在JavaScript中什么时候使用null或未定义?(复制)-WhenisnullorundefinedusedinJavaScript?[duplicate]

79  

The DOM methods getElementById(), nextSibling(), childNodes[n], parentNode() and so on return null (defined but having no value) when the call does not return a node object.

DOM方法getElementById()、nextSibling()、childNodes[n]、parentNode()等在调用不返回节点对象时返回null(定义但没有值)。

The property is defined, but the object it refers to does not exist.

属性被定义，但是它引用的对象不存在。

This is one of the few times you may not want to test for equality-

这是少数几次你可能不想测试平等-

if(x!==undefined) will be true for a null value

对于空值，if(x!= undefined)为真

but if(x!= undefined) will be true (only) for values that are not either undefined or null.

但如果(x != undefined)将对非无定义或null的值为true。

118  

I find that some of these answers are vague and complicated, I find the best way to figure out these things for sure is to just open up the console and test it yourself.

我发现其中的一些答案是模糊的和复杂的，我发现确定这些事情的最好方法是打开控制台并自己测试它。

var x;x == null            // truex == undefined       // truex === null           // falsex === undefined      // truevar y = null;y == null            // truey == undefined       // truey === null           // truey === undefined      // falsetypeof x             // 'undefined'typeof y             // 'object'var z = {abc: null};z.abc == null        // truez.abc == undefined   // truez.abc === null       // truez.abc === undefined  // falsez.xyz == null        // truez.xyz == undefined   // truez.xyz === null       // falsez.xyz === undefined  // truenull = 1;            // throws error: invalid left hand assignmentundefined = 1;       // works fine: this can cause some problems

So this is definitely one of the more subtle nuances of Javascript. As you can see, you can override the value of undefined, making it somewhat unreliable compared to null. Using the == operator, you can reliably use null and undefined interchangeably as far as I can tell. However, because of the advantage that null cannot be redefined, I might would use it when using ==.

因此，这绝对是Javascript更微妙的细微差别之一。如您所见，您可以重写undefined的值，使它与null相比有些不可靠。使用==运算符，就我所知，您可以可靠地使用null和undefined作为交换。但是，由于null不能重新定义的优点，我可能在使用==时使用它。

For example, variable != null will ALWAYS return false if variable is equal to either null or undefined, whereas variable != undefined will return false if variable is equal to either null or undefined UNLESS undefined is reassigned beforehand.

例如，如果变量= null或undefined，则变量!= null将返回false，而如果变量= null或undefined则返回false，除非事先重新分配undefined。

You can reliably use the === operator to differentiate between undefined and null, if you need to make sure that a value is actually undefined (rather than null).

如果您需要确保一个值实际上是未定义的(而不是null)，您可以可靠地使用=== =运算符来区分未定义和null。

According to the ECMAScript 5 spec:

• Both Null and Undefined are two of the six built in types.
• Null和Undefined都是构建在类型中的六个类型中的两个。

4.3.9 undefined value

4.3.9未定义的值

primitive value used when a variable has not been assigned a value

当变量未被赋值时使用的原始值

4.3.11 null value

4.3.11空值

primitive value that represents the intentional absence of any object value

表示有意没有任何对象值的原始值。

49  

You get undefined for the various scenarios:

对于各种场景，你没有定义:

You declare a variable with var but never set it.

使用var声明一个变量，但从不设置它。

var foo; alert(foo); //undefined.

You attempt to access a property on an object you've never set.

您试图访问从未设置的对象上的属性。

var foo = {};alert(foo.bar); //undefined

You attempt to access an argument that was never provided.

您试图访问一个从未提供的参数。

function myFunction (foo) {  alert(foo); //undefined.}

As cwolves pointed out in a comment on another answer, functions that don't return a value.

正如cwolf在另一个答案的评论中指出的，函数不返回值。

function myFunction () {}alert(myFunction());//undefined

A null usually has to be intentionally set on a variable or property (see comments for a case in which it can appear without having been set). In addition a null is of type object and undefined is of type undefined.

通常必须在变量或属性上故意设置null(在没有设置的情况下可以出现，请参阅注释)。另外，一个null类型的对象和未定义的类型是未定义的。

I should also note that null is valid in JSON but undefined is not:

我还要注意，null在JSON中是有效的，但未定义的不是:

JSON.parse(undefined); //syntax errorJSON.parse(null); //null

9  

I might be missing something, but afaik, you get undefined only

我可能漏掉了一些东西，但是，你只能得到不确定的结果

Update: Ok, I missed a lot, trying to complete:

更新:好的，我错过了很多，试图完成:

You get undefined...

你未定义……

... when you try to access properties of an object that don't exist:

…当您试图访问一个不存在的对象的属性时:

var a = {}a.foo // undefined

... when you have declared a variable but not initialized it:

…当您声明了一个变量但没有初始化它时:

var a;// a is undefined

... when you access a parameter for which no value was passed:

…当您访问一个没有传递值的参数时:

function foo (a, b) {    // something}foo(42); // b inside foo is undefined

... when a function does not return a value:

…当一个函数不返回值时:

function foo() {};var a = foo(); // a is undefined

It might be that some built-in functions return null on some error, but if so, then it is documented. null is a concrete value in Javascript, undefined is not.

一些内置函数可能会在某些错误上返回null，但如果是，则会对其进行文档化。null是Javascript中的一个具体值，未定义的则不是。

Normally you don't need to distinguish between those. Depending on the possible values of a variable, it is sufficient to use if(variable) to test whether a value is set or not (both, null and undefined evaluate to false).

通常你不需要区分它们。根据变量的可能值，使用if(变量)来测试是否设置了一个值就足够了(null和undefined evaluate to false)。

Also different browsers seem to be returning these differently.

同样，不同的浏览器返回这些信息的方式也不同。

Please give a concrete example.

请给出一个具体的例子。

3  

Regarding this topic the specification (ecma-262) is quite clear

关于这个主题，规范(ecma-262)非常清楚

I found it really useful and straightforward, so that I share it:- Here you will find Equality algorithm- Here you will find Strict equality algorithm

我发现它非常有用和简单，所以我分享它:-在这里你会发现平等算法-在这里你会发现严格的平等算法。

I bumped into it reading "Abstract equality, strict equality, and same value" from mozilla developer site, section sameness.

我从mozilla开发者网站“相似度”中偶然发现了“抽象的平等、严格的平等和相同的价值”。

I hope you find it useful.

我希望你觉得它有用。

0  

A property, when it has no definition, is undefined. null is an object. It's type is null. undefined is not an object, its type is undefined.

没有定义的属性是未定义的。零是一个对象。它的类型为空。undefined不是对象，它的类型是undefined。

This is a good article explaining the difference and also giving some examples.

这是一篇很好的文章，解释了其中的差异，并给出了一些例子。

null vs undefined

零和未定义的