Get first enum item from an enum

Question

Is it possible to get the first item of an enum in typescript?

For example, Option.All in this enum:

enum Option {
    All = "all",
    Mine = "mine",
}

Tried, but won't compile:

const first: Option = Option[0]; // first should be All. This won't compile - Property '0' does not exist on type 'typeof Option'.

An enum becomes an object, which isn't an ordered data structure. There's only a 0th value if you let the compiler auto-number them from 0. But what's the context, why do you need the first value? — jonrsharpe, Oct 15 '20 at 18:24
What do you mean it's generated? If this isn't happening at compile time it's not really a TypeScript question, you're effectively asking https://stackoverflow.com/q/983267/3001761. — jonrsharpe, Oct 15 '20 at 18:27
Well the generated code is Typescript, but out of my control. I can't change it, but need a (first) value. I could loop the enum, so there is an first in some sense. — Julian, Oct 15 '20 at 18:29

Zachary Haber · Accepted Answer · 2020-10-15T18:54:37.897

7

Option is essentially an Object.

This is how it is defined when compiled to JS:

var Option;
(function (Option) {
    Option["All"] = "all";
    Option["Mine"] = "mine";
})(Option || (Option = {}));

You can use Object.keys() or Object.values() to be able to convert an Object to an array.

Thus, in order to make first = 'All', you can use:

enum Option {
    All = "all",
    Mine = "mine",
}

const first = Object.keys(Option)[0];
console.log(first)

If you want to play around with how things look between TS and JS, you can use the TS Playground. I've loaded this link in with the code I showed here.

If you want the firstValue to be of type Option, use Object.values(Option)[0].

Typescript playground seems to have an issue with the name Option, so if you change it to Options, it shows correctly that first is of type Options: Playground

enum Options {
    All = "all",
    Mine = "mine",
}

const first = Object.values(Options)[0];
console.log(first) // 'all' of type Options

My local environment doesn't show that same weirdness.

edited Oct 15 '20 at 18:54

answered Oct 15 '20 at 18:23

Zachary Haber

10,376
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31

`first` is here of type `string`, I need the Enum value. Maybe I should called it Enum item? – Julian Oct 15 '20 at 18:37
The 2nd playground link is broken? – Julian Oct 15 '20 at 18:52
I fixed the link. Not sure how that happened. – Zachary Haber Oct 15 '20 at 18:55
1

Excellent, this works: `const first: Options = Object.values(Options)[0];` – Julian Oct 15 '20 at 18:55

score 2 · Answer 2 · answered Oct 15 '20 at 18:23

2

Please try this:

Option[Object.keys(Option)[0]]

answered Oct 15 '20 at 18:23

Thunderbolt Engineer

1,524
7
12

That gives: Element implicitly has an 'any' type because index expression is not of type 'number'.[Playground](https://typescript-play.js.org/#code/KYOwrgtgBAsgngeQA4BcCWB7EUDeAoKQqAQQBtSoBeKAIgENyaAaAomNEYK2iD4ZvAF88eAMZYAziigAzNACcpALliJUmbNXjJ0WANoIARgCtgolADoA1sDgSAFNvVYAlHoAMAXU9A) – Julian Oct 15 '20 at 18:27
@Julian You could write `Option[Object.keys(Option)[0] as keyof typeof Option]` – Johannes Riecken Dec 29 '22 at 09:13

Get first enum item from an enum

2 Answers2