如何在JavaScript中检查变量是否为空和/或未定义 [英] How to check if a variable is both null and /or undefined in JavaScript

问题描述

可能重复：

在JavaScript中检测未定义的对象属性

如何确定变量是'undefined'还是'null'

是否有一个标准函数来检查JavaScript中的null，undefined或blank变量？

在我的代码中，我的条件看起来像

In my code, I have a condition that looks like

if (variable !== null && variable !== undefined) {
}

但不是分两步完成，即检查它是否未定义且不为空。是否有一步检查取代此检查。

But instead of doing it in two steps, i.e checking if it is not defined and not null. Is there a one step checking that replaces this check.

推荐答案

变量不能同时为 null 和 undefined 同时。但是，您问题的直接答案是：

A variable cannot be both null and undefined at the same time. However, the direct answer to your question is:

if (variable != null)

一个 = ，而不是两个。

JavaScript规范中的抽象相等比较算法中有两个特殊条款，专门用于一个操作数 null 的情况，另一个是 undefined ，结果是 true for == 和 false for ！= 。因此，如果变量的值是 undefined ，那么它不是！= null ，如果它不是null，那么显然不是！= null 。

There are two special clauses in the "abstract equality comparison algorithm" in the JavaScript spec devoted to the case of one operand being null and the other being undefined, and the result is true for == and false for !=. Thus if the value of the variable is undefined, it's not != null, and if it's not null, it's obviously not != null.

现在，根本没有定义标识符的情况，作为 var 或 let ，作为函数参数，或作为全局上下文的属性是不同。对此类标识符的引用在运行时被视为错误。您可以尝试引用并捕获错误：

Now, the case of an identifier not being defined at all, either as a var or let, as a function parameter, or as a property of the global context is different. A reference to such an identifier is treated as an error at runtime. You could attempt a reference and catch the error:

var isDefined = false;
try {
  (variable);
  isDefined = true;
}
catch (x) {}

我个人认为是然而，有问题的做法。对于可能存在或可能存在某些其他库或某些类似情况的全局符号，您可以测试窗口属性（在浏览器JavaScript中） ：

I would personally consider that a questionable practice however. For global symbols that may or may be there based on the presence or absence of some other library, or some similar situation, you can test for a window property (in browser JavaScript):

var isJqueryAvailable = window.jQuery != null;

或

var isJqueryAvailable = "jQuery" in window;

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