JS Foundations #9 — Data Types & Type Checking

JavaScript has 7 primitive types and 1 object type. The typeof operator, instanceof, and Object.prototype.toString each give you different information — and each has quirks. This article covers the type system and every type-checking tool, including the traps interviewers specifically test.

1. The 8 Types

Primitives (7):  string  number  bigint  boolean  undefined  symbol  null
Object    (1):   everything else — arrays, functions, dates, maps, sets, classes...

Primitives are immutable and compared by value. Objects are compared by reference.

2. typeof — The Primary Tool

The Two Classic typeof Traps

Trap 1: typeof null === 'object' — This is a bug from JavaScript's first implementation that can't be fixed without breaking the web. Always check value === null separately.

Trap 2: typeof [] === 'object' — Arrays are objects, so typeof can't distinguish them. Use Array.isArray().

3. instanceof — Prototype Chain Check

Checks if an object's prototype chain contains a specific constructor's .prototype.

The Cross-Realm Problem

instanceof fails when objects come from a different JavaScript realm (iframe, VM context). Each realm has its own Array, Date, etc.

This is why libraries prefer Object.prototype.toString.call() for type checking.

4. Object.prototype.toString.call(value) — Most Reliable

Returns [object InternalType] where InternalType is the engine's internal class.

This works across realms — it always reflects the engine's internal class.

5. Array.isArray(value) — Array-Specific Check

This is the only reliable way to detect arrays. It works across realms (unlike instanceof Array).

6. Checking for Specific Primitive Types

7. NaN, Infinity, and -0

NaN Detection

Infinity

-0

8. Object.is(a, b) — Most Precise Equality

Object.is is SameValue equality — the strictest comparison. Use it when you need exact equivalence.

Quick Reference

Interview Tips

• Always check null before typeof — typeof null === 'object' will trip you.
• Use Array.isArray — never typeof for arrays.
• Use Number.isNaN — not the global isNaN.
• Use Object.prototype.toString.call for generic type tags — it's the nuclear option that always works.

Related Articles

• Implement deepEqual()
• JS Foundations #5 — Equality & Type Coercion