app.js

const todoStore = new Store([]);

const add = newTodo => todos => todos.concat(newTodo);
const remove = oldTodo => todos => todos.filter(todo => todo !== oldTodo);

todoStore.subscribe(state => {
  // rerender a UI with new state
  console.log('next state', state);
})

todoStore.transition(add('hello'))    // ['hello']
todoStore.transition(add('world'))    // ['hello', 'world']
todoStore.transition(remove('hello')) // ['world']

store.js

/**
 * State Container
 *
 * A state container is a stateful store that manages the state of a given subject.
 * The store is purely functional, exposing a method `transitionState` that
 * accepts a function, that when called with the current state, calculates the
 * next state as its return value. This becomes the next state of the store.
 *
 * When the state changes, observers are notified with the next and last states.
 * Observers have an opportunity to perform UI updates.
 *
 * Example Usage:
 *
 * const store = new Store([]);
 *
 * const add = input => lastState => lastState.concat(input);
 * const remove = input => lastState => lastState.filter(i => i !== input);
 *
 * store.subscribe(state => console.log('next state': state))
 *
 * store.transition(add('hello'));   // next state: ['hello']
 * store.transition(add('world'));   // next state: ['hello', world']
 * store.transition(remove('hello)); // next state: ['world']
 * 
 *
 */
function Store (initialState) {
  var state = initialState;
  var observers = [];
  
  function transitionState (transitionFunction) {
    var lastState = state;
    var nextState = transitionFunction(state);
    if (nextState !== lastState) {
      state = nextState;
      observers.forEach(observer => observer(nextState, lastState));
    }
    return state;
  }
  
  function subscribe (observer) {
    // subscribed
    observers.push(observer);

    // unsubscribe
    return () => observers = observers.filter(o => o !== observer);
  }
  
  function getState () {
    return state;
  }
  
  return {
    transition: transitionState,
    subscribe: subscribe,
    getState: getState
  };
}