import type {IsStringLiteral} from 'type-fest';
type CapitalizedString<T extends string> = IsStringLiteral<T> extends true ? Capitalize<T> : string;
// https://github.com/yankeeinlondon/native-dash/blob/master/src/capitalize.ts
function capitalize<T extends Readonly<string>>(input: T): CapitalizedString<T> {
return (input.slice(0, 1).toUpperCase() + input.slice(1)) as CapitalizedString<T>;
}
const output = capitalize('hello, world!');
//=> 'Hello, world!'
// String types with infinite set of possible values return `false`.
import type {IsStringLiteral} from 'type-fest';
type AllUppercaseStrings = IsStringLiteral<Uppercase<string>>;
//=> false
type StringsStartingWithOn = IsStringLiteral<`on${string}`>;
//=> false
// This behaviour is particularly useful in string manipulation utilities, as infinite string types often require separate handling.
type Length<S extends string, Counter extends never[] = []> =
IsStringLiteral<S> extends false
? number // return `number` for infinite string types
: S extends `${string}${infer Tail}`
? Length<Tail, [...Counter, never]>
: Counter['length'];
type L1 = Length<Lowercase<string>>;
//=> number
type L2 = Length<`${number}`>;
//=> number
Returns a boolean for whether the given type is a
stringliteral type.Useful for: - providing strongly-typed string manipulation functions - constraining strings to be a string literal - type utilities, such as when constructing parsers and ASTs
The implementation of this type is inspired by the trick mentioned in this StackOverflow answer.