how to achieve overload in typescript

Question

I konw there is a way in typescript like this:

function foo(arg: number): string;
function foo(arg: string): number;
function foo(): any {
  if (typeof arguments[0] === "string") {
    return +arguments[0];
  } else {
    return arguments[0].toString();
  }
}

foo(1)       // '1'
foo("hello") // NaN 
foo("2")     // 2

But when I have same number of parameters and each parameter is custom type, like this

interface Dog {
  run: number;
}

interface Bird {
  fly: number;
}

function foo(arg: Dog): number;
function foo(arg: Bird): number;
function foo(): any {
}

How can I tell the difference between these parameters

Answer 1

TypeScript is JavaScript, so you basically have to do the same thing. In order to make your compiler happy, you can implement type guards.

https://www.typescriptlang.org/docs/handbook/advanced-types.html#user-defined-type-guards

function isDog(pet: Dog | Bird): pet is Dog {
  return 'run' in pet
}

You can use these in your code to tell TypeScript that you've validated the type for runtime.

function foo(arg?: Dog | Bird): void | number {
  if(arg) {
    if(isDog(arg)) {
      return arg.run;
    } else {
      return arg.fly;
    }
  }
}