Glossary / Primitive values

Primitive values

JavaScript has seven primitive types: undefined, null, Boolean, Number, BigInt, String, and Symbol. Primitives are immutable values compared primarily by value, while objects are mutable reference values compared by identity. A variable does not have a permanent type; it refers to a value, and the same binding can refer to values of different types over time.

Type-by-type model

Undefined commonly indicates an uninitialized binding, missing property, or omitted argument.
Null conventionally represents intentional absence. typeof null returning "object" is a historic compatibility quirk.
Boolean has two values, true and false, but conditions also coerce other values by truthiness.
Number is IEEE 754 binary64, including finite values, NaN, and infinities. Integers are exactly safe only up to 2^53 - 1 in magnitude.
BigInt represents arbitrary-precision integers, but cannot implicitly mix with Number and is not JSON-serializable by default.
String is an immutable sequence of UTF-16 code units; code units are not always user-perceived characters.
Symbol is a unique primitive often used as a property key and for language protocols.

Values, bindings, and boxing

let value = 4;
value = "four"; // The binding changes which value it refers to.

const text = "hi";
text.toUpperCase(); // "HI"; the primitive itself was not mutated.

Property access on some primitives behaves as if a temporary wrapper were used, but wrapper objects such as new String("hi") are distinct mutable objects and should generally be avoided. const protects a binding from reassignment; it does not make an object or its properties immutable.

Comparisons and conversions

Primitive values generally compare by value, but Number has special cases: NaN !== NaN, and 0 === -0. Object.is and SameValueZero comparisons differ at these edges. Explicit conversion (Number, String, Boolean, BigInt) is clearer than relying on coercion, but conversion is not validation: Number("") is zero and parsing can accept formats you did not intend.

Practical guidance

Validate external values at boundaries with a schema or explicit checks. Model absence consistently (null versus omitted property). Choose Number, BigInt, or decimal-safe representation based on the domain. Avoid wrapper objects and avoid assumptions that strings or numeric values have properties they do not actually possess.

Interview questions