Provide a function or value as input and consistently output the value

I'm struggling to develop a function that accepts another function which, when called, should return a specific type of value. If the value itself is passed, it should just return that value.

Unfortunately, I am facing difficulty in determining the final output type.

type Test<T> = (() => T) | T

class List{
    constructor(protected fnOrNumber: Test<number>) {
        //fnOrNumber will sometimes be number or function that returns number
    }

    get list(): number{
        return resolve(this.fnOrNumber)
        // ^^^
        //Type 'Test<number>' is not assignable to type 'number'.
        //Type '() => number' is not assignable to type 'number'.
    }

}

//function to resolve the final value
function resolve<T>(v: T): T{
    if (typeof v === 'function') {
        return v() 
    }
    return v
}

Typescript Playground

Answer №1

When implementing your resolve function, the signature resolve<T>(v: T): T suggests that it will return the same type that is passed to it. For example, if you pass a function to it, it should return a function based on its declaration.

If you have already defined a type as Test<T> = (() => T) | T, then you can modify the resolve function like this:

function resolve<T>(v: Test<T>): T{
    if (typeof v === 'function') {
        return (v as () => T)()
    }
    return v
}

The use of type assertion is necessary because typeof v === 'function' does not automatically narrow down to v: () => T.

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