Create a TypeScript (string) enum from a union type?

Question

If I have a TypeScript union like so:

type SomeUnion = 'Foo' | 'Bar' | 'Baz';

Is there a way I can convert this into a string-based enum? Like the the following:

enum SomeUnionBasedEnum {
  Foo = 'Foo',
  Bar = 'Bar',
  Baz = 'Baz'
}

I am trying to model an enum that represents the permissions in our system, but those permissions are in some cases automatically generated union types. I want a way to combine them into a single enum because enums provide a better developer experience by limiting the intellisense to the permissible values. For example AppPermisionEnum.<available enum values>. I'd also be open to ways to combine enums into a single enum, but I don't think that's possible. My end goal is to have an enum comprised of values from a couple different sources.

But string literal types will already provide intelisense. If you type "" in a position where a string literal is expected you will get intelisense as to possible values... — Titian Cernicova-Dragomir
– Titian Cernicova-Dragomir, Commented May 28, 2019 at 22:50

Titian Cernicova-Dragomir · Accepted Answer · 2019-05-28 23:05:11Z

10

Like I pointed out in the comments string literal types already offer good intelisense support, so I personally would just use those and not bother creating anything else, the developer experience is already great.

The code below is a solution in search of a problem but we can do something close to what you want.

You can't create an enum, but you can create an object that has all the properties of the union and each property has the same value as the key, using a mapped type. You will need to maintain these objects manually, but typescript will issue a warning if ever they are out of sync so maintaining them should not be an issue.

type SomeUnion = 'Foo' | 'Bar' | 'Baz';

function createEnumObject<T extends string>(o: { [P in T]: P }) {
    return o;
}

const SomeUnion = createEnumObject<SomeUnion>({
    Foo: "Foo",
    Bar: "Bar",
    Baz: "Baz"
})

let o: SomeUnion = SomeUnion.Bar;

Converting a union (ie a type) to an object (ie a value) without extra code is in principle not possible, types are erased at compile time so can't be relied upon for anything at run-time and typescript has a strict policy against type directed emit.

answered May 28, 2019 at 23:05

Titian Cernicova-Dragomir

253k37 gold badges464 silver badges394 bronze badges

Sign up to request clarification or add additional context in comments.

2 Comments

mxiong Over a year ago

Union type is nice and all, but I'm looking into making an enum for the main purpose of maintainability in the event that a string needs to be renamed. I might use the string literal "apple" in multiple places in the codebase, but using an enum gives me an easy way to rename things in a way that a global string find and replace would fail.

Titian Cernicova-Dragomir Over a year ago

@mxiong enums have their advantages for sure. While renaming might not work, you should get errors everywhere the old version of the string is used

Collectives™ on Stack Overflow

Create a TypeScript (string) enum from a union type?

1 Answer 1

2 Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

2 Comments

Your Answer

Sign up or log in

Post as a guest

Related