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

// when createStore is called with a reducer function, it returns 4 methods. We will just create the empty methods now as a first.
function createStore(reducer){
  function getState(){
    
  }
  function dispatch(){
    
  }
  function subscribe(){
    // invoking this returned function will remove the subscriber function
    return function(){
      
    }
  }
  return {
    getState: getState,
    dispatch: dispatch,
    subscribe: subscribe,
  }
}