Function createReducer

modules/store/src/reducer_creator.ts:114–140

(initialState: S, ...ons: ReducerTypes<S, readonly ActionCreator[]>[])

 * ```
 */
export function createReducer<
  S,
  A extends Action = Action,
  // Additional generic for the return type is introduced to enable correct
  // type inference when `createReducer` is used within `createFeature`.
  // For more info see: https://github.com/microsoft/TypeScript/issues/52114
  R extends ActionReducer<S, A> = ActionReducer<S, A>,
>(initialState: S, ...ons: ReducerTypes<S, readonly ActionCreator[]>[]): R {
  const map = new Map<string, OnReducer<S, ActionCreator[]>>();
  for (const on of ons) {
    for (const type of on.types) {
      const existingReducer = map.get(type);
      if (existingReducer) {
        const newReducer: typeof existingReducer = (state, action) =>
          on.reducer(existingReducer(state, action), action);
        map.set(type, newReducer);
      } else {
        map.set(type, on.reducer);
      }
    }
  }

  return function (state: S = initialState, action: A): S {
    const reducer = map.get(action.type);
    return reducer ? reducer(state, action) : state;
  } as R;
}