redux-create-store.md

// notes from https://egghead.io/lessons/react-redux-implementing-store-from-scratch

const createStore = (reducer) => {
  let state;
  let listeners = []; // keeps track of all the change listeners 
  
  const getState = () => state;
  
  const dispatch = (action) => {
    // calculate the new state by calling the reducer and update the state 
    state = reducer(state, action);
    // call every change listener because an update has been made
    listeners.forEach(listener => listener());
  };
  
  const subscribe = (listener) => {
    // register a callback that will be called every time an action has been dispatched
    listeners.push(listener);
    
    // instead of adding a dedicated unsubscribe method, 
    // return a function that removes this listener from the listeners array
    return () => {
      listeners = listeners.filter(l => l!== listener);
    }
  };
  
  // dispatch a dummy action just to populate the initial state 
  dispatch({});
  
  return {getState, dispatch, subscribe};
}