Redux (flux) in 12 lines of code with an example.

createStore.js

const createStore = reducer => {
    const subscribers = [];
    let state = reducer(undefined, {type: "__init"});
    return {
        dispatch: action => {
            state = reducer(state, action);
            subscribers.forEach(fun => fun());
        },
        subscribe: fun => subscribers.push(fun),
        getState: () => state
    }
}

example.js

// Create a simple counter reducer 
const counterReducer = (state = 0, action) => {
  switch (action.type) {
    case "INCREMENT": return state + 1;
    case "DECREMENT": return state - 1;
  }
  return state; // This is important so __init works
};

// Initialize store and dispatch
const store = createStore(counterReducer);
store.dispatch({type: "INCREMENT"});
store.dispatch({type: "INCREMENT"});
console.log(store.getState()); // Prints "2"

// Subscriber example
const render = () => console.log(`counter: ${store.getState()}`);
store.subscribe(render);
store.dispatch({type: "INCREMENT"}); // Prints "counter: 3"