blog-post-introduction-to-redux-for-web-application-development-09.js

function createStore(reducer, preloadedState) {
  var currentState = preloadedState;
  var listeners = [];

  function getState() {
    return currentState;
  }

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

  // Dispatch action to store
  function dispatch(action) {
    currentState = reducer(currentState, action);
    
    for (var i = 0; i < listeners.length; i++) {
      listeners[i]();
    }
  }

  // Execute reducer for the first time
  dispatch({ type: '__INIT__' });

  return {
    getState: getState,
    subscribe: subscribe,
    // Expose the API
    dispatch: dispatch,
  };
}