blocks|key|1091689|text|这是因为val不是null，而是以字符串形式包含'null'。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|1091690|尝试使用“null”进行检查|1091691|if+('null'+!=+val)|code-block|syntax|javascript|1091692|有关何时起作用以及为什么起作用的解释，请参阅the+details+below。|1091693|entityMap|0|LINK|mutability|MUTABLE|url|https://stackoverflow.com/a/34817844/884640^0|9|4|O|6|0|0|0|M|H|0|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|@$9|14|A|15|1|16]]|E|$]]|$1|O|3|-4|5|6|7|17|8|@]|D|@]|E|$]]]|P|$Q|$5|R|S|T|E|$U|V]]]]

It's because val is not <code>null</code>, but contains <code>'null'</code> as a string.

Try to check with 'null'

<pre><code>if ('null' != val)
</code></pre>

For an explanation of when and why this works, see <a href="https://stackoverflow.com/a/34817844/884640">the details below</a>.

blocks|key|1091887|text|这将为你做这个把戏|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1091888|if+(!!val)+{
++++alert("this+is+not+null")
}+else+{
++++alert("this+is+null")
}|code-block|syntax|javascript|1091889|entityMap^0|0|0^^$0|@$1|2|3|4|5|6|7|I|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|J|8|@]|9|@]|A|$E|F]]|$1|G|3|-4|5|6|7|K|8|@]|9|@]|A|$]]]|H|$]]

this will do the trick for you

<pre><code>if (!!val) {
 alert("this is not null")
} else {
 alert("this is null")
}
</code></pre>

blocks|key|1307248|text|有3种方法可以检查"not+null“。我的建议是使用Strict+Not版本。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1307249|1.+Strict+Not版本|1307250|if+(val+!==+null)+{+...+}|code-block|syntax|javascript|1307251|Strict+Not版本使用“严格相等比较算法”http://www.ecma-international.org/ecma-262/5.1/#sec-11.9.6。由于严格相等比较算法不会对值进行类型转换，因此!==的性能比!=运算符更快。|offset|length|style|CODE|1307252|2.非严格非版本|1307253|if+(val+!=+'null')+{+...+}|1307254|非严格版本使用“抽象相等比较算法”http://www.ecma-international.org/ecma-262/5.1/#sec-11.9.3。与!==运算符相比，!=的性能较慢，因为抽象相等比较算法会对值进行类型转换。|1307255|3.双非版本|1307256|if+(!!val)+{+...+}|1307257|双重Not版本!!比严格Not版本!==和非严格Not版本!=+(https://jsperf.com/tfm-not-null/6)都具有更快的性能。但是，它会将“False值”(如undefined和NaN+)类型转换为False+(http://www.ecma-international.org/ecma-262/5.1/#sec-9.2)，这可能会导致意外的结果，而且由于没有显式声明null，因此可读性较差。|1307258|entityMap|0|LINK|mutability|MUTABLE|url|http://www.ecma-international.org/ecma-262/5.1/#sec-11.9.6|1|http://www.ecma-international.org/ecma-262/5.1/#sec-11.9.3|2|https://jsperf.com/tfm-not-null/6|3|http://www.ecma-international.org/ecma-262/5.1/#sec-9.2^0|0|0|0|2Y|3|35|2|O|1M|0|0|0|0|25|3|2E|2|H|1M|1|0|0|0|7|2|H|3|T|2|2K|9|2U|3|5I|4|X|X|2|3B|1J|3|0^^$0|@$1|2|3|4|5|6|7|1E|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|1F|8|@]|9|@]|A|$]]|$1|D|3|E|5|F|7|1G|8|@]|9|@]|A|$G|H]]|$1|I|3|J|5|6|7|1H|8|@$K|1I|L|1J|M|N]|$K|1K|L|1L|M|N]]|9|@$K|1M|L|1N|1|1O]]|A|$]]|$1|O|3|P|5|6|7|1P|8|@]|9|@]|A|$]]|$1|Q|3|R|5|F|7|1Q|8|@]|9|@]|A|$G|H]]|$1|S|3|T|5|6|7|1R|8|@$K|1S|L|1T|M|N]|$K|1U|L|1V|M|N]]|9|@$K|1W|L|1X|1|1Y]]|A|$]]|$1|U|3|V|5|6|7|1Z|8|@]|9|@]|A|$]]|$1|W|3|X|5|F|7|20|8|@]|9|@]|A|$G|H]]|$1|Y|3|Z|5|6|7|21|8|@$K|22|L|23|M|N]|$K|24|L|25|M|N]|$K|26|L|27|M|N]|$K|28|L|29|M|N]|$K|2A|L|2B|M|N]|$K|2C|L|2D|M|N]]|9|@$K|2E|L|2F|1|2G]|$K|2H|L|2I|1|2J]]|A|$]]|$1|10|3|-4|5|6|7|2K|8|@]|9|@]|A|$]]]|11|$12|$5|13|14|15|A|$16|17]]|18|$5|13|14|15|A|$16|19]]|1A|$5|13|14|15|A|$16|1B]]|1C|$5|13|14|15|A|$16|1D]]]]

There are 3 ways to check for &quot;not null&quot;. My recommendation is to use the Strict Not Version.
<h1>1. Strict Not Version</h1>
<pre><code>if (val !== null) { ... }
</code></pre>
The Strict Not Version uses the &quot;Strict Equality Comparison Algorithm&quot; <a href="http://www.ecma-international.org/ecma-262/5.1/#sec-11.9.6" rel="noreferrer">http://www.ecma-international.org/ecma-262/5.1/#sec-11.9.6</a>. The <code>!==</code> has faster performance, than the <code>!=</code> operator because the Strict Equality Comparison Algorithm doesn't typecast values.
<h1>2. Non-strict Not Version</h1>
<pre><code>if (val != 'null') { ... }
</code></pre>
The Non-strict version uses the &quot;Abstract Equality Comparison Algorithm&quot; <a href="http://www.ecma-international.org/ecma-262/5.1/#sec-11.9.3" rel="noreferrer">http://www.ecma-international.org/ecma-262/5.1/#sec-11.9.3</a>. The <code>!=</code> has slower performance, than the <code>!==</code> operator because the Abstract Equality Comparison Algorithm typecasts values.
<h1>3. Double Not Version</h1>
<pre><code>if (!!val) { ... }
</code></pre>
The Double Not Version <code>!!</code> has faster performance, than both the Strict Not Version <code>!==</code> and the Non-Strict Not Version <code>!=</code> (<a href="https://jsperf.com/tfm-not-null/6" rel="noreferrer">https://jsperf.com/tfm-not-null/6</a>). However, it will typecast &quot;Falsey&quot; values like <code>undefined</code> and <code>NaN</code> into False (<a href="http://www.ecma-international.org/ecma-262/5.1/#sec-9.2" rel="noreferrer">http://www.ecma-international.org/ecma-262/5.1/#sec-9.2</a>) which may lead to unexpected results, and it has worse readability because <code>null</code> isn't explicitly stated.

blocks|key|1091776|text|将!==用作!=将把您带入一个非传递性JavaScript真值表怪异的世界。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|1091777|entityMap^0|1|3|6|2|0^^$0|@$1|2|3|4|5|6|7|H|8|@$9|I|A|J|B|C]|$9|K|A|L|B|C]]|D|@]|E|$]]|$1|F|3|-4|5|6|7|M|8|@]|D|@]|E|$]]]|G|$]]

Use <code>!==</code> as <code>!=</code> will get you into a world of nontransitive JavaScript truth table weirdness.

blocks|key|1091716|text|您应该使用严格的not+equals比较运算符!==，这样如果用户输入"null"，您就不会得到else。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|1091717|entityMap^0|8|A|N|3|Z|6|1C|4|0^^$0|@$1|2|3|4|5|6|7|H|8|@$9|I|A|J|B|C]|$9|K|A|L|B|C]|$9|M|A|N|B|C]|$9|O|A|P|B|C]]|D|@]|E|$]]|$1|F|3|-4|5|6|7|Q|8|@]|D|@]|E|$]]]|G|$]]

You should be using the strict <code>not equals</code> comparison operator <code>!==</code> so that if the user inputs <code>"null"</code> then you won't get to the <code>else</code>.

blocks|key|1307288|text|检查https://softwareengineering.stackexchange.com/a/253723|type|unstyled|depth|inlineStyleRanges|entityRanges|offset|length|data|1307289|if(value)+{
}|code-block|syntax|javascript|1307290|如果value不是，则计算结果为true：|1307291|null
undefined
NaN
empty+string+("")
0
false|1307292|entityMap|0|LINK|mutability|MUTABLE|url|https://softwareengineering.stackexchange.com/a/253723^0|2|1I|0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|U|8|@]|9|@$A|V|B|W|1|X]]|C|$]]|$1|D|3|E|5|F|7|Y|8|@]|9|@]|C|$G|H]]|$1|I|3|J|5|6|7|Z|8|@]|9|@]|C|$]]|$1|K|3|L|5|F|7|10|8|@]|9|@]|C|$G|H]]|$1|M|3|-4|5|6|7|11|8|@]|9|@]|C|$]]]|N|$O|$5|P|Q|R|C|$S|T]]]]

Check <a href="https://softwareengineering.stackexchange.com/a/253723">https://softwareengineering.stackexchange.com/a/253723</a>

<pre><code>if(value) {
}
</code></pre>

will evaluate to true if value is not:

<pre><code>null
undefined
NaN
empty string ("")
0
false
</code></pre>

blocks|key|1307030|text|这可能是因为val的值实际上是字符串"null"，而不是值null。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|1307031|entityMap^0|6|3|I|6|T|4|0^^$0|@$1|2|3|4|5|6|7|H|8|@$9|I|A|J|B|C]|$9|K|A|L|B|C]|$9|M|A|N|B|C]]|D|@]|E|$]]|$1|F|3|-4|5|6|7|O|8|@]|D|@]|E|$]]]|G|$]]

It is possibly because the value of <code>val</code> is actually the string <code>"null"</code> rather than the value <code>null</code>.

blocks|key|1307106|text|这应该可以很好地工作..|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1307107|+++if(val!=+null)
++++{
+++++++alert("value+is+"%2Bval.length);+//--+this+returns+4
++++}
++++else
++++{
+++++++alert("value*+is+null");
++++}|code-block|syntax|javascript|1307108|entityMap^0|0|0^^$0|@$1|2|3|4|5|6|7|I|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|J|8|@]|9|@]|A|$E|F]]|$1|G|3|-4|5|6|7|K|8|@]|9|@]|A|$]]]|H|$]]

This should work fine..

<pre><code> if(val!= null)
 {
 alert("value is "+val.length); //-- this returns 4
 }
 else
 {
 alert("value* is null");
 }
</code></pre>

blocks|key|1307125|text|这将会起作用：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1307126|if+(val)+{
++++alert("Not+null");
}+else+{
++++alert("Null");
}|code-block|syntax|javascript|1307127|entityMap^0|0|0^^$0|@$1|2|3|4|5|6|7|I|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|J|8|@]|9|@]|A|$E|F]]|$1|G|3|-4|5|6|7|K|8|@]|9|@]|A|$]]]|H|$]]

This will work:

<pre><code>if (val) {
 alert("Not null");
} else {
 alert("Null");
}
</code></pre>

blocks|key|1091991|text|如果您希望能够将0作为有效值包含在内：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1091992|if+(!!val+%7C%7C+val+===+0)+{+...+}|code-block|syntax|javascript|1091993|entityMap^0|0|0^^$0|@$1|2|3|4|5|6|7|I|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|J|8|@]|9|@]|A|$E|F]]|$1|G|3|-4|5|6|7|K|8|@]|9|@]|A|$]]]|H|$]]

If you want to be able to include 0 as a valid value:

<pre><code>if (!!val || val === 0) { ... }
</code></pre>

Below is a code snippet, where we retrieve a form value.
Before further processing check if the value is not null..

<pre><code>var val = document.FileList.hiddenInfo.value;
alert("val is " + val); // this prints null which is as expected
if (val != null)
{
 alert("value is "+val.length); // this returns 4
}
else
{
 alert("value* is null");
}
</code></pre>

Any ideas why it happens so.. ??

javascript check for not null

下面是一段代码片段，我们在其中检索表单值。在进一步处理之前，请检查该值是否不为null。var val = document.FileList.hiddenInfo.value;alert("val is " + val);  // this prints null which is as expectedif (val != null){   alert("value is "+val.leng

问javascript检查not null
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