Typescript infer object keys based on array

Question

I'm trying to create an object with specific keys based on the value of the array.

Here is the code:

type AllowedKeys = "foo" | "bar" | "baz"

interface Options {
    keys: AllowedKeys[]
}

interface AllTypesDeclaration {
    foo: string[];
    baz: object[];
    bar: number[];
}

function createObj(arg: Options): Pick<AllTypesDeclaration, typeof arg.keys[number]> {
    return {}
}

const myObj = createObj({ keys: ['baz'] })

// This should work:
myObj.baz

// This should fail:
myObj.foo
myObj.bar

As you can see I tried to do it using Pick, but that doesn't seem to work.

Demo: here

brunnerh · Accepted Answer · 2020-10-21 13:15:04Z

3

You will need generics for that, otherwise the type of keys will not be restricted by the input.

function createObj<T extends AllowedKeys[]>(arg: { keys: T })
    : Pick<AllTypesDeclaration, typeof arg.keys[number]> {
    return {}
}

answered Oct 21, 2020 at 13:15

brunnerh

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