Typescript map string literal types to uppercase

Question

type a = 'one' | 'two'

I would like to have a type b like

type b = 'ONE' | 'TWO'

So I tried

type a = 'one' | 'two'
type b = {[P in a]: P['toUpperCase']}

But that does not do what I want it to do.

Thank you for reading :)

I would be very impressed if this was possible. But I suspect it might not be. — VLAZ
– VLAZ, Commented Jun 25, 2020 at 19:08
The only solution I can think of is storing this type a = 'one' | 'two' on a separate file and the read the file content, then programmaticly change the values to uppercase. — user11534547
– user11534547, Commented Jun 26, 2020 at 7:32
@IAMTHEBEST I guess it works but seems a bit ugly. You might be able to have some sort of build step that transforms types, say takes all from lowercase.d.ts and generates an uppercase.d.ts with all the string literal types in uppercase there. But I'm not sure I really like this. Seems really easy to get in a tight bind with this if you need, say, first later capitalised only or anything other that deviates. — VLAZ
– VLAZ, Commented Jun 26, 2020 at 7:36

Derek Nguyen · Accepted Answer · 2022-12-01 09:12:26Z

32

You can now do this with the introduction of Template Literal Types:

type A = 'one' | 'two'
type B = Uppercase<A>

let b: B = 'one' // Type '"one"' is not assignable to type '"ONE" | "TWO"'.

TS Playground

In addition to Uppercase<StringType>, there are also the following helper types:

Lowercase
Capitalize
Uncapitalize

They can be used within a Template Literal Types, like below:

type Fruit = 'Apple' | 'Banana'
type FruitField = `fr_${Uncapitalize<Fruit>}`

const fruit: Record<FruitField, boolean> = {
    'fr_apple': true,
    'fr_banana': false,
    'fr_Apple': true, // error
    'fr_peach': false // error
}

TS Playground

edited Dec 1, 2022 at 9:12

answered Oct 19, 2020 at 4:55

Derek Nguyen

11.5k1 gold badge44 silver badges69 bronze badges

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

2 Comments

Andrew Shepherd Over a year ago

The syntax has changed slightly for v4.1.0 beta, it's now {uppercase A}. typescriptlang.org/play?ts=4.1.0-beta#code/…

Geoff Davids Over a year ago

With release v4.2 it's switched back to the way OP originally wrote it

Lorenzo Imperatrice · Accepted Answer · 2022-12-01 09:21:08Z

5

Here you have the link to TS Playground

And here the code:

type A = 'one' | 'two'
type B = Uppercase<A>

let b: B = 'one' // Error

P.S. I don't get why in the previous comment has been used a more complicated form to write the same thing

EDIT: Derek Nguyen (best answer) has now changed his answer with the newer and more simple solution.

edited Dec 1, 2022 at 9:21

answered Oct 25, 2022 at 14:02

Lorenzo Imperatrice

99710 silver badges25 bronze badges

1 Comment

Derek Nguyen Over a year ago

Thanks for pointing that out, IIRC during the 4.1 beta the manipulation types (uppercase<>, lowercase<>, etc.) only worked within the template literal

Collectives™ on Stack Overflow

Typescript map string literal types to uppercase

2 Answers 2

2 Comments

1 Comment

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

2 Comments

1 Comment

Your Answer

Sign up or log in

Post as a guest

Related