A type guard for number should narrow also the type for enum values

[NoelAbrahams]

Hi,

TS Version: 1.4

At present there doesn't appear to be a way to define an enum in a union type and have it narrowed correctly. Here's an example:

function f1(x: number | string){
  if (typeof x === "number") {
    return x + 10;
  }
  else {
      return x.length;
  }
}

enum Colour { red, blue }

function f2(x: Colour | string) {
  if (typeof x === "number") {
    return x + 10; // Error because x is Colour | string
  }
  else {
      return x.length;
  }
}

var x = f1(1);
var y = f2(Colour.blue);

The type guard typeof x === 'number' should probably narrow the type correctly and not report an error in f2.

[ahejlsberg]

Agreed, that should work as you suggest.