Complete JavaScript Cheat sheet

JavaScript is one of the most popular and powerful programming languages. It is mainly used to make websites interactive, dynamic, and user-friendly. With JavaScript, we can control webpage behavior—such as responding to button clicks, validating forms, updating content without reloading the page, and building full web applications. JavaScript is not limited to browsers only; it is also used for backend development (Node.js), mobile apps, desktop apps, game development & even software automation testing. This cheat sheet is designed for absolute beginners, covering all essential JavaScript concepts with simple explanations and easy-to-understand syntax.

Basic Syntax

Printing Output:

console.log("Hello JS!");

Comments:

// Single-line comment

/*
 Multi-line 
 comment
*/

Variables (let, const, var)

Modern JavaScript → Use let and const

let age = 20;      // changeable
const name = "Manan"; // NOT changeable
var oldWay = true;    // older JS, avoid using

Data Types

Primitive Types:

let name = "Manan Academy";     // string
let age = 17;                  // number
let isHappy = true;           // boolean
let x = undefined;           // undefined
let y = null;               // null
let bigNum = 123n;         // BigInt
let symbol = Symbol();    // symbol

Operators

Arithmetic:

+, -, *, /, %, **    // (power)

Example:

5 + 2;  // 7
5 % 2;  // 1

Comparison:

==   // equal (loose)
===  // equal (strict)
!=   // not equal
> < >= <=

Logical:

&&  // AND
||  // OR
!   // NOT

Strings

Template Literals:

let name = "Manan";
console.log(`Hello ${name}`);

Common String Methods:

text.length
text.toUpperCase()
text.toLowerCase()
text.includes("hi")
text.replace("a", "b")
text.slice(0, 4)

Arrays

let fruits = ["apple", "banana", "mango"];
fruits[0];     // "apple"
fruits.length; // 3

Array Methods:

fruits.push("orange");   // add
fruits.pop();            // remove last
fruits.shift();          // remove first
fruits.unshift("kiwi");  // add first
fruits.indexOf("banana");

Objects

Syntax Example:

let user = {
  name: "Manan",
  age: 17,
  isAdmin: false
};

console.log(user.name);
console.log(user["age"]);

Add new key:

user.country = "Bangladesh";

Conditionals (if / else)

let age = 17;

if (age >= 18) {
  console.log("Adult");
} else {
  console.log("Minor");
}

else if:

if (score > 80) {
  console.log("A");
} else if (score > 60) {
  console.log("B");
}

Switch Case

Example:

let day = 3;

switch (day) {
  case 1: console.log("Sun"); break;
  case 2: console.log("Mon"); break;
  case 3: console.log("Tue"); break;
  default: console.log("Unknown");
}

Loops

For Loop

Example:

for (let i = 1; i <= 5; i++) {
  console.log(i);
}

While Loop

Example:

let i = 1;
while (i <= 5) {
  console.log(i);
  i++;
}

For-of (best for arrays)

Example:

for (let fruit of fruits) {
  console.log(fruit);
}

For-in (objects)

Example:

for (let key in user) {
  console.log(key, user[key]);
}

Functions

Function Declaration Example:

function greet() {
  console.log("Hello!");
}

Function with parameter:

function add(a, b) {
  return a + b;
}

Arrow Function Syntax (modern JS):

const sayHi = () => console.log("Hi!");

DOM Manipulation

Select elements:

document.getElementById("title");
document.querySelector(".class");
document.querySelectorAll("p");

Change content:

document.getElementById("title").textContent = "New Title";

Change style:

document.getElementById("box").style.color = "red";

Add event:

button.addEventListener("click", () => {
  alert("Button clicked!");
});

JSON

Example:

let jsonData = '{"name": "Manan", "age": 17}';

let obj = JSON.parse(jsonData);   // JSON → JS object
let str = JSON.stringify(obj);    // object → JSON

Async JavaScript

setTimeout Example:

setTimeout(() => {
  console.log("Waited 2 seconds");
}, 2000);

Fetch API (GET request) Example:

fetch("https://api.example.com")
  .then(res => res.json())
  .then(data => console.log(data))
  .catch(err => console.log(err));

Classes (OOP Basics)

Example:

class Person {
  constructor(name, age) {
    this.name = name;
    this.age = age;
  }

  greet() {
    console.log(`Hi, I am ${this.name}`);
  }
}

let p = new Person("Manan", 17);
p.greet();

Useful Built-in Methods

// Numbers
Number.parseInt("20");
Number.parseFloat("20.5");

//Math
Math.random()
Math.round()
Math.floor()
Math.ceil()
Math.max()
Math.min()