如何让 TypeScript 通用 function 类型参数是 object 的键值的返回类型

Question

I have a several object types:

type Slave = {
  myKey:string
}

type AnotherSlave = {
  anotherKey:string
}

And a master type that contains some keys, and these object types as those keys' values as below:

type Master = {
  key1: Slave
  key2: AnotherSlave
}

I have a generic function:

myFunc<T = keyof Master>(key:T){
  const someObj = {} // external call that returns "any" type
  return someObj; // I want to cast it to some strongly type here.
}

The someObj is guaranteed to have the type from Master[T] 's value, in other words:

If the key is key1 , someObj is the type of Slave . If the key is key2 , someObj is the type of AnotherSlave . I've wrote the types depending on what's coming from the external call of someObj anyway so it's guaranteed to have that type.

I naively tried return someObj as Master[T] but it errs Type 'T' cannot be used to index type 'Master' .

How can I make the function return strongly typed key's value type? I'm on TypeScript 4.6.3.

Answer 1

You can specify a return value and use type assertion on the returned someObj

function myFunc<T extends keyof Master>(key:T): Master[T] {
  const someObj = {}
  return someObj as Master[T];
}

const test = myFunc("key1") // <-- type is Slave

Playground Link