Types

JavaScript has seven primitive types and one composite. Values carry their type; variables do not. typeof x returns a string tag. The operator’s edge-case file is built around the coercion rules between these types.

For literals, see Syntax. For operators, see Operators.

The catalog

Type	What it holds
undefined	The default value of an uninitialised binding.
null	The intentional absence of a value.
boolean	true or false.
number	Double-precision IEEE 754 float.
bigint	Arbitrary-precision integer (literal suffix n).
string	UTF-16 code units; immutable.
symbol	Unique, primitive identifier.
object	Everything else; plain objects, arrays, functions, dates, errors, maps, sets, classes, promises.

Primitives

undefined

The default of an uninitialised binding.

undefined

null

The intentional absence of a value.

null

boolean

Truthiness rules and the falsy list.

boolean

number

IEEE 754 doubles; NaN, Infinity, Number.MAX_SAFE_INTEGER.

number

bigint

Arbitrary-precision integer; literal suffix n.

bigint

string

Immutable UTF-16 sequences; template literals, tagged templates.

string

symbol

Unique primitive identifiers; well-known protocols.

symbol

Composite

object

Everything that is not a primitive: plain objects, arrays, functions, dates, errors.

object

Conversion

Casting

Conversion between the primitive types and the == / === coercion contrast.

Casting

References

• Syntax for literal forms.

• Operators for == vs === and coercion semantics.

• Data Structures for Array, Map, Set, TypedArray.

• OOP for classes and prototypes.

• MDN, Data structures.