Typescript map string literal types to uppercase

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 🙂

Answer

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