Introduction to infer

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Infer

👨‍💼 The infer keyword is one of TypeScript's most powerful features. It lets you extract types from within other types.

Think of infer as declaring a type variable that TypeScript fills in by pattern matching:

// "If T has a payload property, call that payload P"
type PayloadType<T> = T extends { payload: infer P } ? P : never

type A = PayloadType<{ payload: 'ping' }> // 'ping'
type B = PayloadType<{ payload: 42 }> // 42
type C = PayloadType<string> // never - no payload to infer

The pattern is always:

T extends SomePattern<infer X> ? X : Fallback

Where infer X captures the type at that position.

🐨 Open

index.ts

and:

Create ArrayElement<T> to extract element types from arrays
Create PromiseResult<T> to extract resolved types from Promises
Create FunctionReturn<T> to extract return types from functions

📜 TypeScript Handbook - Inferring Within Conditional Types

06. Conditional Types/02. Introduction to infer (💪 problem)

Introduction to infer