JavaScript Cheat Sheet

This cheat sheet provides a quick reference for the most common syntax and features in the JavaScript programming language.

Syntax and Variables

Variables can be declared with let (block-scoped, mutable), const (block-scoped, constant reference), and var (function-scoped).

// This is a comment
 
let name = "Alice"; // Mutable variable
const age = 30;      // Constant variable
var score = 100;     // Legacy variable declaration

Data Types

Primitives

let myString = "Hello";
let myNumber = 42.5;
let myBoolean = true;
let myNull = null;
let myUndefined = undefined;
let mySymbol = Symbol("id");

Objects

const person = {
    name: "Bob",
    age: 25
};

Data Structures

Arrays: Ordered, mutable collections.

const fruits = ["apple", "banana"];
fruits.push("cherry");
console.log(fruits[0]); // "apple"

Objects: Unordered, mutable key-value pairs.

const car = { make: "Toyota", model: "Camry" };
console.log(car.make); // "Toyota"

Operators

Arithmetic & Comparison

// Arithmetic: + - * / % **
// Comparison: == != === !== > < >= <=
// Use strict equality (===) to avoid type coercion.
"5" == 5;  // true
"5" === 5; // false

Logical & Ternary

(5 > 2) && (2 > 5); // false (AND)
(5 > 2) || (2 > 5); // true (OR)
 
const result = (age > 18) ? "adult" : "minor";

Control Flow

Conditional Statements

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

Loops

// For loop
for (let i = 0; i < 5; i++) {
    console.log(i);
}
 
// For...of loop (for iterables like arrays)
for (const fruit of fruits) {
    console.log(fruit);
}
 
// While loop
let i = 0;
while (i < 5) {
    console.log(i);
    i++;
}

Functions

Function Declaration:

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

Arrow Function (ES6+):

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

Array Methods

Common methods for working with arrays.

const numbers = [1, 2, 3, 4, 5];
 
// .map(): Create a new array by transforming each element
const squares = numbers.map(n => n * n);
 
// .filter(): Create a new array with elements that pass a test
const evens = numbers.filter(n => n % 2 === 0);
 
// .reduce(): Reduce the array to a single value
const sum = numbers.reduce((acc, n) => acc + n, 0);
 
// .forEach(): Execute a function for each element
numbers.forEach(n => console.log(n));

Asynchronous JavaScript

Handle operations like API calls without blocking the main thread.

Promises:

fetch("https://api.example.com/data")
    .then(response => response.json())
    .then(data => console.log(data))
    .catch(error => console.error("Error:", error));

Async/Await (ES2017+):

async function fetchData() {
    try {
        const response = await fetch("https://api.example.com/data");
        const data = await response.json();
        console.log(data);
    } catch (error) {
        console.error("Error:", error);
    }
}