TypeScript - weird behaviour when iterating over an enum

Question

I noticed this just now when trying to iterate over an enum.

Say you have:

enum Gender {
    Male = 1,
    Female = 2
}

and you do:

for (let gender in Gender) {
    console.log(gender)
}

This will run 4 (?) times. First printing string (!) representations of 1 and 2, then printing the strings Male and Female.

I can only suppose this is intended. My question is why this is the case? What's the reasoning behind this (in my opinion) weird implementation?

Answer 1

JS has no enum. TS compiles your enum to:

var Gender;
(function (Gender) {
    Gender[Gender["Male"] = 1] = "Male";
    Gender[Gender["Female"] = 2] = "Female";
})(Gender || (Gender = {}));

Where you can see has 4 keys (1,2,Male,Female).

You can use this site for checking the TS to JS compilation output.