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Implement Redux from scratch for fun and for profit

Redux is a global state management library! Read more about redux here.

Passing state around can get out of hand! Generally keeping the data as close to where it is needed is good.

const Header = ({ users }) => <div>
  There are {users.length} users on the site!

const User => ({ name }) => <div>{name}</div>;

const UserList = ({ users }) => <div>
  {, i) => <User user={user} key={i} />)}

class App extends React.Component {
  state = { users: [] };

  componentWillMount() {
    fetchUsers().then((users) => this.setState({ users }));

  render() {
	const { users } = this.state;
	return (<div>
	  <Header users={users} />
	  <UserList users={users} />

Enter Redux!

  • State
  counter: 0;
  • Actions
{ type: string, data: object }
  • Action Creators
const incrementAction = (count) => ({ type: 'increment', count });
  • Actions mutate state with Reducers
const counterReducer = (state = 0, action) =>
  action.type === 'increment' ? state + 1 : state;


If say you understand something you should be able to explain it to somebody. How about explaining it to a computer?

There's actually not much going on here:

nick:~/dev/_learning/redux master > wc -l *.mjs src/*
      61 test.mjs
       9 src/applyMiddlewares.mjs
       9 src/combineReducers.mjs
      13 src/index.mjs
      36 src/store.mjs
       9 src/thunk.mjs
     137 total

Coding things is a great way to demystify them!

Redux Spec

At each step it's helpful to make a test file that runs your implementation!

Here's the order we reccomend implementing Redux in:

1. the Store class

Most of redux's functionality is baked into this class! Everything else is a helper function built around this class.

class Store
  constructor(rootReducer, initialState = {})
  getState(): state
  subscribe(listener): (unsubscribe: Function)
  replaceReducer(nextReducer): void
  dispatch(action): void

Redux allows you to listen to state changes with listeners. Subscribe adds a callback function (and returns a function to unsubscribe that listener).

Dispatch calls the rootReducer on the current state, updates the state to be the result of that pure function and then calls all the listeners.

2. combineReducers

Each reducer only recieves a slice of the state (its name!)

const combineReducers = (reducers) => (state, action) => nextState;


const counterReducer = (state = 0, action) =>
  action.type === 'increment' ? state + 1 : state;

combineReducers({ counter: countReducer });

3. createStore

This method should be pretty trivial so far!

const createStore = (reducer, preloadedState = {}, enhancer = null) => instanceof Store;

But wait... what's an enhancer?

4. applyMiddlewares

We need to extend createStore to allow the enhancer function. But what's an enhancer? It's just a middleware!

OK, great, but like what's a middleware? It's a function that enhances dispatch (similar to how combineReducers takes a bunch of reducers and makes them act as one).

A middleware has the following signature:

const middleware = (store) => (next) => (action) => {};

When a middleware calls next, then the next middleware in the chain gets called.

This function has a weird signature!!!!!! Took us a while to finally get this one right (without cheating!)

This is where 80% of our time was spent implmenenting redux!

const applyMiddlewares = (middlewares) => (store) => (next) => (action) => {};

5. Bonus! redux-thunk

It turns out redux thunk is just a middleware! Redux thunk is used to dispatch beefed up actions that look like actions but have a lot more functionality baked in (for example: api calls, async functions, or dispatching multiple actions).

Implement a middleware that would allow you to write actions that look like this:

(keep in mind your store should still support plain object actions)

const thunkAction = () => (dispatch, getState) => {
  dispatch({ type: 'increment' });
  dispatch({ type: 'increment' });
  // simulate an api call:
  setTimeout(() => dispatch({ type: 'increment' }), 400);
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