blocks|key|845253|text|未定义是指事物的概念不存在；它没有类型，并且以前从未在该作用域中被引用过；null是已知事物存在但没有值的地方。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|845254|entityMap^0|0^^$0|@$1|2|3|4|5|6|7|D|8|@]|9|@]|A|$]]|$1|B|3|-4|5|6|7|E|8|@]|9|@]|A|$]]]|C|$]]

undefined is where no notion of the thing exists; it has no type, and it's never been referenced before in that scope; null is where the thing is known to exist, but it has no value.

blocks|key|629844|text|在JavaScript中，值null表示有意不存在任何对象值。null表示缺少标识，表示变量指向无对象。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|629845|全局undefined属性表示原始值undefined。undefined是自动分配给变量的原始值。undefined的意思是说引用不存在。|629846|entityMap^0|E|4|V|4|0|2|9|I|9|S|9|1E|9|0^^$0|@$1|2|3|4|5|6|7|J|8|@$9|K|A|L|B|C]|$9|M|A|N|B|C]]|D|@]|E|$]]|$1|F|3|G|5|6|7|O|8|@$9|P|A|Q|B|C]|$9|R|A|S|B|C]|$9|T|A|U|B|C]|$9|V|A|W|B|C]]|D|@]|E|$]]|$1|H|3|-4|5|6|7|X|8|@]|D|@]|E|$]]]|I|$]]

In JavaScript, the value <code>null</code> represents the intentional absence of any object value. <code>null</code> expresses a lack of identification, indicating that a variable points to no object.
The global <code>undefined</code> property represents the primitive value <code>undefined</code>.
<code>undefined</code> is a primitive value automatically assigned to variables.
<code>undefined</code> is meant to say that the reference is not existing.

blocks|key|629731|text|我完全不同意使用null或未定义的用法是不必要的。未定义是使整个原型链过程保持活力的东西。所以只有null的编译器不能检查这个属性是否等于null，或者它没有在端点原型中定义。在其他动态类型语言中(F.E.Python)如果你想访问未定义的属性，它会抛出异常，但对于基于原型的语言，编译器也应该检查父原型，这里是未定义的最需要的地方。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|629732|使用null的全部含义是将变量或属性与对象绑定，该对象是单例的，并且具有空的含义，并且null的使用也具有性能目的。这两个代码有不同的执行时间。|629733|var+p1+=+function(){this.value+=+1};
var+big_array+=+new+Array(100000000).fill(1).map((x,+index)=>{
++++p+=+new+p1();
++++if(index+>+50000000){
+++++++p.x+=+"some_string";
++++}

++++return+p;
});
big_array.reduce((sum,+p)=>+sum+%2B+p.value,+0)

var+p2+=+function(){this.value+=+1,+p.x+=+null};
var+big_array+=+new+Array(100000000).fill(1).map((x,+index)=>{
++++p+=+new+p2();
++++if(index+>+50000000){
+++++++p.x+=+"some_string";
++++}

++++return+p;+
});
big_array.reduce((sum,+p)=>+sum+%2B+p.value,+0)|code-block|syntax|javascript|629734|entityMap^0|0|0|0^^$0|@$1|2|3|4|5|6|7|K|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|L|8|@]|9|@]|A|$]]|$1|D|3|E|5|F|7|M|8|@]|9|@]|A|$G|H]]|$1|I|3|-4|5|6|7|N|8|@]|9|@]|A|$]]]|J|$]]

I completely disagree that usage null or undefined is unnecessary.
undefined is thing which keeping alive whole prototype chaining process.
So compiler only with null can't check if this property just equal to null, or its not defined in endpoint prototype. In other dynamic typed languages(f.e. Python) it throws exception if you want access to not defined property, but for prototype-based languages compiler should also check parent prototypes and here are the place when undefined need most.

Whole meaning of using null is just bind variable or property with object which is singleton and have meaning of emptiness,and also null usage have performance purposes. This 2 code have difference execution time. 

<pre><code>var p1 = function(){this.value = 1};
var big_array = new Array(100000000).fill(1).map((x, index)=&gt;{
 p = new p1();
 if(index &gt; 50000000){
 p.x = "some_string";
 }

 return p;
});
big_array.reduce((sum, p)=&gt; sum + p.value, 0)

var p2 = function(){this.value = 1, p.x = null};
var big_array = new Array(100000000).fill(1).map((x, index)=&gt;{
 p = new p2();
 if(index &gt; 50000000){
 p.x = "some_string";
 }

 return p; 
});
big_array.reduce((sum, p)=&gt; sum + p.value, 0)
</code></pre>

blocks|key|629796|text|这里有一个原因：var+undefined+=+1是合法的javascript，但是var+null+=+1是一个语法错误。不同之处在于，null是一种语言关键字，而undefined由于某种原因不是。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|629797|如果您的代码依赖于与undefined的比较，就好像它是一个关键字(if+(foo+==+undefined)+--这是一个很容易犯的错误)，那么它只能工作，因为没有人定义过具有该名称的变量。所有这些代码都容易受到使用该名称意外或恶意定义全局变量的人的攻击。当然，我们都知道在javascript中意外定义全局变量是完全不可能的……|629798|entityMap^0|8|H|16|C|1X|4|2B|9|0|A|9|Y|L|0^^$0|@$1|2|3|4|5|6|7|J|8|@$9|K|A|L|B|C]|$9|M|A|N|B|C]|$9|O|A|P|B|C]|$9|Q|A|R|B|C]]|D|@]|E|$]]|$1|F|3|G|5|6|7|S|8|@$9|T|A|U|B|C]|$9|V|A|W|B|C]]|D|@]|E|$]]|$1|H|3|-4|5|6|7|X|8|@]|D|@]|E|$]]]|I|$]]

Here's a reason: <code>var undefined = 1</code> is legal javascript, but <code>var null = 1</code> is a syntax error. The difference is that <code>null</code> is a language keyword, while <code>undefined</code> is, for some reason, not.

If your code relies on comparisons to <code>undefined</code> as if it's a keyword (<code>if (foo == undefined)</code> -- a very easy mistake to make) that only works because nobody has defined a variable with that name. All that code is vulnerable to someone accidentally or maliciously defining a global variable with that name. Of course, we all know that accidentally defining a global variable is totally impossible in javascript...

blocks|key|845525|text|根据我们最近遇到的一次破坏，下面的示例说明了为什么我更喜欢使用undefined而不是null，除非有其他特定的原因：|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|845526|function+myfunc+(myArg)+{
++++if+(typeof+myArg+===+'string')+{
++++++++console.log('a',+myArg);
++++}+else+if+(typeof+abc+===+'object')+{
++++++++console.log('b',+myArg);
++++++++if+(myArg.id)+{
+++++++++++console.log('myArg+has+an+id');
++++++++}+else+{
+++++++++++console.log('myArg+has+an+id');
++++++++}
++++}+else+{
++++++++console.log('no+value');
++++}
}|code-block|syntax|javascript|845527|下面的值可以很好地发挥作用：|845528|'abc'
{}
undefined
{+id:+'xyz'+}|845529|另一方面，假设null和undefined在这里是等价的，这打破了代码。原因是null是object类型，而as+undefined是undefined类型。所以这里的代码中断了，因为你不能在null上测试成员。|845530|我见过大量代码外观相似的案例，null只是在自找麻烦：|845531|if+(typeof+myvar+===+'string')+{
++console.log(myvar);
}+else+if+(typeof+myvar+===+'object')+{
++console.log(myvar.id);++
}|845532|这里的修复方法是显式地测试null：|845533|if+(typeof+myvar+===+'string')+{
++console.log(myvar);
}+else+if+(myvar+!==+null+&&+typeof+myvar+===+'object')+{
++console.log(myvar.id);++
}|845534|我的态度是针对一种语言的弱点和该语言程序员的典型行为进行编码，因此这里的哲学是默认使用“未定义的”。|845535|entityMap^0|V|9|17|4|0|0|0|0|7|4|C|9|13|4|18|6|1L|9|1V|9|2O|4|0|F|4|0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|12|8|@$9|13|A|14|B|C]|$9|15|A|16|B|C]]|D|@]|E|$]]|$1|F|3|G|5|H|7|17|8|@]|D|@]|E|$I|J]]|$1|K|3|L|5|6|7|18|8|@]|D|@]|E|$]]|$1|M|3|N|5|H|7|19|8|@]|D|@]|E|$I|J]]|$1|O|3|P|5|6|7|1A|8|@$9|1B|A|1C|B|C]|$9|1D|A|1E|B|C]|$9|1F|A|1G|B|C]|$9|1H|A|1I|B|C]|$9|1J|A|1K|B|C]|$9|1L|A|1M|B|C]|$9|1N|A|1O|B|C]]|D|@]|E|$]]|$1|Q|3|R|5|6|7|1P|8|@$9|1Q|A|1R|B|C]]|D|@]|E|$]]|$1|S|3|T|5|H|7|1S|8|@]|D|@]|E|$I|J]]|$1|U|3|V|5|6|7|1T|8|@]|D|@]|E|$]]|$1|W|3|X|5|H|7|1U|8|@]|D|@]|E|$I|J]]|$1|Y|3|Z|5|6|7|1V|8|@]|D|@]|E|$]]|$1|10|3|-4|5|6|7|1W|8|@]|D|@]|E|$]]]|11|$]]

Based on a recent breakage we ran into, the example below shows why I prefer to use <code>undefined</code> over <code>null</code>, unless there is a specific reason to do otherwise:
<pre class="lang-js prettyprint-override"><code>function myfunc (myArg) {
 if (typeof myArg === 'string') {
 console.log('a', myArg);
 } else if (typeof abc === 'object') {
 console.log('b', myArg);
 if (myArg.id) {
 console.log('myArg has an id');
 } else {
 console.log('myArg has an id');
 }
 } else {
 console.log('no value');
 }
}
</code></pre>
The following values will play nicely:
<pre><code>'abc'
{}
undefined
{ id: 'xyz' }
</code></pre>
On the other hand the assumption of <code>null</code> and <code>undefined</code> being equivalent here breaks the code. The reason being is that <code>null</code> is of type of <code>object</code>, where as <code>undefined</code> is of type <code>undefined</code>. So here the code breaks because you can't test for a member on <code>null</code>.
I have seen a large number of cases with code of similar appearance, where <code>null</code> is just asking for problems:
<pre><code>if (typeof myvar === 'string') {
 console.log(myvar);
} else if (typeof myvar === 'object') {
 console.log(myvar.id); 
}
</code></pre>
The fix here would be to explicitly test for null:
<pre><code>if (typeof myvar === 'string') {
 console.log(myvar);
} else if (myvar !== null &amp;&amp; typeof myvar === 'object') {
 console.log(myvar.id); 
}
</code></pre>
My attitude is to code for the weaknesses of a language and the typical behaviours of programmers of that language, hence the philosophy here of going with 'undefined' bey default.

blocks|key|629911|text|这里已经有了一些很好的答案，但不是我想要的那个。null和undefined在“技术上”都做了同样的事情，都是假的，但当我通读代码时，我看到一个"null“，那么我就认为它是一个用户定义的null，某个东西被显式设置为不包含任何值，如果我通读代码时看到"undefined”，那么我就认为它是从未初始化或赋值的代码。通过这种方式，代码可以向您传达某些东西是由未初始化的内容还是空值引起的。正因为如此，你真的不应该手动将“未定义”赋值给某些东西，否则它会扰乱你(或其他开发人员)阅读代码的方式。如果另一个开发人员看到“未定义”，他们不会直观地认为是你让它没有定义，他们会假设它没有初始化，而实际上它是初始化的。对我来说，这是最大的问题，当我阅读代码时，我想看看它告诉我什么，我不想猜测和弄清楚东西是否“实际上”被初始化了。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|629912|更不用说在typescript中使用它们意味着两件不同的事情。使用：|629913|interface+Example+{
++++name?:+string
}|code-block|syntax|javascript|629914|表示名称可以是未定义的，也可以是字符串，但不能为空。如果您希望它为空，则必须显式使用：|629915|interface+Example+{
++name:+string+%7C+null
}|629916|即使这样，你也会被迫至少用"null“来初始化它。|629917|当然，只有在tsconfig.json中使用"strictNullChecks":+true时才是正确的。|629918|entityMap^0|O|4|T|9|0|0|0|0|0|0|6|D|M|O|0^^$0|@$1|2|3|4|5|6|7|W|8|@$9|X|A|Y|B|C]|$9|Z|A|10|B|C]]|D|@]|E|$]]|$1|F|3|G|5|6|7|11|8|@]|D|@]|E|$]]|$1|H|3|I|5|J|7|12|8|@]|D|@]|E|$K|L]]|$1|M|3|N|5|6|7|13|8|@]|D|@]|E|$]]|$1|O|3|P|5|J|7|14|8|@]|D|@]|E|$K|L]]|$1|Q|3|R|5|6|7|15|8|@]|D|@]|E|$]]|$1|S|3|T|5|6|7|16|8|@$9|17|A|18|B|C]|$9|19|A|1A|B|C]]|D|@]|E|$]]|$1|U|3|-4|5|6|7|1B|8|@]|D|@]|E|$]]]|V|$]]

There are already some good answers here but not the one that I was looking for. <code>null</code> and <code>undefined</code> both &quot;technically&quot; do the same thing in terms of both being falsy, but when I read through code and I see a &quot;null&quot; then I'm expecting that it's a user defined null, something was explicitly set to contain no value, if I read through code and see &quot;undefined&quot; then I assume that it's code that was never initialized or assigned by anything. In this way code can communicate to you whether something was caused by uninitialized stuff or null values. Because of that you really shouldn't assign &quot;undefined&quot; manually to something otherwise it messes with the way you (or another developer) can read code. If another developer sees &quot;undefined&quot; they're not going to intuitively assume it's you who made it undefined, they're going to assume it's not been initialized when in fact it was. For me this is the biggest deal, when I read code I want to see what it's telling me, I don't want to guess and figure out if stuff has &quot;actually&quot; been initialized.
Not even to mention that using them in typescript means two different things. Using:
<pre><code>interface Example {
 name?: string
}
</code></pre>
Means that name can be undefined or a string, but it can't be null. If you want it null you have to explicitly use:
<pre><code>interface Example {
 name: string | null
}
</code></pre>
And even then you'll be forced to initialize it at least with &quot;null&quot;.
That's of course only true if you're using <code>&quot;strictNullChecks&quot;: true</code> in <code>tsconfig.json</code>.

blocks|key|846467|text|要编写简单的代码，您需要降低复杂性和变化性。当对象上的变量或属性没有值时，它是未定义的，为了使值为空，您需要为其分配一个空值。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|846468|entityMap^0|0^^$0|@$1|2|3|4|5|6|7|D|8|@]|9|@]|A|$]]|$1|B|3|-4|5|6|7|E|8|@]|9|@]|A|$]]]|C|$]]

To write simple code you need to keep complexity and variation down. When a variable or a property on an object does not have a value it is undefined , and for a value to be null you need to assign it a null value.

I've been writing JavaScript for quite a long time now, and I have never had a reason to use <code>null</code>. It seems that <code>undefined</code> is always preferable and serves the same purpose programmatically. What are some practical reasons to use <code>null</code> instead of <code>undefined</code>?

What reason is there to use null instead of undefined in JavaScript?

编译器

Python

我写JavaScript已经有很长一段时间了，我从来没有理由使用null。似乎undefined总是更可取的，并且以编程的方式服务于相同的目的。使用null而不是undefined的一些实际原因是什么

问在JavaScript中使用null而不是undefined的原因是什么？
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