Typescript Enum Object.values() return value

Question

Why does Object.values() and Object.keys() always give both the keys and the values?

Consider the following code:

enum Enum {
    FOO,
    BAR
}

console.log(Object.values(Enum));
console.log(Object.keys(Enum));

The output of this would be:

[ 'FOO', 'BAR', 0, 1 ]
[ '0', '1', 'FOO', 'BAR' ]

Why does it do that and how do I only get the keys and values?

Solution

That's how enum types are transpiled by TypeScript.

enum Enum {
    FOO,
    BAR
}

will become

"use strict";
var Enum;
(function (Enum) {
    Enum[Enum["FOO"] = 0] = "FOO";
    Enum[Enum["BAR"] = 1] = "BAR";
})(Enum || (Enum = {}));

Notice that both numeric keys and string keys are provided for easy mapping to and from both types, enabling you to do something like this:

const value = Enum.FOO;  // inferred type Enum.FOO
const key = Enum[value]; // inferred type string

If you want to get an array of only the numeric or string keys, you can do this:

const numericKeys = Object.keys(Enum).map(x => parseInt(x)).filter(x => !isNaN(x));
const stringKeys = Object.keys(Enum).filter(x => isNaN(parseInt(x)));

Or for the numeric or string values (requires the es2017 library in your tsconfig):

const numericValues = Object.values(Enum).filter(x => typeof x === "number");
const stringValues = Object.values(Enum).filter(x => typeof x === "string");

Comments

[Aleksey L.]

More info on enums: stackoverflow.com/questions/28818849/…. How to get keys/values: github.com/Microsoft/TypeScript/issues/…

[Lehks]

Okay, thank you very much. Is there a way to only get the values then? I suppose it would be possible to get that result by using Array.splice() but I am not sure if the order is always the same (although it seems that way).

[Aleksey L.]

const values = Object.values(Enum).filter(v => typeof v === 'number');