TypeScript：如何从函数类型中获取许多参数

Question

Is there any good way to get a number of parameters from function types?

For example, I have two callback functions as function type and a setter function for them. I want to get a number of parameters from function types and replace the magic numbers (2, 3) with it.

type Callback1 = (a: number, b: string) => void;
type Callback2 = (a: number, b: string, b: boolean) => void;

setCallback(callback: Callback1 | Callback2): void {
    if (callback.length == 2) {
        this.callback = callback as Callback1;
    } else if (callback.length == 3) {
        this.callback = callback as Callback2;
    }
}

Answer 1

Use the length property.

let x = function (a,b){}
function bar (a,b){}
console.log(x.length, bar.length); // 2,2 

More

https://developer.mozilla.org/en/docs/Web/JavaScript/Reference/Global_Objects/Function/length

Answer 2

This works for me. If the definition of the function types changes, compilation fails at the dummy function definitions. This is much safer than the magic numbers above.

type Callback1 = (a: number, b: string) => void;
type Callback2 = (a: number, b: string, b: boolean) => void;

private readonly dummyCallback1: Callback1 = (a: number, b: string) => {};
private readonly dummyCallback2: Callback2 = (a: number, b: string, b: boolean) => {};

setCallback(callback: Callback1 | Callback2): void {
    if (callback.length == this.dummyCallback1.length) {
        this.callback = callback as Callback1;
    } else if (callback.length == this.dummyCallback2.length) {
        this.callback = callback as Callback2;
    }
}

Answer 3

This is much simpler. Thanks @torazaburo

type Callback = (a: number, b: string, c?: boolean) => void;

setCallback(callback: Callback): void {
    this.callback = callback as Callback1;
}


setCallback(0, "a");
setCallback(0, "a", true);