javascript 检查未定义的值不起作用？
声明：本页面是StackOverFlow热门问题的中英对照翻译，遵循CC BY-SA 4.0协议，如果您需要使用它，必须同样遵循CC BY-SA许可，注明原文地址和作者信息，同时你必须将它归于原作者（不是我）：StackOverFlow
原文地址: http://stackoverflow.com/questions/6295665/
Warning: these are provided under cc-by-sa 4.0 license. You are free to use/share it, But you must attribute it to the original authors (not me):
StackOverFlow
Checking undefined value not working?
提问by VansFannel
I have the following javascript code:
我有以下 javascript 代码：
var currentIds = localStorage.getItem('currentPairsIds');
if ((typeof currentIds === "undefined") ||
(currentIds == null))
$.myNameSpace.currentIDs = new Array(3);
else
$.myNameSpace.currentIDs = currentIds.Split(',');
I'm debugging with Firebug and although currentIdshasn't got any value it always executes elsestatement.
我正在用 Firebug 进行调试，虽然currentIds没有任何价值，但它总是执行else语句。
UPDATE:
I'm getting this value from HTML5 storage.
更新：
我从 HTML5 存储中获取此值。
What am I doing wrong?
我究竟做错了什么？
回答by VansFannel
This is how I have solved my problem:
这是我如何解决我的问题：
var currentIds = localStorage.getItem('currentPairsIds');
if ((currentIds === undefined) ||
(currentIds == null) || (currentIds == "undefined"))
$.myNameSpace.currentIDs = new Array(3);
else
$.myNameSpace.currentIDs = currentIds.split(',');
localStorage.getItem('currentPairsIds');returns the string "undefined".
localStorage.getItem('currentPairsIds');返回字符串"undefined"。
There is another error in Split()function. The right version is without any capital letter.
功能上还有一个错误Split()。正确的版本没有任何大写字母。
回答by maerics
I would use a direct comparison instead of the notoriously odd "typeof" operator:
我会使用直接比较而不是臭名昭著的“typeof”运算符：
if ((currentIds === undefined) || (currentIds === null)) {
//...
回答by Juan Mendes
It's not working because localStorage.getItemreturns nullif the item is not defined, it does not return undefinedhttp://dev.w3.org/html5/webstorage/#dom-storage-getitem
这不是因为工作localStorage.getItem的回报null，如果没有定义的项目，它没有返回undefinedhttp://dev.w3.org/html5/webstorage/#dom-storage-getitem
Example: http://jsfiddle.net/mendesjuan/Rsu8N/1/
示例：http: //jsfiddle.net/mendesjuan/Rsu8N/1/
var notStored = localStorage.getItem('ffff');
alert(notStored); // null
alert(typeof notStored); // object, yes, null is an object.
Therefore you should just be testing
因此你应该只是测试
alert(notStored === null);
回答by Andrew
[Edit Edit Edit Edit :P]
[编辑编辑编辑编辑：P]
currentIds = "undefined"
implies
暗示
typeof currentIds == "String"
Also see, Detecting Undefined, === isn't necessary for string comparison.
另请参阅，检测未定义，=== 不是字符串比较所必需的。
回答by Nik Sumeiko
I think you have to make checking for undefinedcomparing with ==instead of ===.
Example:
我认为您必须检查undefined与==而不是===. 例子：
typeof currentIds == "undefined"
This will make sure, the variable is really undefined or not.
这将确保变量是否真的未定义。
回答by bthn
In my case LocalStorage.getItem() was converting it to "undefined" as string. I also needed to check if it s "undefined" as a string.
在我的情况下 LocalStorage.getItem() 将其转换为“未定义”作为字符串。我还需要检查它是否“未定义”为字符串。
var myItem = LocalStorage.getItem('myItem');
if(myItem != "undefined" && myItem != undefined && myItem != null){
...
}
