Redux reimplemented in 12 lines of code

const createStore = (reducer) => {
  let onChange
  let state = reducer(undefined, {})
  return {
    getState: () => state,
    subscribe: (fn) => onChange = fn,
    dispatch: (action) => {
      state = reducer(state, action)
      onChange()
    }
  }
}

I like to reimplement libraries I use, this allows me to better understand how they work, above is my minimal implementation of Redux. This works with the example on the Redux homepage below.

const counter = (state = 0, action) => {
  switch (action.type) {
    case 'INCREMENT':
      return state + 1
    case 'DECREMENT':
      return state - 1
    default:
      return state
    }
}

const store = createStore(counter)

store.subscribe(() => console.log(store.getState()))

store.dispatch({type: 'INCREMENT'}) // 1
store.dispatch({type: 'INCREMENT'}) // 2
store.dispatch({type: 'DECREMENT'}) // 1

Obviously this only has a fraction of the real Redux API.