📘 JavaScript Methods Cheatsheet

Most common used methods in Javascript and simple code snippets.

1. Array Methods

map() – transform each item, return new array

[1, 2, 3].map((n) => n * 2); // [2, 4, 6]

filter() – pick only items that match condition

[1, 2, 3, 4].filter((n) => n % 2 === 0); // [2, 4]

reduce() – combine all items into one value

[1, 2, 3].reduce((sum, n) => sum + n, 0); // 6

forEach() – run a function on each item (no return)

['a', 'b'].forEach((v) => console.log(v)); // logs "a" then "b"

find() – return the first match

[10, 20, 30].find((n) => n > 15); // 20

slice() – copy part of an array (no mutation)

['a', 'b', 'c'].slice(1, 3); // ["b","c"]

splice() – remove/replace items (mutates array)

const arr = ['a', 'b', 'c'];
arr.splice(1, 1); // removes "b" → arr = ["a","c"]

concat() – merge arrays

[1, 2].concat([3, 4]); // [1,2,3,4]

includes() – check if value exists

[1, 2, 3].includes(2); // true

some() – does any item match?

[1, 3, 5].some((n) => n % 2 === 0); // false

every() – do all items match?

[2, 4, 6].every((n) => n % 2 === 0); // true

2. String Methods

split() – break into array

'one,two'.split(','); // ["one", "two"]

replace() – swap first match

'hello world'.replace('world', 'JS'); // "hello JS"

replaceAll() – swap all matches

'aba'.replaceAll('a', 'x'); // "xbx"

substring() – extract part by indexes

'JavaScript'.substring(0, 4); // "Java"

trim() – remove spaces both ends

'  hi  '.trim(); // "hi"

trimStart() / trimEnd() – remove spaces one side

'  hi  '.trimStart(); // "hi  "
'  hi  '.trimEnd(); // "  hi"

charAt() – get character at position

'JS'.charAt(1); // "S"

includes() – check substring

'hello'.includes('he'); // true

3. Object Methods

Object.keys() – get property names

Object.keys({ a: 1, b: 2 }); // ["a","b"]

Object.values() – get property values

Object.values({ a: 1, b: 2 }); // [1,2]

Object.entries() – get key-value pairs

Object.entries({ a: 1, b: 2 }); // [["a",1],["b",2]]

Object.assign() – shallow copy/merge

Object.assign({}, { a: 1 }, { b: 2 }); // {a:1, b:2}

Object.hasOwnProperty() – check if property exists directly

({ x: 1 }).hasOwnProperty('x'); // true

Object.freeze() – make object immutable

const obj = Object.freeze({ a: 1 });
// obj.a = 2 → no effect

Object.create() – create new object with prototype

const proto = {
  greet() {
    return 'hi';
  },
};
const o = Object.create(proto);
o.greet(); // "hi"

4. Number Methods

parseInt() – convert string to integer

parseInt('42', 10); // 42

parseFloat() – convert string to decimal

parseFloat('3.14'); // 3.14

toFixed() – format decimals (string result)

(12.345).toFixed(2); // "12.35"

toString() – convert number to string/base

(255).toString(16); // "ff"

5. Date Methods

getDate() – day of month

new Date('2025-09-13').getDate(); // 13

getDay() – weekday (0=Sun, 6=Sat)

new Date('2025-09-13').getDay(); // 6 (Saturday)

getFullYear() – year

new Date().getFullYear(); // 2025

toISOString() – standard ISO format

new Date('2025-09-13').toISOString(); // "2025-09-13T00:00:00.000Z"

6. Function Methods

call() – run with custom this

function hi(msg) {
  return `${msg}, ${this.name}`;
}
hi.call({ name: 'Ana' }, 'Hello'); // "Hello, Ana"

apply() – like call but args array

hi.apply({ name: 'Bo' }, ['Hey']); // "Hey, Bo"

bind() – create new function with fixed this

const bound = hi.bind({ name: 'Cam' });
bound('Hi'); // "Hi, Cam"

7. Async & Fetch

Promise.resolve() / Promise.reject() – instantly create promise

Promise.resolve(123).then(console.log); // 123
Promise.reject('err').catch(console.log); // "err"

Promise.all() – wait all succeed (fail fast)

Promise.all([Promise.resolve(1), Promise.resolve(2)]).then(console.log); // [1,2]

Promise.allSettled() – wait all, even failures

Promise.allSettled([Promise.resolve('ok'), Promise.reject('fail')]).then(
  console.log
);
// [{status:"fulfilled", value:"ok"}, {status:"rejected", reason:"fail"}]

Promise.race() – first settled wins

Promise.race([
  new Promise((r) => setTimeout(() => r('fast'), 100)),
  new Promise((r) => setTimeout(() => r('slow'), 500)),
]).then(console.log); // "fast"

async/await – cleaner async code

async function getData() {
  return await Promise.resolve('done');
}
getData().then(console.log); // "done"

fetch() – GET request

fetch('https://jsonplaceholder.typicode.com/todos/1')
  .then((res) => res.json())
  .then(console.log);

fetch() – POST request with JSON

fetch('/api', {
  method: 'POST',
  headers: { 'Content-Type': 'application/json' },
  body: JSON.stringify({ msg: 'Hello' }),
})
  .then((res) => res.json())
  .then(console.log);