1.13.g — Data Types: Primitives & Non-Primitives

In one sentence: JavaScript values are either primitives (immutable “atoms” stored by value in assignments) or objects (mutable collections stored by reference) — and typeof helps you inspect types at runtime (with a few quirks).

Navigation: ← 1.13.f — Naming & Clean Code · 1.13.h — Mini Practice Programs →

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1. Primitives (this section’s focus)

Type	Example	Notes
number	3, -0.5, NaN, Infinity	All numbers are IEEE-754 doubles — no separate int type in normal code
string	"hello", 'hi', `tick`	Immutable — methods return new strings
boolean	true, false	Result of comparisons and logic
null	null	Intentional “no value”
undefined	undefined	Variable declared but not assigned, or missing property
bigint	123n	Arbitrary-precision integers (specialized)
symbol	Symbol("id")	Unique keys (advanced)

Primitive values themselves are immutable: "ab".toUpperCase() returns a new string; it does not mutate "ab".

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2. Non-primitive: objects (reference type)

Object (including plain objects {}, arrays [], functions, dates, etc.) — stored by reference:

const a = { count: 1 };
const b = a;
b.count = 2;
console.log(a.count);  // 2 — a and b point to the same object

Two variables can alias the same object in memory.

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3. null vs undefined

	null	undefined
Typical meaning	“I explicitly set this to nothing”	“Not set yet” / “missing”
Common source	Your code or APIs documenting “empty”	Unassigned variable, missing object property

Both are falsy in boolean contexts (if (!value)).

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4. typeof operator

typeof 42;           // "number"
typeof "42";        // "string"
typeof true;        // "boolean"
typeof undefined;   // "undefined"
typeof null;        // "object"  ← long-standing spec quirk; use === null to test null
typeof {};          // "object"
typeof [];          // "object"  ← arrays are objects; use Array.isArray(x)

Use typeof for quick checks; combine with === null or Array.isArray when precision matters.

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5. Type coercion (preview)

JS will coerce types in loose operations ("5" - 1 → 4). Prefer === and !== (strict equality) to avoid surprise coercions in beginner code.

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6. Key takeaways

1. number, string, boolean, null, undefined are the core primitives you use daily.
2. Objects (including arrays) are mutable and assigned by reference.
3. typeof null is "object" — memorize the quirk for interviews and debugging.

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Explain-It Challenge

Explain without notes:

1. What does immutable mean for a string, and give a tiny example.
2. Why does changing b in const a = {}; const b = a; b.x = 1 also affect a?
3. Why is typeof [] "object", and what should you use instead to detect an array?

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Navigation: ← 1.13.f — Naming & Clean Code · 1.13.h — Mini Practice Programs →