基本的区别是undefined和null代表不同的概念。

如果只有null可用，您将无法确定null是否被故意设置为值，或者该值是否还没有设置，除非您使用了繁琐的错误捕获:例如

var a;

a == null; // This is true
a == undefined; // This is true;
a === undefined; // This is true;

然而，如果你故意将值设置为null，与undefined的严格相等将失败，从而允许你区分null值和undefined值:

var b = null;
b == null; // This is true
b == undefined; // This is true;
b === undefined; // This is false;

查看这里的参考资料，而不是依赖于人们轻蔑地说“总之，未定义是一个特定于javascript的混乱，这让每个人都感到困惑”之类的垃圾。仅仅因为你感到困惑，并不意味着它就是一团糟。

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/undefined

这种行为也不是JavaScript特有的，它完成了布尔结果可以为真、假、未知(null)、无值(undefined)或出错(error)的广义概念。

http://en.wikipedia.org/wiki/Undefined_value

检查空值的最佳方法是

if ( testVar !== null )
{
    // do action here
}

对于undefined

if ( testVar !== undefined )
{
    // do action here
}

你可以用undefined来赋值一个变量。

testVar = undefined;
//typeof(testVar) will be equal to undefined.

我对Javascript的undefined和null有点困惑。

null的行为通常类似于其他脚本语言的带外‘null’、‘nil’或‘None’对象的概念。

另一方面，undefined是一个奇怪的JavaScript怪癖。它是一个表示带外值的单例对象，本质上是第二个相似但不同的null。它出现了:

When you call a function with fewer arguments than the arguments list in the function statement lists, the unpassed arguments are set to undefined. You can test for that with eg.: function dosomething(arg1, arg2) { if (arg2===undefined) arg2= DEFAULT_VALUE_FOR_ARG2; ... } With this method you can't tell the difference between dosomething(1) and dosomething(1, undefined); arg2 will be the same value in both. If you need to tell the difference you can look at arguments.length, but doing optional arguments like that isn't generally very readable. When a function has no return value;, it returns undefined. There's generally no need to use such a return result. When you declare a variable by having a var a statement in a block, but haven't yet assigned a value to it, it is undefined. Again, you shouldn't really ever need to rely on that. The spooky typeof operator returns 'undefined' when its operand is a simple variable that does not exist, instead of throwing an error as would normally happen if you tried to refer to it. (You can also give it a simple variable wrapped in parentheses, but not a full expression involving a non-existant variable.) Not much use for that, either. This is the controversial one. When you access a property of an object which doesn't exist, you don't immediately get an error like in every other language. Instead you get an undefined object. (And then when you try to use that undefined object later on in the script it'll go wrong in a weird way that's much more difficult to track down than if JavaScript had just thrown an error straight away.) This is often used to check for the existence of properties: if (o.prop!==undefined) // or often as truthiness test, if (o.prop) ...do something... However, because you can assign undefined like any other value: o.prop= undefined; that doesn't actually detect whether the property is there reliably. Better to use the in operator, which wasn't in the original Netscape version of JavaScript, but is available everywhere now: if ('prop' in o) ...

总之，undefined是javascript特有的混乱，这让所有人都感到困惑。除了可选函数参数之外，JS没有其他更优雅的机制，应该避免使用undefined。它不应该成为语言的一部分;null对于(2)和(3)来说可以很好地工作，(4)是一个错误的特性，因为在一开始JavaScript没有异常。

if (!testvar)实际上做什么?它是否测试为undefined和null或只是undefined?

这样的“真实性”测试检查假，未定义，null, 0, NaN和空字符串。但在这种情况下，它确实是没有定义的。在我看来，它应该更明确地说，如果(testvar!==undefined)。

一旦一个变量被定义，我可以把它清除回未定义(因此删除变量)。

你当然可以给它赋值为undefined，但这不会删除变量。只有delete对象。属性操作符实际上删除了一些东西。

Delete实际上指的是属性，而不是变量本身。浏览器允许你直接删除变量，但这不是一个好主意，在ECMAScript第五版的严格模式下也行不通。如果你想释放对某个东西的引用，这样它就可以被垃圾收集，更常见的说法是variable= null。

我可以传递undefined作为参数吗?

Yes.

for if (something)和if (!something)通常用于检查某物是否有定义。例如:

if (document.getElementById)

标识符被转换为布尔值，因此undefined被解释为false。当然，还有其他值(如0和")也被解释为false，但要么标识符不应该有这样的值，要么您乐于将这样的值视为未定义。

Javascript有一个删除操作符，可以用来删除对象的成员。根据变量的作用域(即，如果它是全局的或非全局的)，你可以删除它，使其未定义。

没有可以用作未定义文字的未定义关键字。你可以在函数调用中省略形参以使它们未定义，但这只能通过向函数发送更少的形参来使用，你不能在中间省略形参。

要回答您的第一个问题，not操作符(!)将强制将给定的值转换为布尔值。因此null, 0, false, NaN和“”(空字符串)都将显示为false。

如何在命令行中设置一个变量为undefined:

在Ubuntu 12.10上的Java自带的js javascript命令行终端中将一个变量设置为undefined。

el@defiant ~ $ js

js> typeof boo
"undefined"

js> boo
typein:2: ReferenceError: boo is not defined

js> boo=5
5

js> typeof boo
"number"

js> delete(boo)
true

js> typeof boo
"undefined"

js> boo
typein:7: ReferenceError: boo is not defined

如果你在javascript中设置一个变量为undefined:

把这个放到myjs.html中:

<html>
<body>
    <script type="text/JavaScript">
        document.write("aliens: " + aliens);
        document.write("typeof aliens: " + (typeof aliens));
        var aliens = "scramble the nimitz";
        document.write("found some aliens: " + (typeof aliens));
        document.write("not sayings its aliens but... " + aliens);
        aliens = undefined;
        document.write("aliens deleted");
        document.write("typeof aliens: " + (typeof aliens));
        document.write("you sure they are gone? " + aliens);
    </script>
</body>
</html>

输出如下:

aliens: undefined
typeof aliens: undefined
found some aliens: string
not sayings its aliens but... scramble the nimitz
aliens deleted
typeof aliens: undefined
you sure they are gone? undefined

警告!当你把你的变量设置为undefined时，你就是在把你的变量设置为另一个变量。如果一些鬼鬼祟祟的人运行undefined = 'rm -rf /';然后，无论何时将变量设置为undefined，都将收到该值。

您可能想知道我如何在开始时输出未定义的值异形，并使其仍然运行。这是因为javascript提升:http://www.adequatelygood.com/JavaScript-Scoping-and-Hoisting.html