How to compare Enums in TypeScript

Question

In TypeScript, I want to compare two variables containing enum values. Here\'s my minimal code example:

enum E {
  A,
  B
}

let e1: E = E.A
let e2: E = E.B

if (         


        

Answer 1

There is another way: if you don't want generated javascript code to be affected in any way, you can use type cast:

let e1: E = E.A
let e2: E = E.B


if (e1 as E === e2 as E) {
  console.log("equal")
}

In general, this is caused by control-flow based type inference. With current typescript implementation, it's turned off whenever function call is involved, so you can also do this:

let id = a => a

let e1: E = id(E.A)
let e2: E = id(E.B)

if (e1 === e2) {
  console.log('equal');
}

The weird thing is, there is still no error if the id function is declared to return precisely the same type as its agument:

function id(t: T): T { return t; }