TypeScript Type Operators and Keywords – Complete List with Examples

These are the building blocks the utility types are made from. Most are type-level only; satisfies and as const affect how a value's type is inferred, and enum is one of the few TypeScript features that generates runtime code.

Name	Description	Since
`keyof`	Produce a union of an object type's property names: keyof { a: 1; b: 2 } is 'a' \| 'b'.	2.1
`typeof (type query)`	In a type position, typeof takes the type of a value or function, so you don't have to write it twice.
`Type[Key]`	Look up the type of a property with Type['key'], or of array elements with Type[number].	2.1
`conditional types (T extends U ? X : Y)`	Choose between two types based on a condition, like a ternary for types. The basis of many utility types.	2.8
`infer`	Inside a conditional type, infer captures part of a type into a new type variable, e.g. the element type of an array.	2.8
`mapped types ({ [K in Keys]: … })`	Create a new object type by transforming every property of another, the way Partial and Readonly are built.	2.1
`template literal types`	Build string literal types with template syntax: `on${string}` or combinations of unions like `${Size}-${Color}`.	4.1
`satisfies`	Check that a value matches a type without widening it, so you keep the exact inferred type and still get errors.	4.9
`as const`	Make a value deeply read-only with literal types: strings stay 'admin' instead of string, arrays become readonly tuples.	3.4
`unknown`	The type-safe 'anything': a value of type unknown must be narrowed (checked) before you can use it, unlike any.	3.0
`never`	The type with no values: for functions that never return, impossible branches, and exhaustiveness checks.	2.0
`type predicates (x is T)`	A function returning 'value is Type' tells TypeScript that the value has that type wherever the function returns true.	1.6
`assertion functions (asserts x is T)`	A function declared 'asserts value is Type' throws if the check fails, and narrows the type after the call.	3.7
`enum`	Define a set of named constants. Prefer union types or 'as const' objects in new code; use enums when you need a runtime object.	1.0

Name

Description

Since

keyof

Produce a union of an object type's property names: keyof { a: 1; b: 2 } is 'a' | 'b'.

2.1

typeof (type query)

In a type position, typeof takes the type of a value or function, so you don't have to write it twice.

Type[Key]

Look up the type of a property with Type['key'], or of array elements with Type[number].

2.1

conditional types (T extends U ? X : Y)

Choose between two types based on a condition, like a ternary for types. The basis of many utility types.

2.8

infer

Inside a conditional type, infer captures part of a type into a new type variable, e.g. the element type of an array.

2.8

mapped types ({ [K in Keys]: … })

Create a new object type by transforming every property of another, the way Partial and Readonly are built.

2.1

template literal types

Build string literal types with template syntax: `on${string}` or combinations of unions like `${Size}-${Color}`.

4.1

satisfies

Check that a value matches a type without widening it, so you keep the exact inferred type and still get errors.

4.9

as const

Make a value deeply read-only with literal types: strings stay 'admin' instead of string, arrays become readonly tuples.

3.4

unknown

The type-safe 'anything': a value of type unknown must be narrowed (checked) before you can use it, unlike any.

3.0

never

The type with no values: for functions that never return, impossible branches, and exhaustiveness checks.

2.0

type predicates (x is T)

A function returning 'value is Type' tells TypeScript that the value has that type wherever the function returns true.

1.6

assertion functions (asserts x is T)

A function declared 'asserts value is Type' throws if the check fails, and narrows the type after the call.

3.7

enum

Define a set of named constants. Prefer union types or 'as const' objects in new code; use enums when you need a runtime object.

1.0