Dynamic types depending on number of parameters in IDE

Question

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

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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.

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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 Google Apps Script IDE. I would like to know what the internal mechanism is, and if it can be extended to VSCode using only Typescript.

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Answer 1

You're after function overloading, which in your case would be implemented like so:

function test(a: number, b: number): number;
function test(a: string): string;

function test(...args: any[]): string | number {
  // implementation based on args
}

Which presents like this in your IDE - notice the 1/2 to denote the possible overloads, which you can cycle though:

Answer 2

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");
}