Redux store

reduxStore.js

    function createStore(reducer) {
      // Store should have 4 parts.
      // 1. The state.
      // 2. Get the state.
      // 3. Listen to state change.
      // 4. Update the state.

      let state;
      let listeners = [];

      const getState = () => state;
      const subscribe = (listener) => {
        listeners.push(listener);

        return () => {
          listeners = listeners.filter(l => l !== listener);
        };
      };

      const dispatch = (action) => {
        state = reducer(state, action);
        listeners.forEach(listener => listener());
      };

      return {
        getState,
        subscribe,
        dispatch,
      };
    }

// Combine reducers.

    function app(state = {}, action) {
      return {
        todos: todos(state.todos, action),
        goals: goals(state.goals, action),
      };
  }