每次您尝试为localStorage/sessionStorage保存/加载某些内容并且您知道它是一个json对象时，请始终根据情况对其进行字符串化/解析.

each time you try to save/load something for the localStorage/sessionStorage and you know it is a json object, always stringify/parse it depending on the case.

在这里，您的代码已修复，可以工作.

here you have your code fixed to work.

注意::我尝试创建一个代码段，但由于我们无法访问沙箱的sessionStorage，因此无法正常工作.

NOTE: I tried to create a snippet but it didn't work because we can not access to the sessionStorage of a sandbox.

注意2::始终检查要解析的数据，如果存储中不存在该记录，它将返回null.

NOTE2: always check what data are you going to parse, if the record doesnt exist on the storage, it will return null.

var data = {
  "A43D": {
    "FIRSTNAME": "Mike",
    "EMAIL": "mjohns@gmail.com",
    "LASTNAME": "Johns"
  },
  "4E83": {
    "FIRSTNAME": "Steve",
    "EMAIL": "scook@gmail.com",
    "LASTNAME": "Cook"
  }
}

//here we save the item in the sessionStorage.
sessionStorage.setItem("customersData", JSON.stringify(data));


//now we retrieve the object again, but in a string form.
var customersDataString = sessionStorage.getItem("customersData");
console.log(customersDataString);

//to get the object we have to parse it.
var customersData = JSON.parse(customersDataString);
console.log(customersData);