如何检查本地存储中的键值？

Question

The user should only be able to store information if there is not a preexisting value stored to the key. How can i check if there is already a value, and alert the user that there is already a value? I know i have to use an if statement. If there is a value alert the user. Else set item...(save the object). But how do i check for the value?

function addEvent() {
    var announcement = {
        title: document.getElementById('title').value,
        group: document.getElementById('group').value,
        author: document.getElementById('author').value,
        type: document.getElementById('type').value,
        date: document.getElementById('date').value,
        time: document.getElementById('time').value,
        gender: document.getElementById('gender').value,
        grade: document.getElementById('grade').value,
        message: document.getElementById('message').value
    };
    var objS = JSON.stringify(announcement);

    localStorage.setItem('announcement', objS);

    alert('Announcement successfully stored!');
}

function showA() {
    var getObj = localStorage.getItem('announcement');
    var objP = JSON.parse(getObj);

    document.getElementById('showAnnouncement').innerHTML = "Announcement"+"<br/>Title:"+objP.title+"<br/>Group:"+objP.group+"<br />Author:"+objP.author+"<br />Type:"+objP.type+"<br/>Date:"+objP.date+"<br />Time:"+objP.time+"<br/>Gender:"+objP.gender+"<br/>Grades:"+objP.grade+"<br />Message:"+objP.message;
}

Answer 1

I think the following code snippet can help you!

var getObj = localStorage.getItem('announcement');
var obj = JSON.parse(getObj);
for (key in obj) {
  if(obj[key] !== ''){
   alert(key + 'is existed');
  }else{
   var newValue = document.getElementById(key).value;
   obj[key] = newValue == '' ? '' : newValue;
  }
}
var newObjS = JSON.stringify(obj);

localStorage.setItem('announcement', newObjS);