如何检查javascript中是否存在变量？ [英] how to check if a variable exist in javascript?

问题描述

我不是在询问变量是否未定义或是否为 null 。我想检查变量是否存在。这可能吗？

I am not asking if the variable is undefined or if it is null. I want to check if the variable exists or not. Is this possible?

推荐答案

typeof 技术不起作用，因为它们没有区分何时根本没有声明变量，何时声明变量但未赋值，或声明并设置等于undefined。

The typeof techniques don't work because they don't distinguish between when a variable has not been declared at all and when a variable has been declared but not assigned a value, or declared and set equal to undefined.

但是如果你尝试使用未在if条件中声明的变量（或在赋值的右侧），您会收到错误。所以这应该有效：

But if you try to use a variable that hasn't been declared in an if condition (or on the right-hand side of an assignment) you get an error. So this should work:

var exists = true;
try {
    if (someVar)
        exists = true;
} catch(e) { exists = false; }
if (exists)
   // do something - exists only == true if someVar has been declared somewhere.

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