⚡ Lesson 03 of 30

Variables & Data Types

Explore var, let, const, and JavaScript's seven primitive data types with hands-on examples.

Declaring Variables

JavaScript has three keywords for declaring variables. Prefer const by default; use let when the value will change; avoid var in modern code.

const pi = 3.14159;       // cannot be reassigned
let score = 0;            // can be reassigned
score = 42;
// var oldStyle = "avoid"; // function-scoped, hoisted

Primitive Types

JavaScript has seven primitive types:

let name   = "Alice";         // string
let age    = 30;              // number
let price  = 9.99;            // number (no int/float split)
let active = true;            // boolean
let empty  = null;            // null
let undef;                    // undefined
let id     = Symbol("id");    // symbol

BigInt

For integers larger than 2⁵³−1, use the BigInt type by appending n:

const big = 9007199254740993n;
console.log(typeof big); // "bigint"

typeof Operator

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

console.log(typeof "hello");   // "string"
console.log(typeof 42);        // "number"
console.log(typeof true);      // "boolean"
console.log(typeof undefined); // "undefined"
console.log(typeof null);      // "object" (historical quirk)

Type Coercion

JavaScript automatically converts types in some situations — this is called implicit coercion. Understanding it prevents subtle bugs:

console.log("5" + 3);   // "53"  (number coerced to string)
console.log("5" - 3);   // 2    (string coerced to number)
console.log(Boolean(0)); // false
console.log(Boolean(""));// false

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