Javascript 如果我不知道变量是否存在，如何将变量与未定义进行比较？
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How to compare variables to undefined, if I don’t know whether they exist?
提问by ncubica
In JavaScript you can declare a variable and if it's undefined, you can check variable == undefined; I know that, but how can you compare a value that you don't know yet if it's in memory?
在 JavaScript 中，您可以声明一个变量，如果是undefined，您可以检查variable == undefined；我知道，但是如果它在内存中，你怎么能比较一个你还不知道的值呢？
For example, I have a class which is created when the user clicks a button. Before this, the class is undefined?—?it doesn't exist anywhere; how can I compare it?
例如，我有一个在用户单击按钮时创建的类。在此之前，该类是未定义的？—？它在任何地方都不存在；我该如何比较？
Is there a way without using try–catch?
有没有不使用try- 的方法catch？
回答by Makram Saleh
The best way is to check the type, because undefined/null/falseare a tricky thing in JS.
So:
最好的办法是检查类型，因为undefined/ null/false在JS一件棘手的事情。所以：
if(typeof obj !== "undefined") {
// obj is a valid variable, do something here.
}
Note that typeofalways returns a string, and doesn't generate an error if the variable doesn't exist at all.
请注意，typeof始终返回一个字符串，如果该变量根本不存在，则不会生成错误。
回答by Timmy
if (obj === undefined)
{
// Create obj
}
If you are doing extensive javascript programming you should get in the habit of using === and !== when you want to make a type specific check.
如果你正在做大量的 javascript 编程，当你想要进行类型特定的检查时，你应该养成使用 === 和 !== 的习惯。
Also if you are going to be doing a fair amount of javascript, I suggest running code through JSLint http://www.jslint.comit might seem a bit draconian at first, but most of the things JSLint warns you about will eventually come back to bite you.
此外，如果您打算使用大量的 javascript，我建议通过 JSLint http://www.jslint.com运行代码，起初它可能看起来有点严厉，但 JSLint 警告您的大部分事情最终都会出现回来咬你。
回答by Delan Azabani
if (document.getElementById('theElement')) // do whatever after this
For undefined things that throw errors, test the property name of the parent object instead of just the variable name - so instead of:
对于抛出错误的未定义事物，测试父对象的属性名称而不仅仅是变量名称 - 所以而不是：
if (blah) ...
do:
做：
if (window.blah) ...
回答by Mr.Hunt
!undefinedis true in javascript, so if you want to know whether your variable or object is undefined and want to take actions, you could do something like this:
!undefined在 javascript 中是 true，所以如果你想知道你的变量或对象是否未定义并想要采取行动，你可以做这样的事情：
if(<object or variable>) {
//take actions if object is not undefined
} else {
//take actions if object is undefined
}
回答by Thevs
if (!obj) {
// object (not class!) doesn't exist yet
}
else ...
