Imagine that I have a function that behaves differently when passed one parameters vs two. For example:

function test(a: string | number, b?: number ) {
  if(typeof b !== 'undefined' && typeof a === "number"){
    return a * b
  }
  if (typeof a === "string" && typeof b === "undefined"){
    return a.toUpperCase() 
  }
  throw new Error("Incorrect Params");
}

Function behavior:

• When only a is present and it is a string, convert it to uppercase.

• When both a and b are present, both must be numbers, and multiply them.

• Any other type of combination throws an error

The question is, can it be typed in such a way that this behavior is reflected in the IDE?

That is, when only one parameter is passed to it, the IDE will show this

But when passed two, or a comma is added, the behavior changes to show this:

This would allow to avoid error during code development.

Clarification: The exposed function is only an example and not a real use case. Still I think this would be very useful and I have seen it implemented in the

CodePudding user response：

After reviewing the marked as duplicated question a possible solution will be (not the most optimal because I have to change the behavior of the function):

function test(...args: [a: string] | [a: number, b: number]): number | string {
  let a = args[0];
  let b = args[1];
  if (typeof b !== "undefined" && typeof a === "number") {
    return a * b;
  }
  if (typeof a === "string" && typeof b === "undefined") {
    return a.toUpperCase();
  }
  throw new Error("Incorrect Params");
}