Reducer Extensions

extendReducer.js

/**
 * A higher order function that takes a reducer and an extension
 * and returns a reducer amplified with provided extensions on
 * existing or new action types.
 *
 * Extensions are objects containing the action `type` attribute
 * and an `actionTypeExtension` function which takes `state` and
 * the `action` and must return an object to append to the reducer's
 * new state.
 *
 * Example of an extension:
 * {
 *   type: ACTION_SUCCESS,
 *   actionTypeExtension: (state, action) => ({
 *     foo: action.payload.foo
 *   })
 * }
 *
 * @param {Function} reducer - The reducer to extend
 * @param {Array<Object>} extensions - An array of extensions actions
 * with which to amplify the reducer
 *
 * @returns {Function} - The extended reducer
 */
const extendedReducer = (reducer, extensions) => (state, action) => {
  if (!extensions || !action) {
    return reducer(state, action);
  }

  const { type } = action;
  const extension = extensions.find(x => x.type === type);

  if (extension && extension.type === type) {
    if (typeof extension.actionTypeExtension !== 'function') {
      throw new Error('Expecting extension to contain an `actionTypeExtension` method.');
    }

    return {
      ...reducer(state, action),
      ...extension.actionTypeExtension(state, action),
    };
  }

  return reducer(state, action);
};

export default extendedReducer;