javascript 检查JS对象是否存在
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check if JS object exists
提问by Naigel
I need to discover at runtime if an object with a given name exists, but this name is stored in a variable as a string.
我需要在运行时发现具有给定名称的对象是否存在，但该名称作为字符串存储在变量中。
For example, in my Javascript I have an object named test, then at runtime I write the word "text" in a box, and I store this string in a variable, let's name it input. How can I check if a variable named with the string stored in input variable exist?
例如，在我的 Javascript 中，我有一个名为 test 的对象，然后在运行时我将单词“text”写入一个框中，并将此字符串存储在一个变量中，让我们将其命名为 input。如何检查以输入变量中存储的字符串命名的变量是否存在？
回答by Alex K.
If the object is in the global scope;
如果对象在全局范围内；
var exists = (typeof window["test"] !== "undefined");
var exists = (typeof window["test"] !== "undefined");
回答by Amadan
If you're in a browser (i.e. not Node):
如果您使用浏览器（即不是 Node）：
var varName = 'myVar';
if (typeof(window[varName]) == 'undefined') {
console.log("Variable " + varName + " not defined");
} else {
console.log("Variable " + varName + " defined");
}
However, let me say that I would find it veryhard to justify writing this code in a project. You should know what variables you have, unless you expect people to write plugins to your code or something.
然而，让我说，我会觉得很很难证明在一个项目中编写这些代码。您应该知道您拥有哪些变量，除非您希望人们为您的代码编写插件或其他内容。
回答by Niet the Dark Absol
if( window[input])...
All global variables are properties of the windowobject. []notation allows you to get them by an expression.
所有全局变量都是window对象的属性。[]符号允许您通过表达式获取它们。
回答by Zach
If you want to see whether it exists as a global variable, you can check for a member variable on the window object. window is the global object, so its members are globals.
如果要查看它是否作为全局变量存在，可以在window 对象上检查成员变量。window 是全局对象，所以它的成员是全局对象。
if (typeof window['my_objname_str'] != 'undefined')
{
// my_objname_str is defined
}
回答by quevedo
I'm writing my code but not allways can know if a method exists. I load my js files depending on requests. Also I have methods to bind objects "via" ajax responses and need to know, if they must call the default callbacks or werther or not particular a method is available. So a test like :
我正在编写我的代码，但并非总能知道方法是否存在。我根据请求加载我的 js 文件。此外，我有“通过”ajax 响应绑定对象的方法，并且需要知道，如果它们必须调用默认回调或不特定的方法可用。所以像这样的测试：
function doesItExist(oName){
var e = (typeof window[oName] !== "undefined");
return e;
}
is suitable for me.
适合我。
