Argument of type 'Function' is not assignable to parameter of type '(…args: any[]) => void'

Question

function on(event: string, listener: Function) {
    console.log('on event: ', event, 'typeof listener:', typeof (listener));
    listener();
}

function on1(event: string, listener: (...args: any[]) => void) {
    console.log('on event: ', event, 'typeof listener:', typeof (listener));
    listener();
}

function createCallback(a: number): Function {
    let f = (a: number) => {
        console.log('return a:', a);
        return (a: number) => { return a + 1 };
    };
    return f(a);
}

let f = createCallback(1);
console.log('type of f', typeof (f));
// on("start", f);
on1("start", f);

In the above code, on("start", f) works fine but on1("start", f); returns error

main.ts:22:14 - error TS2345: Argument of type 'Function' is not assignable to parameter of type '(...args: any[]) => void'.
  Type 'Function' provides no match for the signature '(...args: any[]): void'.

22 on1("start", f);
                ~

If I remove the Function type assertion of createCallback, on1("start", f) works, isn't Function a general type to represent closures?

The original question comes from this commit, https://github.com/DefinitelyTyped/DefinitelyTyped/commit/96545154cc6488643a7064a6dc4ec9726c7af12a#diff-7d84e08967cded0b99ed4328aab0a1a8L291

I don't understand why he changes Function to (...args: any[]) => void which makes my code break.

“isn't Function a general type to represent closures?” — right, it’s a general type, but your function on1 requires a more specific type. Just like Any is a general type, and can’t be bound to a more specific type (but can itself bind anything). — Konrad Rudolph

Titian Cernicova-Dragomir Titian Cernicova-Dragomir · Accepted Answer · 2018-08-08T11:01:48

There is a suggestion to allow something like this, for Function to be assignable to (...args: any[]) => any. The reason that this is not the current behavior is:

The original intention of Function is to not be callable. in other words, Function to function types should be like unknown to other types, but not callable. we have since relaxed this restriction giving Function a callable behavior in the compiler through special casing. We have talked about making this a --noImplicitAny error since, it is really unsafe to call Functions.

I suggest you up-vote the suggestion if this is important to you.

I would generally stay way from Function it's not a type-safe away to express function signatures. For your case I would use the following:

function on(event: string, listener: (...args: any[]) => void) {
    console.log('on event: ', event, 'typeof listener:', typeof (listener));
    listener();
}

function createCallback(a: number)  { // we can omit the return type or be explicit and type it as (a: number) => number
    let f = (a: number) => {
        console.log('return a:', a);
        return (a: number) => { return a + 1 };
    };
    return f(a);
}

let f = createCallback(1); // (a: number) => number 
on("start", f);

Argument of type 'Function' is not assignable to parameter of type '(…args: any[]) => void'

1 Answers