TypeScript：`'keyof R'` 不可分配给类型'string'，因为它可以是'string | 编号 | 象征'

Question

I have a function that takes a key of an object, is there a way to narrow the type of the key to only string

function foo<R, K extends keyof R>(obj: R, key: K, array: string[]) {
  array.push(key); // Error🚨: Type 'string | number | symbol' is not assignable to type 'string'.
    
  // omit implementation details...
  return obj[key];
}

So TS compiler says that 'keyof R' is not assignable to type 'string' because it can be either 'string | number | symbol'. I wonder if there is a way to narrow its type to only string so we can exclude number and symbol from it.

I tried this but it seems wrong syntactically

function foo<R, K extends Extract(keyof R, string)>(obj: R, key: K, array: string[]) {
  array.push(key);

  // omit implementation details...
  return obj[key];
}

Answer 1

Your generic syntax is wrong, Extract(A, B) needs to be Extract<A, B> , so function foo<R, K extends Extract<keyof R, string>>(obj: R, key: K, array: string[]) .

That said there is really no reason to have your function generic on two things when you care about one object:

function foo<R>(obj: R, key: Extract<keyof R, string>, array: string[]): void {
  array.push(key);
}

Would work just as well and is much simpler.

You need the second type if you want to return the object's value at the key, and you end up with:

function foo<R, T extends Extract<keyof R, string>>(obj: R, key: T, array: string[]): R[T] {
  array.push(key);
  return obj[key];
}
foo({x: 3, y: 5}, 'x', array);