Typescript - callback with generic prop type

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?

Lesiak · Accepted Answer · 2020-09-30 14:05:33Z

2

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?

edited Sep 30, 2020 at 14:05

answered Sep 30, 2020 at 11:34

Lesiak

26.5k2 gold badges54 silver badges79 bronze badges

Sign up to request clarification or add additional context in comments.

Collectives™ on Stack Overflow

Typescript - callback with generic prop type

1 Answer 1

Comments

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

Comments

Your Answer

Sign up or log in

Post as a guest

Linked

Related