    getAge(today, birthday) {
      // ..
    }
  }
  
  // good
  class Person {
    constructor(fullName, email, birthday) {
      this.fullName = fullName;
      this.email = email;
      this.setAge(birthday);
    }
  
    setAge(birthday) {
      const today = new Date();
      const age = getAge(today, birthday);
      this.age = age;
    }
  
    getAge(today, birthday) {
      // ..
    }
  }

• 19.10 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);
  }

• 19.11 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]);

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

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

• 19.13 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://rapha.cc/', 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://rapha.cc/',
    data: { name: 'John' },
  })
    .done(() => console.log('Congratulations!'))
    .fail(() => console.log('You have failed this city.'));

• 19.14 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;
  }

• 19.15 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];

• 19.16 Enforce spacing inside of computed property brackets. 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]];

• 19.17 Avoid spaces between functions and their invocations. eslint: func-call-spacing

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

• 19.18 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 trailing spaces at the end of lines. eslint: no-trailing-spaces

• 19.20 Avoid multiple empty lines, only allow one newline at the end of files, and avoid a newline at the beginning of files. eslint: no-multiple-empty-lines

  // bad - multiple empty lines
  var x = 1;
  
  var y = 2;
  
  // bad - 2+ newlines at end of file
  var x = 1;
  var y = 2;
  
  // bad - 1+ newline(s) at beginning of file
  
  var x = 1;
  var y = 2;
  
  // good
  var x = 1;
  var y = 2;

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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',
  };

• 20.2 Additional trailing comma: Yup. eslint: comma-dangle

  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 chart', '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'];
  
  // bad
  function createHero(firstName, lastName, inventorOf) {
    // does nothing
  }
  
  // good
  function createHero(firstName, lastName, inventorOf) {
    // does nothing
  }
  
  // good (note that a comma must not appear after a "rest" element)
  function createHero(firstName, lastName, inventorOf, ...heroArgs) {
    // does nothing
  }

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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 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

  Why? The parseInt function produces an integer value dictated by interpretation of the contents of the string argument according to the specified radix. Leading whitespace in string is ignored. If radix is undefined or 0, it is assumed to be 10 except when the number begins with the character pairs 0x or 0X, in which case a radix of 16 is assumed. This differs from ECMAScript 3, which merely discouraged (but allowed) octal interpretation. Many implementations have not adopted this behavior as of 2013. And, because older browsers must be supported, always specify a radix.

  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 RaphaStyleGuide = {
    es6: {},
  };
  
  export default RaphaStyleGuide;

• 23.9 Acronyms and initialisms should always be all uppercased, 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 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',
  };
  
  export const FOO = 'bar';

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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
  });

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ECMAScript 5 Compatibility

• 27.1 Refer to Kangax’s ES5 compatibility table.

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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

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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

• 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 tests more brittle.
• We primarily jest at Rapha. 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.

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Performance

• On Layout & Web Performance
• String vs Array Concat
• Try/Catch Cost In a Loop
• Bang Function
• jQuery Find vs Context, Selector
• innerHTML vs textContent for script text
• Long String Concatenation
• Are JavaScript functions like map(), reduce(), and filter() optimized for traversing arrays?

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Resources

Learning ES6+

• Latest ECMA spec
• ExploringJS
• ES6 Compatibility Table
• Comprehensive Overview of ES6 Features

Tools

• Code Style Linters
  • ESlint
  • Airbnb Style .eslintrc
  • @neutrinojs/airbnb

Other Style Guides

• Principles of Writing Consistent, Idiomatic JavaScript
• StandardJS
• Google JavaScript Style Guide
• jQuery Core Style Guidelines

Other Styles

• Naming this in nested functions - Christian Johansen
• Conditional Callbacks - Ross Allen
• Popular JavaScript Coding Conventions on GitHub - JeongHoon Byun
• Multiple var statements in JavaScript, not superfluous - Ben Alman

Further Reading

• Understanding JavaScript Closures - Angus Croll
• Basic JavaScript for the impatient programmer - Dr. Axel Rauschmayer
• You Might Not Need jQuery - Zack Bloom & Adam Schwartz
• ES6 Features - Luke Hoban
• Frontend Guidelines - Benjamin De Cock

Books

• JavaScript: The Good Parts - Douglas Crockford
• JavaScript Patterns - Stoyan Stefanov
• Pro JavaScript Design Patterns - Ross Harmes and Dustin Diaz
• High Performance Web Sites: Essential Knowledge for Front-End Engineers - Steve Souders
• Maintainable JavaScript - Nicholas C. Zakas
• JavaScript Web Applications - Alex MacCaw
• Pro JavaScript Techniques - John Resig
• Smashing Node.js: JavaScript Everywhere - Guillermo Rauch
• Secrets of the JavaScript Ninja - John Resig and Bear Bibeault
• Human JavaScript - Henrik Joreteg
• Superhero.js - Kim Joar Bekkelund, Mads Mobæk, & Olav Bjorkoy
• JSBooks - Julien Bouquillon
• Third Party JavaScript - Ben Vinegar and Anton Kovalyov
• Effective JavaScript: 68 Specific Ways to Harness the Power of JavaScript - David Herman
• Eloquent JavaScript - Marijn Haverbeke
• You Don’t Know JS: ES6 & Beyond - Kyle Simpson

Blogs

• JavaScript Weekly
• JavaScript, JavaScript...
• Bocoup Weblog
• Adequately Good
• NCZOnline
• Perfection Kills
• Ben Alman
• Dmitry Baranovskiy
• nettuts

Podcasts

• JavaScript Air
• JavaScript Jabber

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License

(The MIT License)

Copyright (c) 2022 Rapha Racing Ltd

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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