Data types, typeof and equality — JavaScript (ES5 → ES2025) & TypeScript Basics

1. Data types, typeof and equality

Basic★ Must knowLesson 1 of 16

Primitive types (immutable, compared by value): string, number, bigint, boolean, undefined, symbol, null. Everything else is an object (objects, arrays, functions, dates, maps), compared by reference.

typeof "hi"        // "string"
typeof 42          // "number"
typeof 10n         // "bigint"
typeof undefined   // "undefined"
typeof null        // "object"  ← historical bug
typeof []          // "object"  → use Array.isArray([])
typeof function(){} // "function"

0.1 + 0.2 === 0.3            // false (floating point) → compare with Number.EPSILON or use integers (paise)
NaN === NaN                  // false → Number.isNaN(x)

// == converts types (coercion); === compares type and value
"5" == 5          // true
"5" === 5         // false
null == undefined // true
null === undefined // false
[] == false       // true (weird coercion) → always use ===

Falsy values: false, 0, -0, 0n, "", null, undefined, NaN. Everything else is truthy (including "0", [], {}).