indexedDB中IDBDatabase对象已生成但不能调用createObjectStore方法创建命名空间，提示Uncaught TypeError: Cannot read property 'createObjectStore' of undefined。
下面是代码

    var request = indexedDB.open("jusing"),
        db;
        user = {
            username: "jusing",
            age: 18,
            gender: "male",
            handsome: true
        }
    request.onsuccess = function() {
        db = event.target.result;
    }
    var store = db.createObjectStore("users", {keyPath: "username"});
    store.add(user);