Conditional types in TypeScript ternary logic at the type level

Conditional type syntax

type IsString = T extends string ? true : false;

type A = IsString; // true
type B = IsString; // false

Distributive conditional types

When T is a union, conditional types distribute over each member:

type Flatten = T extends Array ? Item : T;

type A = Flatten;      // string
type B = Flatten;        // number (no change)
type C = Flatten<(string|number)[]>; // string | number

infer keyword

infer captures a type inside a conditional check:

type UnpackPromise = T extends Promise ? R : T;

type A = UnpackPromise>; // string
type B = UnpackPromise;          // number

This is how the built-in Awaited and ReturnType are implemented. The infer variable only exists within the conditional branch where it was matched.