JavaScript Complete Cheatsheet & Notes

Syntax Basics

let, const, var — variable declarations
// comment or /* multi-line */
Case-sensitive & semicolon optional
console.log("Hello") — print output
Block scope: { ... }

Data Types

Primitive: string, number, boolean
null, undefined, symbol, bigint
Reference: object, array, function
typeof — returns variable type

Operators

+, -, *, /, %
=== (strict equality) vs ==
>, <, >=, <=
! (not), && (and), || (or)
+=, -= — assignment shortcuts
++, -- — increment/decrement
? ternary: condition ? a : b

Functions

function greet() { return "Hi" }
const sum = (a, b) => a + b;
arguments object for traditional functions
Default params: (x = 5)
Anonymous / Arrow / Callback functions

Arrays & Objects

const arr = [1, 2, 3];
arr.push(4); arr.pop();
arr.map(x => x * 2)
arr.filter(x => x > 2)
const obj = { name: "John", age: 25 };
obj.name or obj["age"]
Destructuring: const {name} = obj;

Control Flow

if (x > 5) { ... } else { ... }
switch(value) { case 1: ... break; }
for (let i=0; i<5; i++)
while(condition), do...while
break / continue

DOM Manipulation

document.getElementById("id")
document.querySelector(".class")
element.textContent = "Hello"
element.classList.add("active")
element.addEventListener("click", fn)

ES6+ Features

let, const — block scoping
Template Literals — `Hello ${name}`
Destructuring — const {a,b} = obj;
Spread — arr2 = [...arr]
Rest Params — function(...args)
Modules — export / import
Classes — class Person { constructor() {} }

Async JavaScript

setTimeout(fn, 1000)
setInterval(fn, 500)
Promise — new Promise((res, rej) => ...)
async / await
fetch("url").then(res => res.json())
try...catch for error handling

Browser APIs

localStorage.setItem("key", "value")
localStorage.getItem("key")
navigator.geolocation.getCurrentPosition()
fetch() API
history.pushState()