type-samurai/src/last-character.d.ts

Advanced utility types for Typescript

import { If } from "./if";
import { IsEmptyString } from "./string";

export type LastCharacterOptions = {
  includeRest: boolean;
};

type _LastCharacter<
  T extends string,
  Options extends LastCharacterOptions = {
    includeRest: false;
  },
  Previous extends string = ""
> = string extends T
  ? string
  : T extends `${infer First}${infer Rest}`
  ? IsEmptyString<Rest> extends true
    ? If<Options["includeRest"], [First, Previous], First>
    : _LastCharacter<Rest, Options, `${Previous}${First}`>
  : T;

/**
 * Accepts a string argument and returns its first character.
 * If the `includeRest` options is `true`, returns the first character and the rest of the string in the format of: [last, rest]
 * @example
 * ```ts
 * // 'c'
 * type Case1 = LastCharacter<'abc'>
 * // ['c', 'ab']
 * type Case2 = LastCharacter<'abc', {includeRest: true}>
 * ```
 */
export type LastCharacter<
  T extends string,
  Options extends LastCharacterOptions = {
    includeRest: false;
  }
> = _LastCharacter<T, Options>;