与 null 比较 - !== vs != 在 JavaScript 中
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Comparing to null - !== vs != in JavaScript
提问by David
Ok, so I installed Linter on my Sublime editor while working on my node.js app. One of the things that it caught said that I should always use !== to compare an object to null (I usually use != ).
好的，所以我在我的 node.js 应用程序上工作时在我的 Sublime 编辑器上安装了 Linter。它捕获的一件事说我应该始终使用 !== 将对象与 null 进行比较（我通常使用 != ）。
So I changed it...but then I noticed that the !== wasn't working.
所以我改变了它......但后来我注意到 !== 不起作用。
I have this scenario:
我有这个场景：
var x = null;
if (x !== null)
console.log('x is not equal to null');
When I use the !== the console printed that line even though it was obviously not true. When I switched it back to != it behaved normally.
当我使用 !== 时，控制台打印了该行，即使它显然不是真的。当我将其切换回 != 时，它表现正常。
So my question is, why is linter telling me to use !== if it doesn't do what I want it to...
所以我的问题是，为什么 linter 告诉我使用 !== 如果它没有做我想要的......
I know I am missing something.
我知道我错过了一些东西。
UPDATEOk, so it may be a bit more complicated than I originally thought. In my real code I was using !== with the node.js GLOBAL object.
更新好的，所以它可能比我原先想象的要复杂一些。在我的实际代码中，我将 !== 与 node.js GLOBAL 对象一起使用。
console.log('Global User: ' + GLOBAL.User);
if (GLOBAL.User != null)
{
console.log('User is not null');
}
The console line prints even when GLOBAL.User is null...
即使 GLOBAL.User 为空，控制台行也会打印...
Perhaps this object is special?
也许这个对象很特别？
Update 2
更新 2
Ok, so after reading through the comments and looking at my code, I have learned that !== can have issues if the object is undefined rather than null (see this post: Why is null an object and what's the difference between null and undefined?).
好的，所以在阅读评论并查看我的代码后，我了解到 !== 如果对象未定义而不是 null 可能会出现问题（请参阅这篇文章：为什么 null 是一个对象以及 null 和 undefined 之间的区别是什么？）。
So in my case, my global variable could be, depending on when this method is called, undefined, null, or full of data. I am going to go back and update my code so that it is never undefined and then !== will work consistently.
因此，在我的情况下，我的全局变量可能是未定义的、空的或充满数据的，具体取决于调用此方法的时间。我将返回并更新我的代码，以便它永远不会未定义，然后 !== 将始终如一地工作。
Thanks for the help!
谢谢您的帮助！
Thanks, David
谢谢，大卫
回答by loganfsmyth
Your global.Useris undefined, not null. When using ==they evaluate to equal, but with ===the items you are comparing need to be the same type. undefinedhas the type undefinedand nullhas the type object.
你的global.User是undefined，不是null。使用时，==它们评估为相等，但与===您比较的项目需要是相同的类型。undefined有类型undefined并且null有类型object。
undefinedand nullare very similar, but they generally mean two very different things. Usually undefinedis the result when something has had no value assigned to it, whereas nullhas a value, and the value is explicitly set to "nothing".
undefined和null非常相似，但它们通常意味着两个非常不同的东西。通常undefined是当某物没有分配给它的值时的结果，而null有一个值，并且该值显式设置为“无”。
回答by plalx
The only value that doesn't equal itself in JavaScript is NaN. If null === nullis false, then your JavaScript engine has serious problems ;)
JavaScript 中唯一不等于自身的值是NaN. 如果null === null是false，那么您的 JavaScript 引擎存在严重问题；)
To make sure your conditional statement is well written, always use the braces.
为了确保您的条件语句写得很好，请始终使用大括号。
var x = null;
if (x !== null) {
console.log('x is not equal to null');
}
回答by oshribr
It is even more simple
更简单
var x = null;
if (x) 6
if (!x) 7
the result is
结果是
undefined
7
