# Antares UX Coding Style Guide *Inspired by the [AirBNB Style Guide](/~https://github.com/airbnb/javascript/blob/master/README.md)* A style guide helps maintain consistency and readability throughout the codebase, it also allows a much more calmer code review process and quicker onboarding for new contributors. This is a living document, as our codebase grows and evolves we should keep in mind to update this document as well. ## Table of Contents 1. [Types](#types) 2. [References](#references) 3. [Objects](#objects) 4. [Arrays](#arrays) 5. [Destructuring](#destructuring) 6. [Strings](#strings) 7. [Functions](#functions) 8. [Arrow Functions](#arrow-functions) 9. [Classes & Constructors](#classes--constructors) 10. [Modules](#modules) 11. [Iterators and Generators](#iterators-and-generators) 12. [Properties](#properties) 13. [Variables](#variables) 14. [Comparison Operators & Equality](#comparison-operators--equality) 15. [Blocks](#blocks) 16. [Control Statements](#control-statements) 17. [Comments](#comments) 18. [Whitespace](#whitespace) 19. [Commas](#commas) 10. [Semicolons](#semicolons) 21. [Naming Conventions](#naming-conventions) 22. [Accessors](#accessors) 23. [React](#react) ## Types - [1.1](#types--usage) **Usage**: - Always favor type inference over explicit type declaration except for function return types - Always define the return type of functions. This can help catch errors as functions evolve. - Types should be used whenever necessary (no implicit any). - Arrays should be defined as type[] instead of Array\. - Use the `any` type sparringly. It is always better to define an interface. - [1.2](#types--primitives) **Primitives**: When you access a primitive type you work directly on its value. - `string` - `number` - `boolean` - `null` - `undefined` - `symbol` ```typescript const foo = 1; let bar = foo; bar = 9; console.log(foo, bar); // => 1, 9 ``` - [1.3](#types--complex) **Complex**: When you access a complex type you work on a reference to its value. - `object` - `array` - `function` ```typescript const foo = [1, 2]; const bar = foo; bar[0] = 9; console.log(foo[0], bar[0]); // => 9, 9 ``` **[⬆ back to top](#table-of-contents)** ## References - [2.1](#references--prefer-const) Use `const` for all of your references; avoid using `var`. > Why? This ensures that you can’t reassign your references, which can lead to bugs and difficult to comprehend code. ```typescript // bad var a = 1; var b = 2; // good const a = 1; const b = 2; ``` - [2.2](#references--disallow-var) If you must reassign references, use `let` instead of `var`. > Why? `let` is block-scoped rather than function-scoped like `var`. ```typescript // bad var count = 1; if (true) { count += 1; } // good, use the let. let count = 1; if (true) { count += 1; } ``` - [2.3](#references--block-scope) Note that both `let` and `const` are block-scoped. ```typescript // const and let only exist in the blocks they are defined in. { let a = 1; const b = 1; } console.log(a); // ReferenceError console.log(b); // ReferenceError ``` **[⬆ back to top](#table-of-contents)** ## Objects - [3.1](#object--concise) Use property value shorthand. > Why? It is shorter to write and descriptive. ```typescript const lukeSkywalker = 'Luke Skywalker'; // bad const obj: Starwars = { lukeSkywalker: lukeSkywalker, }; // good const obj: Starwars = { lukeSkywalker, }; ``` - [3.2](#objects--grouped-shorthand) Group your shorthand properties at the beginning of your object declaration. > Why? It’s easier to tell which properties are using the shorthand. ```typescript 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, }; ``` - [3.3](#objects--rest-spread) Prefer the object spread operator over [`Object.assign`](https://developer.mozilla.org/en/docs/Web/JavaScript/Reference/Global_Objects/Object/assign) to shallow-copy objects. Use the object rest operator to get a new object with certain properties omitted. ```typescript // very bad const original: Something = { a: 1, b: 2 }; const copy: SomethingElse = Object.assign(original, { c: 3 }); // this mutates `original` ಠ_ಠ delete copy.a; // so does this // bad const original: Something = { a: 1, b: 2 }; const copy: SomethingElse = Object.assign({}, original, { c: 3 }); // copy => { a: 1, b: 2, c: 3 } // good const original: Something = { a: 1, b: 2 }; const copy: SomethingElse = { ...original, c: 3 }; // copy => { a: 1, b: 2, c: 3 } const { a, ...noA } = copy; // noA => { b: 2, c: 3 } ``` **[⬆ back to top](#table-of-contents)** ## Arrays - [4.1](#arrays--literals) Use the literal syntax for array creation. ```typescript // bad const items = new Array(); // good const items: Item[] = []; ``` - [4.2](#arrays--push) Use [Array#push](https://developer.mozilla.org/en/docs/Web/JavaScript/Reference/Global_Objects/Array/push) instead of direct assignment to add items to an array. ```typescript const someStack: Item[] = []; // bad someStack[someStack.length] = 'abracadabra'; // good someStack.push('abracadabra'); ``` - [4.3](#es6-array-spreads) Use array spreads `...` to copy arrays. ```typescript // bad const len = items.length; const itemsCopy = []; let i; for (i = 0; i < len; i += 1) { itemsCopy[i] = items[i]; } // good const itemsCopy: Item[] = [...items]; ``` - [4.4](#arrays--from-iterable) To convert an iterable object to an array, use spreads `...` instead of [`Array.from`](https://developer.mozilla.org/en/docs/Web/JavaScript/Reference/Global_Objects/Array/from). ```typescript const foo = document.querySelectorAll('.foo'); // good const nodes = Array.from(foo); // best const nodes: Node[] = [...foo]; ``` - [4.5](#arrays--callback-return) 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. * ADD CLARITY ```typescript // good [1, 2, 3].map((x: number): string => { const y = x + 1; return `number ${y}`; }); // good [1, 2, 3].map(x => x + 1); // bad - no returned value means `acc` becomes undefined after the first iteration [[0, 1], [2, 3], [4, 5]].reduce((acc, item, index) => { const flatten = acc.concat(item); acc[index] = flatten; }); // good [[0, 1], [2, 3], [4, 5]].reduce((acc, item, index) => { const flatten = acc.concat(item); acc[index] = flatten; return flatten; }); // bad inbox.filter((msg) => { const { subject, author } = msg; if (subject === 'Mockingbird') { return author === 'Harper Lee'; } else { return false; } }); // good inbox.filter((msg) => { const { subject, author } = msg; if (subject === 'Mockingbird') { return author === 'Harper Lee'; } return false; }); ``` - [4.6](#arrays--bracket-newline) Use line breaks after open and before close array brackets if an array has multiple lines ```typescript // bad const arr = [ [0, 1], [2, 3], [4, 5], ]; const objectInArray = [{ id: 1, }, { id: 2, }]; const numberInArray = [ 1, 2, ]; // good const arr: Item[] = [[0, 1], [2, 3], [4, 5]]; const objectInArray: ObjectItem[] = [ { id: 1, }, { id: 2, }, ]; const numberInArray: number[] = [ 1, 2, ]; ``` **[⬆ back to top](#table-of-contents)** ## Destructuring - [5.1](#destructuring--object) Use object destructuring when accessing and using multiple properties of an object. > Why? Destructuring saves you from creating temporary references for those properties. ```typescript // bad function getFullName(user) { const firstName = user.firstName; const lastName = user.lastName; return `${firstName} ${lastName}`; } // good function getFullName(user: User) { const { firstName, lastName } = user; return `${firstName} ${lastName}`; } ``` - [5.2](#destructuring--array) Use array destructuring. ```typescript const arr = [1, 2, 3, 4]; // bad const first = arr[0]; const second = arr[1]; // good const [first, second] = arr; ``` - [5.3](#destructuring--object-over-array) 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. ```typescript // 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) { // some code that returns following object return { left, right, top, bottom }; } // the caller selects only the data they need const { left, top } = processInput(input); ``` **[⬆ back to top](#table-of-contents)** ## Strings - [6.1](#strings--quotes) Use single quotes `''` for strings. ```typescript // 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--line-length) 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. ```typescript // 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](#string--literals) When programmatically building up strings, use template strings instead of concatenation. > Why? Template strings give you a readable, concise syntax with proper newlines and string interpolation features. ```typescript // bad function sayHi(name) { return 'How are you, ' + name + '?'; } // bad function sayHi(name) { return ['How are you, ', name, '?'].join(); } // good function sayHi(name) { return `How are you, ${name}?`; } ``` - [6.4](#strings--eval) Never use `eval()` on a string, it opens too many vulnerabilities. - [6.5](#strings--escaping) Do not unnecessarily escape characters in strings. > Why? Backslashes harm readability, thus they should only be present when necessary. ```typescript // bad const foo = '\'this\' \i\s \"quoted\"'; // good const foo = '\'this\' is "quoted"'; const foo = `my name is '${name}'`; ``` **[⬆ back to top](#table-of-contents)** ## Functions - [7.1](#functions--in-blocks) 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. Use arrow functions. - [7.2](#functions--arguments-shadow) Never name a parameter `arguments`. This will take precedence over the `arguments` object that is given to every function scope. ```typescript // bad function foo(name, options, arguments) { // ... } // good function foo(name, options, args) { // ... } ``` - [7.3](#default--parameters) Use default parameter syntax rather than mutating function arguments. ```typescript // 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 = {}) { // ... } ``` - [7.4](#functions--default-side-effects) Avoid side effects with default parameters. > Why? They are confusing to reason about. ```typescript var b = 1; // bad function count(a = b++) { console.log(a); } count(); // 1 count(); // 2 count(3); // 3 count(); // 3 ``` - [7.5](#functions--defaults-last) Always put default parameters last. ```typescript // bad function handleThings(opts = {}, name) { // ... } // good function handleThings(name, opts = {}) { // ... } ``` - [7.6](#functions--constructor) Never use the Function constructor to create a new function. > Why? Creating a function in this way evaluates a string similarly to `eval()`, which opens vulnerabilities. ```typescript // bad var add = new Function('a', 'b', 'return a + b'); // still bad var subtract = Function('a', 'b', 'return a - b'); ``` - [7.7](#functions--mutate-params) Never mutate parameters. > Why? Manipulating objects passed in as parameters can cause unwanted variable side effects in the original caller. ```typescript // bad function f1(obj) { obj.key = 1; } // good function f2(obj) { const key = Object.prototype.hasOwnProperty.call(obj, 'key') ? obj.key : 1; } ``` - [7.10](#functions--reassign-params) Never reassign parameters. > Why? Reassigning parameters can lead to unexpected behavior, especially when accessing the `arguments` object. It can also cause optimization issues, especially in V8. ```typescript // 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) { // ... } ``` - [7.11](#functions--spread-vs-apply) Prefer the use of the spread operator `...` to call variadic functions. > Why? It’s cleaner, you don’t need to supply a context, and you can not easily compose `new` with `apply`. ```typescript // 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]); ``` - [7.12](#functions--signature-invocation-indentation) 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. ```typescript // bad function foo(bar, baz, quux) { // ... } // good function foo( bar, baz, quux, ) { // ... } // bad console.log(foo, bar, baz); // good console.log( foo, bar, baz, ); ``` - [7.13](#functions--length) We should try our best to keep the number of lines in a given function to less 100. Modularize as much as possible to improve redability and reusability. **[⬆ back to top](#table-of-contents)** ## Arrow Functions - [8.1](#arrows--use-them) When you must use an anonymous function (as when passing an inline callback), use arrow function notation. > Why? It creates a version of the function that executes in the context of `this`, which is usually what you want, and is a more concise syntax. > Why not? If you have a fairly complicated function, you might move that logic out into its own named function expression. ```typescript // 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](#arrows--implicit-return) If the function body consists of a single statement returning an [expression](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Guide/Expressions_and_Operators#Expressions) without side effects, omit the braces and use the implicit return. Otherwise, keep the braces and use a `return` statement. > Why? Syntactic sugar. It reads well when multiple functions are chained together. ```typescript // bad [1, 2, 3].map(number => { return `A string containing the ${number}.`; }); // 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](#arrows--one-arg-parens) If your function takes a single argument omit the parentheses. > Why? Less visual clutter. ```typescript // bad [1, 2, 3].map((x) => x * x); // good [1, 2, 3].map(x => x * x); ``` - [8.4](#arrows--confusing) Avoid confusing arrow function syntax (`=>`) with comparison operators (`<=`, `>=`). ```typescript // 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; }; ``` - [8.5](#whitespace--implicit-arrow-linebreak) Enforce the location of arrow function bodies with implicit returns. ```typescript // bad (foo) => bar; (foo) => (bar); // good (foo) => bar; (foo) => ({ bar: 'hello' }) ``` **[⬆ back to top](#table-of-contents)** ## Classes & Constructors - [9.1](#constructors--extends) Use `extends` for inheritance. > Why? It is a built-in way to inherit prototype functionality without breaking `instanceof`. ```typescript // 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]; } } ``` - [9.2](#constructors--chaining) Methods can return `this` to help with method chaining. ```typescript // 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); ``` - [9.3](#constructors--tostring) It’s okay to write a custom `toString()` method, just make sure it works successfully and causes no side effects. ```typescript class Jedi { constructor(options = {}) { this.name = options.name || 'no name'; } getName() { return this.name; } toString() { return `Jedi - ${this.getName()}`; } } ``` - [9.4](#constructors--no-useless) 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. ```typescript // 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'; } } ``` **[⬆ back to top](#table-of-contents)** ## Modules - [10.1](#modules--use-them) 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. ```typescript // 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](#modules--no-wildcard) Try to avoid wildcard imports. > Why? This makes sure you have a single default export. ```typescript // bad import * as AirbnbStyleGuide from './AirbnbStyleGuide'; // good import AirbnbStyleGuide from './AirbnbStyleGuide'; ``` - [10.3](#modules--no-export-from-import) 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. ```typescript // bad // filename es6.js export { es6 as default } from './AirbnbStyleGuide'; // good // filename es6.js import { es6 } from './AirbnbStyleGuide'; export default es6; ``` - [10.4](#modules--no-duplicate-imports) Only import from a path in one place. > Why? Having multiple lines that import from the same path can make code harder to maintain. ```typescript // 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](#modules--no-mutable-exports) Do not export mutable bindings. > 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. ```typescript // bad let foo = 3; export { foo }; // good const foo = 3; export { foo }; ``` - [10.6](#modules--prefer-default-export) In modules with a single export, prefer default export over named export. > Why? To encourage more files that only ever export one thing, which is better for readability and maintainability. ```typescript // bad export function foo() {} // good export default function foo() {} ``` - [10.7](#modules--imports-first) Put all `import`s above non-import statements. > Why? Since `import`s are hoisted, keeping them all at the top prevents surprising behavior. ```typescript // bad import foo from 'foo'; foo.init(); import bar from 'bar'; // good import foo from 'foo'; import bar from 'bar'; foo.init(); ``` - [10.8](#modules--multiline-imports-over-newlines) 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. ```typescript // bad import {longNameA, longNameB, longNameC, longNameD, longNameE} from 'path'; // good import { longNameA, longNameB, longNameC, longNameD, longNameE, } from 'path'; ``` - [10.9](#modules--no-webpack-loader-syntax) Disallow Webpack loader syntax in module import statements. > Why? Since using Webpack syntax in the imports couples the code to a module bundler. Prefer using the loader syntax in `webpack.config.js`. ```typescript // 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](#table-of-contents)** ## Iterators and Generators - [11.1](#iterators--nope) Don’t use iterators. Prefer JavaScript’s higher-order functions instead of loops like `for-in` or `for-of`. > 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. ```typescript 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); ``` **[⬆ back to top](#table-of-contents)** ## Properties - [12.1](#properties--dot) Use dot notation when accessing properties, when possible. ```typescript const luke = { jedi: true, age: 28, }; // bad const isJedi = luke['jedi']; // good const isJedi = luke.jedi; ``` - [12.2](#properties--bracket) Use bracket notation `[]` when accessing properties with a variable. ```typescript const luke = { jedi: true, age: 28, }; function getProp(prop) { return luke[prop]; } const isJedi = getProp('jedi'); ``` **[⬆ back to top](#table-of-contents)** ## Variables - [13.1](#variables--const) 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. ```typescript // bad superPower = new SuperPower(); // good const superPower = new SuperPower(); ``` - [13.2](#variables--one-const) Use one `const` or `let` declaration per variable. > Why? It’s easier to add new variable declarations this way, and you never have to worry about swapping out a `;` for a `,` or introducing punctuation-only diffs. You can also step through each declaration with the debugger, instead of jumping through all of them at once. ```typescript // 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](#variables--define-where-used) Assign variables where you need them, but place them in a reasonable place. > Why? `let` and `const` are block scoped and not function scoped. ```typescript // 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; } ``` - [13.4](#variables--no-chain-assignment) Don’t chain variable assignments. > Why? Chaining variable assignments creates implicit global variables. ```typescript // 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` ``` - [13.5](#variables--linebreak) Avoid linebreaks before or after `=` in an assignment. > Why? Linebreaks surrounding `=` can obfuscate the value of an assignment. ```typescript // bad const foo = superLongLongLongLongLongLongLongLongFunctionName(); // bad const foo = 'superLongLongLongLongLongLongLongLongString'; // good const foo = ( superLongLongLongLongLongLongLongLongFunctionName() ); // good const foo = 'superLongLongLongLongLongLongLongLongString'; ``` - [13.6](#variables--no-unused-vars) Disallow unused variables. > 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. ```typescript // 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](#table-of-contents)** ## Comparison Operators & Equality - [14.1](#comparison--eqeqeq) Use `===` and `!==` over `==` and `!=`. - [14.2](#comparison--if) 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** ```typescript if ([0] && []) { // true // an array (even an empty one) is an object, objects will evaluate to true } ``` - [14.3](#comparison--shortcuts) Use shortcuts for booleans, but explicit comparisons for numbers. ```typescript // bad if (isValid === true) { // ... } // good if (isValid) { // ... } // bad if (collection.length) { // ... } // good if (collection.length > 0) { // ... } ``` - [14.4](#comparison--nested-ternaries) Ternaries should not be nested and generally be single line expressions. ```typescript // 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; ``` - [14.5](#comparison--unneeded-ternary) Avoid unneeded ternary statements. ```typescript // 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; ``` - [14.6](#comparison--no-mixed-operators) When mixing operators, enclose them in parentheses. The only exception is the standard arithmetic operators (`+`, `-`, `*`, & `/`) since their precedence is broadly understood. > Why? This improves readability and clarifies the developer’s intention. ```typescript // 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](#table-of-contents)** ## Blocks - [15.1](#blocks--braces) Use braces with all multi-line blocks. ```typescript // bad if (test) return false; // bad if (test) return false; // good if (test) { return false; } // bad function foo() { return false; } // good function bar() { return false; } ``` - [15.2](#blocks--cuddled-elses) If you're using multi-line blocks with `if` and `else`, put `else` on the same line as your `if` block’s closing brace. ```typescript // bad if (test) { thing1(); thing2(); } else { thing3(); } // good if (test) { thing1(); thing2(); } else { thing3(); } ``` - [15.3](#blocks--no-else-return) 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. ```typescript // 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](#table-of-contents)** ## Control Statements - [16.1](#control-statements) 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. ```typescript // bad if ((foo === 123 || bar === 'abc') && doesItLookGoodWhenItBecomesThatLong() && isThisReallyHappening()) { 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(); } ``` - [16.2](#control-statements--value-selection) Don't use selection operators in place of control statements. ```typescript // bad !isRunning && startRunning(); // good if (!isRunning) { startRunning(); } ``` **[⬆ back to top](#table-of-contents)** ## Comments - [17.1](#comments--multiline) Use `//` for multi-line comments. ```typescript // good // make() returns a new element // based on the passed in tag name // // @param {String} tag // @return {Element} element function make(tag) { // ... return element; } ``` - [17.2](#comments--singleline) 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. ```typescript // bad const active = true; // is current tab // good // is current tab const active = true; // bad function getType() { console.log('fetching type...'); // set the default type to 'no type' const type = this.type || 'no type'; return type; } // good function getType() { console.log('fetching type...'); // set the default type to 'no type' const type = this.type || 'no type'; return type; } // also good function getType() { // set the default type to 'no type' const type = this.type || 'no type'; return type; } ``` - [17.3](#comments--spaces) Start all comments with a space to make it easier to read. ```typescript // 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; } ``` - [17.4](#comments--actionitems) Prefixing your comments with `TODO ()` helps other developers quickly understand if you're pointing out a problem that needs to be revisited, or if you're suggesting a solution to the problem that needs to be implemented. These are different than regular comments because they are actionable. The actions are `FIXME: -- need to figure this out` or `TODO: -- need to implement`. - [17.5](#comments--todo) Use `// TODO ():` to annotate solutions to problems. ```typescript class Calculator extends Abacus { constructor() { super(); // TODO: total should be configurable by an options param this.total = 0; } } ``` **[⬆ back to top](#table-of-contents)** ## Whitespace - [18.1](#whitespace--spaces) Use soft tabs (space character) set to 4 spaces. ```typescript // bad function foo() { ∙∙let name; } // bad function bar() { ∙let name; } // good function baz() { ∙∙∙∙let name; } ``` - [18.2](#whitespace--before-blocks) Place 1 space before the leading brace. ```typescript // bad function test(){ console.log('test'); } // good function test() { console.log('test'); } // bad dog.set('attr',{ age: '1 year', breed: 'Bernese Mountain Dog', }); // good dog.set('attr', { age: '1 year', breed: 'Bernese Mountain Dog', }); ``` - [18.3](#whitespace--around-keywords) 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. ```typescript // bad if(isJedi) { fight (); } // good if (isJedi) { fight(); } // bad function fight () { console.log ('Swooosh!'); } // good function fight() { console.log('Swooosh!'); } ``` - [18.4](#whitespace--infix-ops) Set off operators with spaces. ```typescript // bad const x=y+5; // good const x = y + 5; ``` - [18.5](#whitespace--chains) 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. ```typescript // bad $('#items').find('.selected').highlight().end().find('.open').updateCount(); // bad $('#items'). find('.selected'). highlight(). end(). find('.open'). updateCount(); // good $('#items') .find('.selected') .highlight() .end() .find('.open') .updateCount(); // bad const leds = stage.selectAll('.led').data(data).enter().append('svg:svg').classed('led', true) .attr('width', (radius + margin) * 2).append('svg:g') .attr('transform', `translate(${radius + margin},${radius + margin})`) .call(tron.led); // good const leds = stage.selectAll('.led') .data(data) .enter().append('svg:svg') .classed('led', true) .attr('width', (radius + margin) * 2) .append('svg:g') .attr('transform', `translate(${radius + margin},${radius + margin})`) .call(tron.led); // good const leds = stage.selectAll('.led').data(data); ``` - [18.6](#whitespace--after-blocks) Leave a blank line after blocks and before the next statement. ```typescript // bad if (foo) { return bar; } return baz; // good if (foo) { return bar; } return baz; // bad const obj = { foo() { }, bar() { }, }; return obj; // good const obj = { foo() { }, bar() { }, }; return obj; // bad const arr = [ function foo() { }, function bar() { }, ]; return arr; // good const arr = [ function foo() { }, function bar() { }, ]; return arr; ``` - [18.7](#whitespace--in-parens) Do not add spaces inside parentheses. ```typescript // bad function bar( foo ) { return foo; } // good function bar(foo) { return foo; } // bad if ( foo ) { console.log(foo); } // good if (foo) { console.log(foo); } ``` - [18.8](#whitespace--in-brackets) Do not add spaces inside brackets. ```typescript // bad const foo = [ 1, 2, 3 ]; console.log(foo[ 0 ]); // good const foo = [1, 2, 3]; console.log(foo[0]); ``` - [18.9](#whitespace--in-braces) Add spaces inside curly braces. ```typescript // bad const foo = {clark: 'kent'}; // good const foo = { clark: 'kent' }; ``` - [18.10](#whitespace--max-len) Avoid having lines of code that are longer than 100 characters (including whitespace). Note: per [above](#strings--line-length), long strings are exempt from this rule, and should not be broken up. > Why? This ensures readability and maintainability. ```typescript // 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.')); ``` - [18.11](#whitespace--block-spacing) 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. ```typescript // bad function foo() {return true;} if (foo) { bar = 0;} // good function foo() { return true; } if (foo) { bar = 0; } ``` - [18.12](#whitespace--comma-spacing) Avoid spaces before commas and require a space after commas. ```typescript // bad var foo = 1,bar = 2; var arr = [1 , 2]; // good var foo = 1, bar = 2; var arr = [1, 2]; ``` - [18.13](#whitespace--func-call-spacing) Enforce spacing between functions and their invocations. ```typescript // bad func (); func (); // good func(); ``` - [18.14](#whitespace--key-spacing) Enforce spacing between keys and values in object literal properties. ```typescript // bad var obj = { "foo" : 42 }; var obj2 = { "foo":42 }; // good var obj = { "foo": 42 }; ``` - [18.15](#whitespace--no-trailing-spaces) Avoid trailing spaces at the end of lines. - [18.16](#whitespace--no-multiple-empty-lines) Avoid multiple empty lines and only allow one newline at the end of files. ```typescript // bad var x = 1; var y = 2; // good var x = 1; var y = 2; ``` **[⬆ back to top](#table-of-contents)** ## Commas - [19.1](#commas--leading-trailing) Leading commas: **Nope.** ```typescript // bad const story = [ once , upon , aTime ]; // good const story = [ once, upon, aTime, ]; // bad const hero = { firstName: 'Ada' , lastName: 'Lovelace' , birthYear: 1815 , superPower: 'computers' }; // good const hero = { firstName: 'Ada', lastName: 'Lovelace', birthYear: 1815, superPower: 'computers', }; ``` - [19.2](#commas--dangling) Additional trailing comma: **Yup.** > Why? This leads to cleaner git diffs. Also, transpilers will remove the additional trailing comma in the transpiled code which means you don’t have to worry about the [trailing comma problem](/~https://github.com/airbnb/javascript/blob/es5-deprecated/es5/README.md#commas) in legacy browsers. ```diff // 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'], }; ``` ```typescript // 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 } // bad createHero( firstName, lastName, inventorOf ); // good createHero( firstName, lastName, inventorOf, ); // good (note that a comma must not appear after a "rest" element) createHero( firstName, lastName, inventorOf, ...heroArgs ); ``` **[⬆ back to top](#table-of-contents)** ## Semicolons - [20.1](#semicolons--required) Required: **Yup.** > Why? When JavaScript encounters a line break without a semicolon, it uses a set of rules called [Automatic Semicolon Insertion](https://tc39.github.io/ecma262/#sec-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. ```typescript // 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](https://stackoverflow.com/questions/7365172/semicolon-before-self-invoking-function/7365214#7365214). **[⬆ back to top](#table-of-contents)** ## Naming Conventions - [21.1](#naming--descriptive) Avoid single letter names. Be descriptive with your naming. ```typescript // bad function q() { // ... } // good function query() { // ... } ``` - [21.2](#naming--camelCase) Use camelCase when naming objects, functions, and instances. ```typescript // bad const OBJEcttsssss = {}; const this_is_my_object = {}; function c() {} // good const thisIsMyObject = {}; function thisIsMyFunction() {} ``` - [21.3](#naming--PascalCase) Use PascalCase only when naming constructors or classes. ```typescript // 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', }); ``` - [21.4](#naming--leading-underscore) Use leading underscore to represent private accessors. > Why? JavaScript does not have the concept of privacy in terms of properties or methods. 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 would lead developers to realize the property or the method is private. ```typescript // good this._firstName = 'Panda'; ``` - [21.5](#naming--self-this) Don’t save references to `this`. Use arrow functions or [Function#bind](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Function/bind). ```typescript // 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); }; } ``` - [21.6](#naming--filename-matches-export) A base filename should exactly match the name of its default export. ```typescript // 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 ``` - [21.7](#naming--camelCase-default-export) Use camelCase when you export-default a function. Your filename should be identical to your function’s name. ```typescript function makeStyleGuide() { // ... } export default makeStyleGuide; ``` - [21.8](#naming--PascalCase-singleton) Use PascalCase when you export a constructor / class / singleton / function library / bare object. ```typescript const AzureFunctionsStyleGuide = { es6: { }, }; export default AzureFunctionsStyleGuide; ``` - [21.9](#naming--uppercase) 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. ```typescript // 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](#table-of-contents)** ## Accessors - [22.1](#accessors--not-required) Accessor functions for properties are not required. - [22.2](#accessors--no-getters-setters) 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')`. ```typescript // bad class Dragon { get age() { // ... } set age(value) { // ... } } // good class Dragon { getAge() { // ... } setAge(value) { // ... } } ``` - [22.3](#accessors--boolean-prefix) If the property/method is a `boolean`, use `isVal()` or `hasVal()`. ```typescript // bad if (!dragon.age()) { return false; } // good if (!dragon.hasAge()) { return false; } ``` - [22.4](#accessors--consistent) It’s okay to create `get()` and `set()` functions, but be consistent. ```typescript class Jedi { constructor(options = {}) { const lightsaber = options.lightsaber || 'blue'; this.set('lightsaber', lightsaber); } set(key, val) { this[key] = val; } get(key) { return this[key]; } } ``` **[⬆ back to top](#table-of-contents)** ## React (TSX/JSX) ### Naming - **Extensions**: Use `.tsx` extension for React components. - **Filename**: Use PascalCase for filenames. E.g., `ReservationCard.jsx`. - **Reference Naming**: Use PascalCase for React components and camelCase for their instances. ```jsx // bad import reservationCard from './ReservationCard'; // good import ReservationCard from './ReservationCard'; // bad const ReservationItem = ; // good const reservationItem = ; ``` - **Component Naming**: Use the filename as the component name. For example, `ReservationCard.jsx` should have a reference name of `ReservationCard`. ```jsx // bad import Footer from './Footer/FooterComp'; // good import Footer from './Footer'; // good import Footer from './Footer/Footer'; ``` - **Higher-order Component Naming**: Use a composite of the higher-order component’s name and the passed-in component’s name as the `displayName` on the generated component. For example, the higher-order component `withFoo()`, when passed a component `Bar` should produce a component with a `displayName` of `withFoo(Bar)`. > Why? A component’s `displayName` may be used by developer tools or in error messages, and having a value that clearly expresses this relationship helps people understand what is happening. ```jsx // bad export default function withFoo(WrappedComponent) { return function WithFoo(props) { return ; } } // good export default function withFoo(WrappedComponent) { function WithFoo(props) { return ; } const wrappedComponentName = WrappedComponent.displayName || WrappedComponent.name || 'Component'; WithFoo.displayName = `withFoo(${wrappedComponentName})`; return WithFoo; } ``` - **Props Naming**: Avoid using DOM component prop names for different purposes. > Why? People expect props like `style` and `className` to mean one specific thing. Varying this API for a subset of your app makes the code less readable and less maintainable, and may cause bugs. ```jsx // bad // bad // good ``` **[⬆ back to top](#table-of-contents)** ### Props - Always use camelCase for prop names. ```jsx // bad // good ``` - Omit the value of the prop when it is explicitly `true`. eslint: [`react/jsx-boolean-value`](/~https://github.com/yannickcr/eslint-plugin-react/blob/master/docs/rules/jsx-boolean-value.md) ```jsx // bad