JavaScript Cheat Sheet

letBlock-scoped mutable variable

let count = 0;
count += 1;

constBlock-scoped constant

const PI = 3.14;
const name = "Mishu";

Primitive typesstring, number, boolean, null, undefined, symbol

const str = "hello";
const num = 42;
const bool = true;

TypeofReturns the type of a value

typeof "hello" // "string"
typeof 42      // "number"

Function declarationStandard function

function greet(name) {
  return `Hello, ${name}!`;
}

Arrow functionConcise function syntax

const add = (a, b) => a + b;

Default paramsParameters with defaults

function multiply(a, b = 1) {
  return a * b;
}

Rest paramsVariable number of arguments

function sum(...nums) {
  return nums.reduce((a, b) => a + b, 0);
}

Array literalCreate an array

const arr = [1, 2, 3];

Array methodsCommon operations

arr.map(x => x * 2);
arr.filter(x => x > 1);
arr.reduce((a, b) => a + b, 0);
arr.find(x => x === 2);

Object literalCreate an object

const obj = {
  key: "value",
  method() { return this.key; }
};

DestructuringUnpack values

const { name, age } = person;
const [first, second] = arr;

Spread operatorSpread iterable into elements

const copy = [...arr];
const merged = { ...obj1, ...obj2 };

If-elseConditional execution

if (x > 0) {
  console.log("positive");
} else {
  console.log("not positive");
}

Loopfor, while, do-while

for (let i = 0; i < 5; i++) { ... }
while (condition) { ... }

SwitchMulti-way branch

switch (day) {
  case 1: return "Mon";
  case 2: return "Tue";
  default: return "Unknown";
}

TernaryInline conditional

const result = x > 0 ? "yes" : "no";