What is the best way to create an object with a generic mapped type?

My defined mapped type looks like this:

type FooOrBar =  "foo" | "bar"

type ObjectWithHi<T extends FooOrBar> = {
        [key in T]: 'hi';
}

I am trying to create a function call that follows this structure:

getFooOrBar<'foo'>('foo')

The function I have attempted to define is as follows:

function getFooOrBar<T extends FooOrBar>(fooOrBar: T): ObjectWithHi<T> {
    return {
        [fooOrBar]: 'hi',
    }
}

Unfortunately, TypeScript gives me an error saying:

Type '{ [x: string]: string; }' is not assignable to type 'ObjectWithHi<T>'.

What might be the issue here?

Answer №1

From my understanding, when working with keys in typescript, they typically get widened to string, number or symbol during object creation. While I may not know all the intricate details or inner workings behind this process, I have found that simply casting the return to ObjectWithHi<T> is often the most straightforward way to achieve the desired outcome.

Additionally, you can enhance the strictness of the check by adding as const to the value:

return {
    [fooOrBar]: 'hi' as const,
} as ObjectWithHi<T>

This approach ensures a more rigorous validation where the value must be exactly hi in order to successfully cast it to ObjectWithHi<T>.

If you're interested in delving deeper into this topic, I recommend checking out this Github issue for further insights: https://github.com/microsoft/TypeScript/issues/5579.

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