Variables and Data Types

Variables store values that your program can read and modify. JavaScript has three ways to declare variables: var, let, and const. Modern JavaScript uses let and const almost exclusively.

Declaring Variables

let name = "Alice";       // can be reassigned
const age = 30;           // cannot be reassigned
var score = 100;          // old-style, avoid in modern code

Use const by default. Switch to let only when you need to reassign the variable. Avoid var — it has quirky scoping behaviour that can cause bugs.

Primitive Data Types

JavaScript has seven primitive types:

// Number — integers and decimals share one type
let count = 42;
let price = 9.99;

// String — text in single, double, or backtick quotes
let greeting = "Hello";
let message = 'World';
let template = `Count is ${count}`;  // template literal

// Boolean
let isLoggedIn = true;
let hasPaid = false;

// undefined — variable declared but not assigned
let result;
console.log(result); // undefined

// null — intentional absence of value
let user = null;

// Symbol — unique identifiers (advanced use)
const id = Symbol("id");

// BigInt — integers larger than Number.MAX_SAFE_INTEGER
const big = 9007199254740993n;

The typeof Operator

Use typeof to inspect a value's type at runtime:

console.log(typeof 42);          // "number"
console.log(typeof "hello");     // "string"
console.log(typeof true);        // "boolean"
console.log(typeof undefined);   // "undefined"
console.log(typeof null);        // "object"  (historical quirk!)
console.log(typeof {});          // "object"
console.log(typeof []);          // "object"
console.log(typeof function(){}); // "function"

Note: typeof null returns "object" — this is a well-known bug in JavaScript that was never fixed for compatibility reasons.

Type Coercion

JavaScript automatically converts types in certain contexts. This can be surprising:

console.log("5" + 3);   // "53"  (number converted to string)
console.log("5" - 3);   // 2     (string converted to number)
console.log(true + 1);  // 2     (true becomes 1)
console.log(false + 1); // 1     (false becomes 0)

To convert explicitly:

Number("42");      // 42
String(100);       // "100"
Boolean(0);        // false
Boolean("hello");  // true
parseInt("3.7");   // 3
parseFloat("3.7"); // 3.7

Naming Variables

JavaScript uses camelCase by convention:

let firstName = "Alice";
let totalPrice = 49.99;
const MAX_RETRIES = 3;   // constants often use UPPER_SNAKE_CASE

Variable names must start with a letter, underscore, or dollar sign. They are case-sensitive — myVar and myvar are different.