TypeScript: Error when passing an array in a variable rather than direcly to a function

Question

The following function workOnFunctions takes an object and an array of keys to that object where all the keys map to a function:

const myObj = {
    a: () => 1,
    b: "test",
    c: 2,
    d: (a : number) => a*2
}

function workOnFunctions <K extends PropertyKey>(o: Record<K, Function>, names: K[]) {
    names.forEach(name => {
        const f =  o[name]; // do something with function f
    })
}

I noticed that it works as intended when I do this:

workOnFunctions(myObj, ["a", "d"]); // works

But when storing the array in a variable and then passing this variable to the function, I get the error

Argument of type '{ a: () => number; b: string; c: number; d: (a: number) => number; }' is not assignable to parameter of type 'Record<string, Function>'.
  Property 'b' is incompatible with index signature.
    Type 'string' is not assignable to type 'Function'.

Code:

const names = ["a", "d"];
workOnFunctions(myObj, names);

Why is that the case? It seems like the compiler can't infer the type of names correctly. When I pass the array directly, it can. I also tried this:

const names = ["a", "d"] as const;
workOnFunctions(myObj, names);

Which does not work either.

My final goal is to write a function that takes an object and an array of keys that point to functions within that object.

EDIT: Partial solution found, but I don't understand why

Looks like this works:

type KeysMatching<T, V> = { [K in keyof T]: T[K] extends V ? K : never }[keyof T]
const names : Array<KeysMatching<typeof myObj, Function>> = ["a", "d"];
workOnFunctions(myObj, names);

I got KeysMatching from here: TypeScript: Accept all Object keys that map to a specific type

But why can the compiler infer this complex type by itself when I directly pass the array? Is there a trick to auto-infer the type also when using a separate array?

John · Accepted Answer · 2020-05-14 00:52:49Z

0

Your error is not because of the names array, but because of your myObject, the "b" and "c" properties does not return a function, which is what you have defined in your workOnFunctions function o: Record<K, Function>.

If you change your myObj with the correct types, it will work

// adding the type :Record<PropertyKey, Function> is optional here, and will work without having to defined the type.
myObj: Record<PropertyKey, Function> = {
    a: () => 1,
    b: () => "test",
    c: () => 2,
    d: (a : number) => a*2
}

answered May 14, 2020 at 0:52

John

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