createStore with no slice.js

function createStore(reducer, initialState) {
  var currentReducer = reducer;
  var currentState = initialState;
  var listeners = [];
  var isDispatching = false;

  function getState() {
    return currentState;
  }

  function subscribe(listener) {
    listeners.push(listener);

    return function unsubscribe() {
      var index = listeners.indexOf(listener);
      listeners.splice(index, 1);
    };
  }

  function dispatch(action) {
    if (isDispatching) {
      throw new Error('Reducers may not dispatch actions.');
    }

    try {
      isDispatching = true;
      currentState = currentReducer(currentState, action);
    } finally {
      isDispatching = false;
    }

    //listeners.slice().forEach(listener => listener());
    
    listeners.forEach(listener => {
    	// this will prevent listeners to run to completion 
      //if prev listener unsubscribes the next listener
    	listener();
    });
    return action;
  }

  dispatch({
    type: '@@redux/INIT'
  });

  return {
    dispatch,
    subscribe,
    getState
  };
}


const counter = (state = 0, action) => {
  if (action.type === 'inc') {
    return state + 1;
  } else {
    return state;
  }
}

const store = createStore(counter);


unsubscribe1 = store.subscribe(() => {
	console.log('listener 1');
  const currentState = store.getState();
  if (currentState === 2) {
    unsubscribe2();
  }
})
const unsubscribe2 = store.subscribe(() => console.log('listener 2'));


store.dispatch({
  type: 'inc'
});
store.dispatch({
  type: 'inc'
});

// if we remove .slice when running listeners.forEach we should get in console
// listener 1
// listener 2
// listener 1

// if we use .slice though, we will get
// listener 1
// listener 2
// listener 1
// listener 2

// meaning .slice will let the listeners run to completion