yay-machine/src/MachineEvent.ts

A modern, simple, lightweight, zero-dependency, TypeScript state-machine library

/**
 * The base type for all machine states.
 * User-defined states do not need to extend this interface, just conform to its shape,
 * and can also have their own arbitrary properties.
 */
export interface MachineEvent<Type extends string = string> {
  /**
   * The unique type (or types if a string-union) of the state.
   */
  readonly type: Type;
}

/**
 * Extracts the specific event type from a union of events.
 */
export type ExtractEvent<
  EventType extends MachineEvent,
  Type extends EventType["type"],
> =
  Extract<EventType, { type: Type }> extends never
    ? ExtractComplexEvent<EventType, Type>
    : Extract<EventType, { type: Type }>;

/**
 * Extracts a concrete event type from type whose `type` field is a `string` union
 */
type ExtractComplexEvent<
  EventType extends MachineEvent,
  Type extends EventType["type"],
> = EventType extends {
  type: infer N;
}
  ? Type extends N
    ? EventType /* & { type: Type } */
    : never
  : never;

/**
 * Extracts the "payload" (ie, everything but the `type` field) of a specific event type.
 */
export type EventPayload<
  EventType extends MachineEvent,
  Type extends EventType["type"] = EventType["type"],
> = keyof Omit<ExtractEvent<EventType, Type>, "type"> extends never
  ? never
  : Omit<ExtractEvent<EventType, Type>, "type">;