JavaScript Cheat Sheet 2026: Learn Once, Remember Forever (ES2026)

console.log('Hello, JS'); // run in browser DevTools console, or: node file.js 

const PI = 3.14159; // default choice let count = 0; // when you need to reassign // avoid var const user = { name: 'A' }; user.name = 'B'; // allowed: const protects the binding, not the contents // user = {}; // TypeError 

console.log(a); // undefined (var is hoisted) var a = 5; console.log(b); // ReferenceError (TDZ) let b = 5; sayHi(); // works: function declarations are fully hoisted function sayHi() { console.log('hi'); } hello(); // TypeError: hello is not a function (var hoisted, value not yet assigned) var hello = () => {}; 

Array.isArray([]); // true value === null; // the right null check value == null; // true for both null and undefined (the one accepted use of ==) 

let a = 1; let b = a; b = 2; // a is still 1 (copied) const o1 = { x: 1 }; const o2 = o1; // same object o2.x = 99; // o1.x is now 99 

0.1 + 0.2 === 0.3; // false (0.30000000000000004) Math.abs(0.1 + 0.2 - 0.3) < Number.EPSILON; // true: compare floats like this Number.MAX_SAFE_INTEGER; // 9007199254740991, beyond this use BigInt NaN === NaN; // false Number.isNaN(NaN); // true (use this, not isNaN()) 9007199254740993n + 1n; // BigInt: arbitrary precision, no mixing with Number 

1 == '1'; // true 1 === '1'; // false null == undefined; // true null === undefined; // false Object.is(NaN, NaN);// true (like === but NaN equals NaN, and +0 differs from -0) 

1 + '2'; // '12' (+ prefers strings) '3' - 1; // 2 (- forces numbers) '5' * '2'; // 10 [] + []; // '' [] + {}; // '[object Object]' true + 1; // 2 Number(''); // 0 Number('12px'); // NaN parseInt('12px', 10); // 12 (always pass the radix) 

const port = config.port ?? 3000; // keeps 0 if port is 0 const port2 = config.port || 3000; // replaces 0 with 3000 (bug) const city = user?.address?.city; // undefined instead of TypeError 

const name = 'World'; `Hello, ${name}! ${2 + 3}`; // template literal, multi-line allowed 

(1234.5678).toFixed(2); // '1234.57' (returns a STRING) Number.isInteger(5); // true Math.round(2.5); // 3 Math.floor(-1.5); // -2 Math.trunc(-1.5); // -1 Math.max(...nums); // Math.max() with no args is -Infinity Math.floor(Math.random() * (max - min + 1)) + min; // random int in [min, max] 

if (x > 0) { } else if (x === 0) { } else { } switch (day) { case 'sat': case 'sun': console.log('weekend'); break; // forget break and it falls through default: console.log('weekday'); } 

for (const [i, value] of arr.entries()) { } for (const [key, value] of Object.entries(obj)) { } for (const [key, value] of map) { } outer: for (const a of A) { // label to break out of nested loops for (const b of B) { if (a === b) break outer; } } 

function add(a, b) { return a + b; } // declaration (hoisted) const sub = function (a, b) { return a - b; }; // expression const mul = (a, b) => a * b; // arrow, implicit return const makeUser = name => ({ name }); // wrap returned object literal in () 

function greet(name = 'friend', ...rest) { // default value, rest parameter return `Hi ${name} (+${rest.length} extra)`; } const [first, second] = [1, 2]; // destructuring function area({ w, h = 1 }) { return w * h; } // destructure a param object 

const user = { name: 'Asha', hi() { return this.name; }, later() { setTimeout(() => console.log(this.name), 100); }, // arrow keeps this }; const f = user.hi; f(); // this is lost f.call(user); // 'Asha' const bound = user.hi.bind(user); bound(); // 'Asha' 

fn.call(obj, a, b); // call now, args listed fn.apply(obj, [a, b]); // call now, args as array fn.bind(obj, a); // returns a new function, call later 

(() => { /* IIFE: runs immediately, rarely needed with modules */ })(); const compose = (...fns) => x => fns.reduceRight((v, f) => f(v), x); // composition const curry = a => b => a + b; // currying function fact(n) { return n <= 1 ? 1 : n * fact(n - 1); } // recursion needs a base case 

function counter() { let n = 0; // private, nobody else can touch it return () => ++n; } const next = counter(); next(); // 1 next(); // 2 

for (var i = 0; i < 3; i++) setTimeout(() => console.log(i)); // 3 3 3 (one shared var) for (let i = 0; i < 3; i++) setTimeout(() => console.log(i)); // 0 1 2 (new let per iteration) 

function debounce(fn, ms = 300) { // run after the user STOPS for ms let t; return (...args) => { clearTimeout(t); t = setTimeout(() => fn(...args), ms); }; } function throttle(fn, ms = 300) { // run at most once per ms let last = 0; return (...args) => { const now = Date.now(); if (now - last >= ms) { last = now; fn(...args); } }; } 

const name = 'Asha'; const user = { name, // shorthand for name: name age: 30, ['is' + 'Admin']: false, // computed key greet() { return `Hi ${this.name}`; }, get label() { return `${this.name} (${this.age})`; }, }; user.name; user['name']; // two ways to read delete user.age; 'name' in user; // true (includes inherited) Object.hasOwn(user, 'name'); // true (own property, modern replacement for hasOwnProperty) 

const { name: userName, age = 18, ...others } = user; // rename, default, rest const updated = { ...user, age: 31 }; // copy + override const merged = { ...defaults, ...overrides }; // later wins 

const prices = { a: 10, b: 20 }; Object.fromEntries(Object.entries(prices).map(([k, v]) => [k, v * 2])); // { a: 20, b: 40 } Object.groupBy([1, 2, 3, 4], n => (n % 2 ? 'odd' : 'even')); // { odd: [1,3], even: [2,4] } 

const nums Source：https://dev.to/devcodehub99/javascript-cheat-sheet-2026-learn-once-remember-forever-es2026-cmb
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