Conditional Types and the infer Keyword

You need to extract a type from inside another type (e.g., the resolved type of a Promise, the return type of a function) without hardcoding it.

Use 'infer' inside a conditional type to capture and name a type variable.

// Extract Promise value type
type Awaited<T> = T extends Promise<infer V> ? V : T;
type R = Awaited<Promise<string>>; // string

// Extract function parameters
type Params<T> = T extends (...args: infer P) => any ? P : never;
type P = Params<(a: string, b: number) => void>; // [string, number]

// Extract array element type
type Element<T> = T extends (infer E)[] ? E : never;
type E = Element<User[]>; // User

// Recursive unwrapping
type DeepAwaited<T> = T extends Promise<infer V>
  ? DeepAwaited<V>
  : T;

'infer' introduces a type variable scoped to the true branch of a conditional type. TypeScript pattern-matches against the extends clause and binds the inferred type to the variable.