如何确定页面中是否定义了JavaScript变量？ [重复]

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JavaScript check if variable exists (is defined/initialized) 29 answers
JavaScript检查变量是否存在（已定义/初始化）29个答案

How can i check in JavaScript if a variable is defined in a page? Suppose I want to check if a variable named "x" is defined in a page, if I do if(x != null), it gives me an error.

如果在页面中定义变量，我如何签入JavaScript？假设我想检查一个名为“x”的变量是否在页面中定义，如果我这样做（x！= null），它会给我一个错误。

7 个解决方案

#1

151

I got it to work using if (typeof(x) != "undefined")

我用if（typeof（x）！=“undefined”）让它工作

#2

To avoid accidental assignment, I make a habit of reversing the order of the conditional expression:

为了避免意外分配，我养成了颠倒条件表达式顺序的习惯：

if ('undefined' !== typeof x) {

#3

The typeof operator, unlike the other operators, doens't throws a ReferenceError exception when used with an undeclared symbol, so its safe to use...

与其他运算符不同，typeof运算符在与未声明的符号一起使用时不会抛出ReferenceError异常，因此可以安全使用...

if (typeof a != "undefined") {
    a();
}

#4

You can do that with:

你可以这样做：

if (window.x !== undefined) { // You code here }

if（window.x！== undefined）{//你在这里编码}

#5

As others have mentioned, the typeof operator can evaluate even an undeclared identifier without throwing an error.

正如其他人所提到的，typeof运算符甚至可以在不抛出错误的情况下评估未声明的标识符。

alert (typeof sdgfsdgsd);

Will show "undefined," where something like

将显示“未定义”，类似的东西

alert (sdgfsdgsd);

will throw a ReferenceError.

将抛出一个ReferenceError。

#6

Assuming your function or variable is defined in the typical "global" (see: window's) scope, I much prefer:

假设您的函数或变量是在典型的“全局”（参见：窗口）范围内定义的，我更喜欢：

if (window.a != null) {
   a();
}

or even the following, if you're checking for a function's existence:

如果您正在检查函数是否存在，甚至是以下内容：

if (window.a) a();

#7

-3

try to use undefined

尝试使用undefined

if (x !== undefined)

This is how checks for specific Browser features are done.

这是检查特定浏览器功能的方法。

#1

151