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?

Amir Popovich · Accepted Answer · 2019-01-08 12:53:33Z

8

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.

answered Jan 8, 2019 at 12:53

Amir Popovich

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2 Comments

Viggo Lundén Over a year ago

That makes sense. But what does the for loop compile to then? Wouldn't it make sense to also compile the loop to reflect the TS enum, and not the underlying JS?

Amir Popovich Over a year ago

The for...in statement iterates over all non-Symbol, enumerable properties of an object. - developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/…

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