test.ts

function a() : string |number
{
    return ""
}

function b(){
  let foo = a()
  if( typeof foo === "number")
  {
      console.log(foo.toFixed())
  }
  if (typeof foo === "string")
  {
      console.log(foo.toLowerCase())
  }
}

In fact, this also compiles fine

function b(){
  let foo = a()
  if( typeof foo === "number")
  {
      console.log(foo.toFixed())
  }
  else
  {
      console.log(foo.toLowerCase())
  }
}

class Character {

  constructor(public name: string, public x: number, public y: number) { }
}

class Butts {
  static doThing(isCoordinate: boolean): { x: number, y: number } | Character {
    if (isCoordinate) return { x: 1, y: 1 };

    return new Character('test', 1, 1);
  }
}

const res = Butts.doThing(true);