如何检查本地存储中的密钥是否有值？

Question

如果没有存储到密钥中的预先存储的值，用户应该只能存储信息。我如何检查是否已有值，并提醒用户已有值？我知道我必须使用if语句。如果有值提醒用户。否则设置项目...（保存对象）。但是，我如何检查价值？

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

我想下面的代码片段可以帮助你！

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);