TypeScript Generic Constraints: Building Type-Safe APIs

Tue, 05 Mar 2024 00:00:00 +0900

TypeScript generics let you write reusable, type-safe code, but without constraints they are too permissive. The extends keyword restricts type parameters to shapes that have specific properties, giving you both flexibility and safety.

The extends Constraint

The foundation of generic constraints is the extends keyword. It ensures a type parameter satisfies a required structure:

function getProperty<T, K extends keyof T>(obj: T, key: K): T[K] {
 return obj[key];
}

Here K extends keyof T guarantees key is a valid property of obj. Attempting to pass an invalid key triggers a compile-time error. Common patterns include constraining to interfaces (T extends { id: string }), union members, or primitive types.

Generics on Commentary of Takao

TypeScript Generic Constraints: Building Type-Safe APIs

The extends Constraint