Using TypeScript to automatically determine the argument type of a function by analyzing the return type of a different function

Question

Using TypeScript to automatically determine the argument type of a function by analyzing the return type of a different function

I am working on an interface with the following structure:

interface Res<R = any> {
    first?(): Promise<R>;
    second(arg: { response: R }): void;
}

However, I noticed that when creating a plain object based on this interface, the response type is not inferred correctly. Here's an example:

const entity: Res = {
    first: () => Promise.resolve({ name: 'Bob' }),
    second: (arg) => {
        console.log(arg.response) // currently inferred as "any", but should be "{ name: string }"
    }
}

Is it feasible to obtain the correct type for arg.response based on the return value of the first() method?

TS Playground

javascript typescript types

Answer 1

Answer №1

To achieve the desired behavior, you cannot rely solely on a variable; instead, you can leverage the inference behavior of functions:

interface Res<R = any> {
    first?(): Promise<R>;
    second(arg: { response: R }): void;
}

function createRes<T>(o: Res<T>) {
    return o
}
const entity = createRes({
    first: () => Promise.resolve({ name: 'Bob' }),
    second: (arg) => {
        console.log(arg.response)
    }
})

Playground Link

Answer 2

To achieve the desired behavior, you cannot rely solely on a variable; instead, you can leverage the inference behavior of functions:

interface Res<R = any> {
    first?(): Promise<R>;
    second(arg: { response: R }): void;
}

function createRes<T>(o: Res<T>) {
    return o
}
const entity = createRes({
    first: () => Promise.resolve({ name: 'Bob' }),
    second: (arg) => {
        console.log(arg.response)
    }
})

Playground Link

Using TypeScript to automatically determine the argument type of a function by analyzing the return type of a different function

Answer №1

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