John J. Camilleri John J. Camilleri - 2 months ago 31
TypeScript Question

How to compare Enums in TypeScript

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


When compiling with
tsc
(v 2.0.3) I get the following error:


TS2365: Operator '===' cannot be applied to types 'E.A' and 'E.B'.


Same with
==
,
!==
and
!=
.
I tried adding the
const
keyword but that seems to have no effect.
The TypeScript spec says the following:


4.19.3 The <, >, <=, >=, ==, !=, ===, and !== operators

These operators require one or both of the operand types to be assignable to the other. The result is always of the Boolean primitive type.


Which (I think) explains the error. But how can I get round it?

Side note

I'm using the Atom editor with atom-typescript, and I don't get any errors/warnings in my editor. But when I run
tsc
in the same directory I get the error above. I thought they were supposed to use the same
tsconfig.json
file, but apparently that's not the case.

Answer

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): T { return t; }
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