TypeScript failed to infer the correct type when calling the value of object (which is a function)

Question

const record = {
    foo: () => ({ foo: 1 }),
    bar: () => ({ bar: 1 }),
}

function getRecord<T extends keyof typeof record>(type: T) {
    return record[type];
}

const obj = getRecord(`foo`);
// if line 7 is:        return record[type];
// typeof obj will be:  () => { foo: number; }

// but if line 7 is:    return record[type]();
// typeof obj will be:  { foo: number; } | { bar: number; }
obj

Playground link

When the return value is not called, TypeScript can successfully infer the return type to be () => { foo: number }, but when the return value is called, the type inference broadened to { foo: number; } | { bar: number; }. Why is this happening?

I would suggest making it clearer that you're looking for the reason why it's not inferring the correct type. — Dane Brouwer
– Dane Brouwer, Commented Nov 20, 2020 at 8:10
if I understand your request, you might want to add the return type explicitly (ReturnType<typeof record[T]>), like this — michele b
– michele b, Commented Nov 20, 2020 at 8:22
Type casting can do the trick, but we want to avoid it as much as possible. Are there any issues about this behavior currently? TypeScript issue tracker is a huge mess and I even can't come up with anything to search with.. — Nikaple
– Nikaple, Commented Nov 20, 2020 at 8:42

Aleksey L. · Accepted Answer · 2020-11-20 08:43:24Z

2

The return type of:

function getRecord<T extends keyof typeof record>(type: T) {
    return record[type];
}

Is (typeof record)[T]. As you can see, the generic type parameter influences the return type of the function when function is called.

In the second example:

function getRecord<T extends keyof typeof record>(type: T) {
    return record[type]();
}

The return type is { foo: number } | { bar: number }. Here the generic parameter doesn't affect it (already used to pre-evaluate the return type).

answered Nov 20, 2020 at 8:43

Aleksey L.

38.4k11 gold badges86 silver badges90 bronze badges

Sign up to request clarification or add additional context in comments.

Collectives™ on Stack Overflow

TypeScript failed to infer the correct type when calling the value of object (which is a function)

1 Answer 1

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

Comments

Your Answer

Sign up or log in

Post as a guest

Related