JavaScript

Constructs

for … of loops through the items of an iterable.

for … in loops through the keys of an object.

Using for … in is discouraged.

- for (const key in obj) {
+ for (const key of Object.keys(obj)) {

Array

Functional

• filter
• reduce(callback, init) — callback: (acc, current, idx, array)
• map
• forEach
• some
• every

Search

['foo', 'bar', 'baz'].includes('bar')
// true

[{ id: 2 }, { id: 3 }, { id: 9 }].find((item) => item.id === 3);
// { id: 3 }

Object

Object.assign

Copy the values of all of the enumerable own properties from one or more source objects to a target object. Returns the target object.

Object.assign(defaults, passed); // Mutates `defaults`
const options = Object.assign({}, defaults, passed); // Doesn't mutate `defaults`

Date

toLocalDateString(locales?, options?)

options

• weekday
  • long
  • short
  • narrow
• era
  • long
  • short
  • narrow
• year
  • numeric
  • 2-digit
• month
  • numeric
  • 2-digit
  • long
  • short
  • narrow
• day
  • numeric
  • two-digit
• hour, minute, second
  • numeric
  • two-digit
• timeZoneName
  • long
  • short

const options = { weekday: "short", day: "numeric", month: "short" };
const today = new Date();
console.log(today.toLocaleDateString(undefined, options)); // Tue, Jun 2