TypeScript 7 reference · Type Operators and Keywords

TypeScript conditional types (T extends U ? X : Y) Operator

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

TypeScript 7 Since 2.8

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Syntax

T extends U ? X : Y

A type-level ifTry it yourself

type IsString<T> = T extends string ? "yes" : "no";
const a: IsString<"hi"> = "yes";
const b: IsString<42> = "no";
console.log(a, b);

Output

yes no

Unwrap arrays with inferTry it yourself

type ElementOf<T> = T extends (infer E)[] ? E : T;
const x: ElementOf<number[]> = 5;
const y: ElementOf<string> = "not an array";
console.log(x, y);

Output

5 not an array

Version	Change
TypeScript 2.8	Added

infer · Inside a conditional type, infer captures part of a type into a new type variable, e.g. the element type of an array.
Exclude<UnionType, ExcludedMembers> · Remove members from a union type. Exclude<'a' | 'b' | 'c', 'a'> is 'b' | 'c'.
mapped types ({ [K in Keys]: … }) · Create a new object type by transforming every property of another, the way Partial and Readonly are built.
TypeScript keyof, typeof and Indexed Access · tutorial