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.)
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'
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