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JavaScript

npx code-complexity . --limit 20 --sort ratio - https://www.kevinpeters.net/the-fastest-way-to-understand-new-code-bases

Copilot instructions - https://github.com/github/awesome-copilot/blob/main/instructions/nodejs-javascript-vitest.instructions.md

In JavaScript typeof null is "object".

Deep JS - https://exploringjs.com/deep-js/

https://stateofjs.com/en-US

Libraries​

Validation​

See comparison here: https://github.com/colinhacks/zod#comparison

JSON (eg JSON request and responses):

JSDoc​

https://jsdoc.app

https://devhints.io/jsdoc

TC39​

https://github.com/tc39

https://tc39.es

ECMAScript Language Specification: https://tc39.es/ecma262/

ECMAScript Proposals: https://github.com/tc39/proposals

https://twitter.com/tc39

== vs === and != vs !==​

info

There is also Object.is() that "does no type conversion and no special handling for NaN, -0, and +0 (giving it the same behavior as === except on those special numeric values)" source

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

https://stackoverflow.com/a/55822553/4034572

null == undefined // true
null === undefined // false

null != undefined // false
null !== undefined // true

console​

Spec: https://console.spec.whatwg.org/

https://developer.mozilla.org/en-US/blog/learn-javascript-console-methods/

Use %i (integer), %f (float), %s (string) for type conversions: https://twitter.com/addyosmani/status/1535706659128868864

CSS style​

console.log(
  '%cSome Text',
  'color: red; font-size: 30px; background-color: yellow;'
)

console.group​

https://developer.mozilla.org/en-US/docs/Web/API/Console/group

console.group('label')
console.log(...)
console.log(...)
console.groupEnd()

console.time​

console.time('label')
// ...
console.timeLog('label')
// ...
console.timeEnd('label')

This is for React Native.

/**
 * Use it to print objects to the console preserving undefined values.
 * Usage: `console.log('User', prettyPrint(user))`.
 * From https://stackoverflow.com/a/50100175/4034572.
 */
export function prettyPrint(object: any): string {
  return JSON.stringify(
    object,
    (k, v) => (v === undefined ? '__undefined' : v),
    2
  ).replace(/"__undefined"/g, 'undefined')
}

Object​

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object

delete operator​

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/delete

Object.entries()​

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/entries

const user = {
  name: 'Joe',
  age: 42,
}
for (const [key, value] of Object.entries(user)) {
  console.log(`${key}: ${value}`)
}

Object.assign()​

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/assign

Can be used to copy an object changing a property without modifying it.

Eg to toggle 'complete' of an object in a Redux reducer without modifying it:

const newTodo = Object.assign({}, todo, {complete: !todo.complete})

This creates a new object , merges all the properties of the 'todo' object into this new object, and then overrides the 'complete' property. The original 'todo' object is not modified, so the reducer is pure.

structuredClone​

https://developer.mozilla.org/en-US/docs/Web/API/structuredClone

Note that it also works with arrays.

See https://www.linkedin.com/feed/update/urn:li:activity:7086378041525825536/

Conditionally add field​

https://stackoverflow.com/questions/11704267/in-javascript-how-to-conditionally-add-a-member-to-an-object

const user = { name: 'Pere' }
if (condition) {
  user.age = 42
}
const user = {
  name: 'Pere',
  ...(condition && { age: 42 }),
}

Classes​

https://webreflection.medium.com/js-classes-are-not-just-syntactic-sugar-28690fedf078

https://medium.com/@dan_abramov/how-to-use-classes-and-sleep-at-night-9af8de78ccb4

Don’t make super calls from methods

Instead of instanceof checks, assert the existence of the methods you plan to use, and trust the user to do the right thing.

Error​

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Error

Error('message') and new Error('message') is the same.

When Error is used like a function -- without new, it will return an Error object. Therefore, a mere call to Error will produce the same output that constructing an Error object via the new keyword would. source

Rethrow an error but with a different message: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Error/Error#rethrowing_an_error_with_a_cause

Handle errors in a simple, stable, consistent way: https://github.com/ehmicky/modern-errors

tip

Error.isError() is a more robust alternative to instanceof Error because it avoids false positives and false negatives.

AbortController​

https://developer.mozilla.org/en-US/docs/Web/API/AbortController

https://www.bennadel.com/blog/4200-using-fetch-abortsignal-and-settimeout-to-apply-retry-mechanics-in-javascript.htm

https://stackoverflow.com/a/65805464/4034572

String​

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/String

danger

substr() is deprecated. Use slice() instead.

startsWith()

endsWith()

Regex​

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

Regex syntax cheatsheet: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Guide/Regular_Expressions/Cheatsheet

https://devdocs.io/javascript-regexp/

Name capture groups: https://twitter.com/addyosmani/status/1386031624232456194

In TypeScript use the type RegExp.

RegExp.prototype.test()​

MDN

function isValidEmail(email: string): boolean {
  const regex = /^[^\s@]+@[^\s@]+\.[^\s@]+$/ // From https://stackoverflow.com/a/9204568/4034572
  return regex.test(email)
}

RegExp.prototype.exec()​

MDN

const regex = /abcd/gi
const string = 'something'
regex.exec(string)
// returns an array with the matches or null

String.prototype.match()​

MDN

const regex = /abcd/gi
const string = 'something'
string.match(regex)
// returns an array with the matches or null

Dates​

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Intl

JS Dates Are About to Be Fixed - https://docs.timetime.in/blog/js-dates-finally-fixed/

Date format​

https://stackoverflow.com/questions/3552461/how-do-i-format-a-date-in-javascript

Date.prototype.toLocaleDateString() at MDN

toLocaleString options: https://www.jsman.net/manual/Standard-Global-Objects/Date/toLocaleString

new Date(search.date).toLocaleDateString() // 5/27/2021
new Date(search.date).toLocaleDateString('es-ES') // 27/5/2021

Intl.DateTimeFormat​

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Intl/DateTimeFormat

Important: does not work on React Native Android. You get the error "Can't find variable: Intl". See solutions at https://stackoverflow.com/questions/56943813/using-intl-properly-in-android-react-native-app. Update November 2022: Hermes does support Intl, see https://hermesengine.dev/docs/intl. Hermes is the default since 0.70 - see https://reactnative.dev/blog/2022/09/05/version-070#hermes-as-default-engine and https://reactnative.dev/blog/2022/07/08/hermes-as-the-default. On Expo, since SDK 47 - see https://blog.expo.dev/expo-sdk-47-a0f6f5c038af.

Example from https://stackoverflow.com/a/3552493/4034572:

export function formatDate(dateNumber: number): string {
  const dateObject = new Date(dateNumber)
  const year = new Intl.DateTimeFormat('en', { year: 'numeric' }).format(
    dateObject
  )
  const month = new Intl.DateTimeFormat('en', { month: 'short' }).format(
    dateObject
  )
  const day = new Intl.DateTimeFormat('en', { day: '2-digit' }).format(
    dateObject
  )
  return `${day} ${month} ${year}`
}

formatDate(1583495987) returns 19 Jan 1970.

Intl.DurationFormat​

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Intl/DurationFormat

New API. Added in preview in Firefox 136 (released March 2025).