Indexed access with keyof

If you have type User = { id: number; email: string }, then keyof User is the union of its key names, "id" | "email". Indexed access goes the other way: User["email"] is string.

Combine the two in a generic function like function getProp<T, K extends keyof T>(obj: T, key: K): T[K], and a key that doesn't exist becomes a compile error, while the result gets exactly the type of the field you asked for.

The common mistake is to write key: keyof T and make the result T[keyof T]. That's the union of the types of all the values, so the email's type becomes string | number. The second type parameter, K, is what remembers which key you passed.