JavaScript Cheat Sheet: Basics

Variables

Declaring Variables

• var (function-scoped, avoid in modern code)
• let (block-scoped, recommended)
• const (block-scoped, constant)

Example:

let name = "Alice";
const age = 25;
var city = "Berlin";

Variable Naming Rules

• Case-sensitive
• Cannot start with a number
• Use letters, $, _

Data Types

Primitive Types

• String
• Number
• Boolean
• Undefined
• Null
• Symbol
• BigInt

Reference Types

• Objects
• Arrays
• Functions

Example:

let name = "Bob";            // String
let score = 95;              // Number
let isActive = true;         // Boolean
let data = null;             // Null
let car;                     // Undefined
let user = { name: "Sam" };  // Object
let list = [1, 2, 3];        // Array

Operators

Arithmetic

+, -, *, /, %, **

Assignment

=, +=, -=, *=, /=

Comparison

==, ===, !=, !==, <, >, <=, >=

Logical

&&, ||, !

Increment / Decrement

++, --

Control Structures

If Statements

if (score > 90) {
  console.log("Great!");
} else if (score > 75) {
  console.log("Good!");
} else {
  console.log("Keep trying!");
}

Switch Statements

let color = "green";
switch (color) {
  case "red":
    console.log("Stop");
    break;
  case "green":
    console.log("Go");
    break;
  default:
    console.log("Unknown color");
}

Loops

For Loop:

for (let i = 0; i < 5; i++) {
  console.log(i);
}

While Loop:

let i = 0;
while (i < 5) {
  console.log(i);
  i++;
}

Functions

Declaring Functions

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

Arrow Functions

const greet = (name) => `Hello, ${name}`;

Function Expressions

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

Objects

Creating Objects

const person = {
  name: "Jane",
  age: 30,
  greet: function() {
    console.log("Hello!");
  }
};

Accessing Properties

console.log(person.name);
person.greet();

Arrays

Creating Arrays

const fruits = ["apple", "banana", "orange"];

Accessing Elements

console.log(fruits[0]); // "apple"

Array Methods

• push()
• pop()
• shift()
• unshift()
• splice()
• slice()
• forEach()
• map()
• filter()