打字稿 - 具有通用道具类型的回调

Question

I have code like below, myFunc requires callback which needed generic parameter.

It looks like I can pass function which not required any parameters and not getting any error notification, any idea why?

const myFunc = <T>(callback: (par: T) => void) => {
  const param = ('test' as any) as T;
  callback(param);
};

const callback = (par: number) => {
  console.log(par);
};

myFunc<string>(callback); // Not working as suspected - OK
myFunc<string>(() => {}); // Why I do not getting any error notification here?

Answer 1

This is clearly described in handbook. Comparing two functions :

 let x = (a: number) => 0; let y = (b: number, s: string) => 0; y = x; // OK x = y; // Error

You may be wondering why we allow 'discarding' parameters like in the example y = x. The reason for this assignment to be allowed is that ignoring extra function parameters is actually quite common in JavaScript. For example, Array#forEach provides three parameters to the callback function: the array element, its index, and the containing array. Nevertheless, it's very useful to provide a callback that only uses the first parameter

See also relevant section of TypeScript FAQ: Why are functions with fewer parameters assignable to functions that take more parameters?