Is it possible to set a default value for a function that depends on two generic types?

Question

typescript

Working code example:

Working code

const displayFunctionCode = <T, P>(callback: (props: T) => P) => {
    console.log(callback);
}

displayFunctionCode<number, number>((n) => n);

Sample code that I would like to work:

Desired code

const displayFunctionCode = (callback: (props: T) => P = (prop: T) => prop) => {
console.log(callback);
}

displayFunctionCode((n) => n);

Is it possible to make ts believe that the return value in a function is exactly of type P, like ?
(prop: T) => props as P;

Answer 1

There is such a construction: - by double casting you can cast anything into anything. Only here you don’t need to do this, this one kills the whole point of using TypeScript. Instead, you need to write conditions in the "default function" so that it works as it is described, and not the hell understand how. foo as unknown as Type