Typescript is failing to return nested types when attempting to return a nested object

My goal is for my function to return a nested type of Content, but it's not working even though the type that should be returned is known. Let's take a look at an example:

type Content = {
    some: {
        extra: string;
        prop: number;
    },
    other: string,
    somenumber: number
}

const factory = (content: Content) => {
    return (key: keyof Content) => {
        return content[key];
    };
};

const content:Content = {
    some: {
        extra: "wow",
        prop: 123
    },
    other: "abc",
    somenumber: 999
};

const el = factory(content);
const some = el("some"); // this works fine, I can pass an existing key as a string
console.log(some.extra); // however, I'm unable to access "extra"

LIVE EXAMPLE

I anticipate that "some" should include both "extra" and "prop," but trying to access "extra" results in a TypeScript error.

Answer №1

To ensure Typescript can accurately deduce the types, define your function with a generic type parameter:

const factory = (content: Content) => {
    return <K extends keyof Content>(key: K) => {
        return content[key];
    };
};

View this practical demonstration.

Answer №2

Here is a simple solution to the issue:

const createGetter = (data: Data) => {
    return <K extends keyof Data>(property: K) => {
        return data[property];
    };
};

The key difference now is that we are restricting the type of property to a specific string literal instead of allowing any key from the Data object.

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