State machine in TypeScript

create-state-machine.ts

/**
 * @example
 * ```ts
 * const POST_STATE = {
 *   DRAFT: 0,
 *   PUBLISHED: 1,
 *   // ...
 * } as const;
 * type PostState = (typeof POST_STATE)[keyof typeof POST_STATE];
 *
 * const POST_EVENT = {
 *   publish: {
 *     from: POST_STATE.DRAFT,
 *     to: POST_STATE.PUBLISHED,
 *   },
 *   withdraw: {
 *     from: MY_STATE.PUBLISHED,
 *     to: MY_STATE.DRAFT,
 *   },
 *   // ...
 * } as const;
 * type PostEvent = keyof typeof POST_EVENT;
 * 
 * const updatePostState = createStateMachine(POST_EVENT);
 * 
 * {
 *   const next = updatePostState(POST_STATE.DRAFT, "publish");
 *   //=> 1
 * }
 *
 * {
 *   const next = updatePostState(POST_STATE.PUBLISHED, "publish");
 *   //=> Error("Invalid transition: 1 -> 1");
 * }
 * ```
 */
export function createStateMachine<Event extends string, State extends number | string>(
  event: Record<Event, { from: State; to: State }>
): (prev: State, evt: Event) => State {
  return (prev, evt) => {
    const e = event[evt];
    if (e == null) {
      throw new Error(`Unknown event: ${evt}`);
    }

    if (prev !== e.from) {
      throw new Error(`Invalid transition: ${prev} -> ${e.to}`);
    }

    return e.to;
  };
}