Define a TypeScript type that takes a lowercase word

Is there any way to define that kind of type in TypeScript? Something like:

type lowercaseWord = /[a-z]/

But that defines a string instead? (I think the code above will define a regex.)

Answer

Yes and no. There is built int (intrinsic) type LowerCase but it works as an utility type. It works as String.prototype.toLowerCase and not as a regex. See example:

type Result = Lowercase<'A'> // 'a'

Playground

Here you can find more types: Capitalize, Uncapitalize, Lowercase, Uppercase

If you want to apply such restriction it is possible to do in the function:

type UpLetters = 'A' | 'B' | 'C' // provide all allowed letters
type LowLetters = Lowercase<UpLetters>

type Letters = UpLetters | LowLetters

type IsAllowed<T extends string> = T extends Letters ? T extends Lowercase<T> ? T : never : never

const lower = <T extends string>(str: IsAllowed<T>) => null

/**
 * Ok
 */
lower('a') // ok

/**
 * Error
 */
lower('A') // error
lower('1') // error
lower('$%^') // error

Please keep in mind that

const x: Lowercase<string> = 'A' // no error

will not trigger an error