Data Types

JavaScript is a dynamically typed language — variables don't have fixed types and can hold any value. The language has seven primitive types and one non-primitive type (Object). Understanding the distinction is fundamental to avoiding bugs and answering interview questions correctly.

The 7 Primitive Types

• string: Textual data enclosed in single quotes, double quotes, or backticks. Strings are immutable — methods like .toUpperCase() return new strings.
• number: 64-bit IEEE 754 floating-point. Handles both integers and decimals. Has special values: Infinity, -Infinity, and NaN (Not a Number). NaN !== NaN is true — use Number.isNaN() to check.
• bigint: Arbitrary-precision integers for values beyond Number.MAX_SAFE_INTEGER (2^53 - 1). Created by appending n to a literal: 123n. Cannot be mixed with regular numbers in arithmetic without explicit conversion.
• boolean: true or false. Used in conditionals and logical operations.
• undefined: Automatically assigned to variables that are declared but not initialized, and to function parameters that weren't passed.
• null: Intentional absence of a value. Unlike undefined, null is explicitly assigned by the developer.
• symbol: Unique, immutable identifiers created with Symbol(). Every Symbol() call creates a globally unique value. Used as object property keys to avoid naming collisions.

Primitive vs Non-Primitive

• Primitives are immutable — you cannot change the value itself, only reassign the variable. They are compared by value: 'hello' === 'hello' is true.
• Objects (including arrays, functions, dates, and regex) are reference types. They are mutable and compared by reference: {} === {} is false because they are two different objects in memory.
• Primitives are passed by value (a copy is made). Objects are passed by reference (the reference is copied, but both point to the same object).

The typeof Operator

• typeof 'hello' → 'string', typeof 42 → 'number', typeof true → 'boolean', typeof undefined → 'undefined', typeof Symbol() → 'symbol', typeof 42n → 'bigint'.
• typeof null → 'object' — this is a well-known bug from JavaScript's first implementation (1995) kept for backwards compatibility. Always check for null with === null.
• typeof function(){} → 'function' — functions are objects, but typeof gives them a special return value.
• typeof [] → 'object' — arrays are objects. Use Array.isArray() to check for arrays.

Key Interview Distinction: null vs undefined undefined means a variable exists but has no value assigned (the engine's default). null means a value was explicitly set to "nothing" (the developer's intent). Both are falsy. null == undefined is true (loose equality), but null === undefined is false (strict equality). Interviewers frequently test whether candidates understand this semantic difference.