Numeric Enum with is considered as a string in type comparison

Issue

While trying to do a comparison between a numeric enum, I noticed an error where the enum value is converted to a string type. Is that an expected behavior?

enum Test {
    a = 0,
    b = 1
}

console.log(Test.a === Test[0]);
// ^ This condition will always return 'false' since the types 'Test' and 'string' have no overlap.(2367)

TypeScript version: v4.6.4

Playground Link

Solution

This seems to be a confusion about what Test[0] is. Numeric enum members in TypeScript get a reverse mapping, where indexing into the enum object with an enum value gives you back the corresponding enum key.

So in

enum Test {
    a = 0,
    b = 1
}

you have Test.a === 0 and therefore Test[0] === "a". And since Test.b === 1, then Test[1] === "b". By comparing Test.a to Test[0], you are comparing a number to a string, and is indeed considered a TypeScript error to make such a comparison.

So you shouldn't write

console.log(Test.a === Test[0]); // error, different types.  Outputs false

But instead possibly one of these:

console.log("a" === Test[0]); // okay, Outputs true
console.log(Test.a === 0); // okay, Outputs true

Playground link to code

Answered By - jcalz