blocks|key|1350479|text|JavaScript将空字符串解释为0，这将导致isNAN测试失败。您可以先对字符串使用parseInt，这不会将空字符串转换为0。然后结果应该会导致isNAN失败。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1350480|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|$]]

JavaScript interprets an empty string as a 0, which then fails the isNAN test. You can use parseInt on the string first which won't convert the empty string to 0. The result should then fail isNAN.

blocks|key|1566152|text|您可能会感到惊讶，也可能不会感到惊讶，但这里有一些测试代码，可以向您展示JavaScript引擎的怪异之处。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1566153|document.write(isNaN(""))+//+false
document.write(isNaN("+"))++//+false
document.write(isNaN(0))++//+false
document.write(isNaN(null))+//+false
document.write(isNaN(false))++//+false
document.write(""+==+false)++//+true
document.write(""+==+0)++//+true
document.write("+"+==+0)++//+true
document.write("+"+==+false)++//+true
document.write(0+==+false)+//+true
document.write("+"+==+"")+//+false|code-block|syntax|javascript|1566154|所以这意味着|1566155|"+"+==+0+==+false|1566156|和|1566157|""+==+0+==+false|1566158|但|1566159|""+!=+"+"|1566160|玩得开心:)|1566161|entityMap^0|0|0|0|0|0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|W|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|X|8|@]|9|@]|A|$E|F]]|$1|G|3|H|5|6|7|Y|8|@]|9|@]|A|$]]|$1|I|3|J|5|D|7|Z|8|@]|9|@]|A|$E|F]]|$1|K|3|L|5|6|7|10|8|@]|9|@]|A|$]]|$1|M|3|N|5|D|7|11|8|@]|9|@]|A|$E|F]]|$1|O|3|P|5|6|7|12|8|@]|9|@]|A|$]]|$1|Q|3|R|5|D|7|13|8|@]|9|@]|A|$E|F]]|$1|S|3|T|5|6|7|14|8|@]|9|@]|A|$]]|$1|U|3|-4|5|6|7|15|8|@]|9|@]|A|$]]]|V|$]]

You may find this surprising or maybe not, but here is some test code to show you the wackyness of the JavaScript engine.

<pre><code>document.write(isNaN("")) // false
document.write(isNaN(" ")) // false
document.write(isNaN(0)) // false
document.write(isNaN(null)) // false
document.write(isNaN(false)) // false
document.write("" == false) // true
document.write("" == 0) // true
document.write(" " == 0) // true
document.write(" " == false) // true
document.write(0 == false) // true
document.write(" " == "") // false
</code></pre>

so this means that

<pre><code>" " == 0 == false
</code></pre>

and 

<pre><code>"" == 0 == false
</code></pre>

but 

<pre><code>"" != " "
</code></pre>

Have fun :)

blocks|key|1566116|text|为了更好地理解它，请打开第43页“应用于字符串类型的ToNumber”中的Ecma-Script+spec+pdf|type|unstyled|depth|inlineStyleRanges|entityRanges|offset|length|data|1566117|如果字符串具有数字语法，可以包含任意数量的空格字符，则可以将其转换为number类型。空字符串的计算结果为0。此外，字符串“Infinity”应该给出|1566118|isNaN('Infinity');+//+false|code-block|syntax|javascript|1566119|entityMap|0|LINK|mutability|MUTABLE|url|http://www.ecma-international.org/publications/standards/Ecma-262.htm^0|11|K|0|0|0|0^^$0|@$1|2|3|4|5|6|7|S|8|@]|9|@$A|T|B|U|1|V]]|C|$]]|$1|D|3|E|5|6|7|W|8|@]|9|@]|C|$]]|$1|F|3|G|5|H|7|X|8|@]|9|@]|C|$I|J]]|$1|K|3|-4|5|6|7|Y|8|@]|9|@]|C|$]]]|L|$M|$5|N|O|P|C|$Q|R]]]]

To understand it better, please open <a href="http://www.ecma-international.org/publications/standards/Ecma-262.htm" rel="noreferrer">Ecma-Script spec pdf</a> on page 43 "ToNumber Applied to the String Type"

if a string has a numerical syntax, which can contain any number of white-space characters, it can be converted to Number type. Empty string evaluates to 0. Also the string 'Infinity' should give

<pre><code>isNaN('Infinity'); // false
</code></pre>

blocks|key|1350526|text|尝试使用：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1350527|alert(isNaN(parseInt("+++")));|code-block|syntax|javascript|1350528|或|1350529|alert(isNaN(parseFloat("++++")));|1350530|entityMap^0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|M|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|N|8|@]|9|@]|A|$E|F]]|$1|G|3|H|5|6|7|O|8|@]|9|@]|A|$]]|$1|I|3|J|5|D|7|P|8|@]|9|@]|A|$E|F]]|$1|K|3|-4|5|6|7|Q|8|@]|9|@]|A|$]]]|L|$]]

Try using:

<pre><code>alert(isNaN(parseInt(" ")));
</code></pre>

Or

<pre><code>alert(isNaN(parseFloat(" ")));
</code></pre>

blocks|key|1350585|text|我认为这是因为Javascript的类型：'+'被转换为0，而'x'不是：|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|1350586|alert('+'+*+1);+//+0
alert('x'+*+1);+//+NaN|code-block|syntax|javascript|1350587|entityMap^0|L|3|V|3|0|0^^$0|@$1|2|3|4|5|6|7|M|8|@$9|N|A|O|B|C]|$9|P|A|Q|B|C]]|D|@]|E|$]]|$1|F|3|G|5|H|7|R|8|@]|D|@]|E|$I|J]]|$1|K|3|-4|5|6|7|S|8|@]|D|@]|E|$]]]|L|$]]

I think it's because of Javascript's typing: <code>' '</code> is converted to zero, whereas <code>'x'</code> isn't:

<pre><code>alert(' ' * 1); // 0
alert('x' * 1); // NaN
</code></pre>

blocks|key|1350761|text|如果你真的想要正确地检查它是否为整数，我建议你使用以下函数：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1350762|function+isInteger(s)
{
+++return+Math.ceil(s)+==+Math.floor(s);
}|code-block|syntax|javascript|1350763|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|$]]

I suggest you to use the following function if you really want a proper check if it is an integer:

<pre><code>function isInteger(s)
{
 return Math.ceil(s) == Math.floor(s);
}
</code></pre>

blocks|key|1566552|text|NaN+!==“不是数字”|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|1566553|NaN是Number类型的值|1566554|这是ECMAScript中isNaN()的定义|1566555|1.+Let+num+be+ToNumber(number).
2.+ReturnIfAbrupt(num).
3.+If+num+is+NaN,+return+true.
4.+Otherwise,+return+false.|code-block|syntax|javascript|1566556|尝试将任何值转换为数字。|1566557|Number("+")+//+0
Number("x")+//+NaN
Number(null)+//+0|1566558|如果要确定该值是否为NaN，则应首先尝试将其转换为数字值。|1566559|entityMap^0|0|3|0|0|3|0|0|0|0|0|A|3|0^^$0|@$1|2|3|4|5|6|7|W|8|@$9|X|A|Y|B|C]]|D|@]|E|$]]|$1|F|3|G|5|6|7|Z|8|@$9|10|A|11|B|C]]|D|@]|E|$]]|$1|H|3|I|5|6|7|12|8|@]|D|@]|E|$]]|$1|J|3|K|5|L|7|13|8|@]|D|@]|E|$M|N]]|$1|O|3|P|5|6|7|14|8|@]|D|@]|E|$]]|$1|Q|3|R|5|L|7|15|8|@]|D|@]|E|$M|N]]|$1|S|3|T|5|6|7|16|8|@$9|17|A|18|B|C]]|D|@]|E|$]]|$1|U|3|-4|5|6|7|19|8|@]|D|@]|E|$]]]|V|$]]

<code>NaN</code> !== "not a number"

<code>NaN</code> is a value of Number Type

this is a definition of isNaN() in ECMAScript

<pre><code>1. Let num be ToNumber(number).
2. ReturnIfAbrupt(num).
3. If num is NaN, return true.
4. Otherwise, return false.
</code></pre>

Try to convert any value to Number.

<pre><code>Number(" ") // 0
Number("x") // NaN
Number(null) // 0
</code></pre>

If you want to determine if the value is <code>NaN</code>, you should try to convert it to a Number value firstly.

blocks|key|1566282|text|我写了这个快速的小函数来帮助解决这个问题。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1566283|function+isNumber(val)+{
+++++return+(val+!=+undefined+&&+val+!=+null+&&+val.toString().length+>+0+&&+val.toString().match(/[%5E0-9\.\-]/g)+==+null);
};|code-block|syntax|javascript|1566284|它只检查非数字(0-9)、不是'-‘或'.’、不是未定义的、null或空的字符，如果没有匹配则返回true。:)|1566285|entityMap^0|0|0|0^^$0|@$1|2|3|4|5|6|7|K|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|L|8|@]|9|@]|A|$E|F]]|$1|G|3|H|5|6|7|M|8|@]|9|@]|A|$]]|$1|I|3|-4|5|6|7|N|8|@]|9|@]|A|$]]]|J|$]]

I wrote this quick little function to help solve this problem.

<pre><code>function isNumber(val) {
 return (val != undefined &amp;&amp; val != null &amp;&amp; val.toString().length &gt; 0 &amp;&amp; val.toString().match(/[^0-9\.\-]/g) == null);
};
</code></pre>

It simply checks for any characters that aren't numeric (0-9), that aren't '-' or '.', and that aren't undefined, null or empty and returns true if there's no matches. :)

blocks|key|1566450|text|如前所述，isNaN函数将在验证空字符串之前将其强制转换为数字，从而将空字符串更改为0(这是一个有效的数字)。但是，我发现当尝试解析空字符串或只包含空格的字符串时，parseInt函数将返回NaN。因此，下面的组合似乎工作得很好：|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|1566451|if+(+isNaN(string)+%7C%7C+isNaN(parseInt(string))+)+console.log('Not+a+number!');|1566452|此检查将适用于正数、负数和带小数点的数，因此我相信它涵盖了所有常见的数值情况。|1566453|entityMap^0|5|5|2A|8|2N|3|0|0|25|0|0^^$0|@$1|2|3|4|5|6|7|L|8|@$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|@$9|T|A|U|B|C]]|D|@]|E|$]]|$1|H|3|I|5|6|7|V|8|@]|D|@]|E|$]]|$1|J|3|-4|5|6|7|W|8|@]|D|@]|E|$]]]|K|$]]

As other explained the <code>isNaN</code> function will coerce the empty string into a number before validating it, thus changing an empty string into 0 (which is a valid number).
However, I found that the <code>parseInt</code> function will return <code>NaN</code> when trying to parse an empty string or a string with only spaces. As such the following combination seems to be working well:

<code>if ( isNaN(string) || isNaN(parseInt(string)) ) console.log('Not a number!');</code>

This check will work for positive numbers, negative numbers and numbers with a decimal point, so I believe it covers all common numerical cases.

blocks|key|1350769|text|这个函数似乎在我的测试中起作用了|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1350770|function+isNumber(s)+{
++++if+(s+===+""+%7C%7C+s+===+null)+{
++++++++return+false;
++++}+else+{
++++++++var+number+=+parseInt(s);
++++++++if+(number+==+'NaN')+{
++++++++++++return+false;
++++++++}+else+{
++++++++++++return+true;
++++++++}
++++}
}|code-block|syntax|javascript|1350771|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 function seemed to work in my tests

<pre><code>function isNumber(s) {
 if (s === "" || s === null) {
 return false;
 } else {
 var number = parseInt(s);
 if (number == 'NaN') {
 return false;
 } else {
 return true;
 }
 }
}
</code></pre>

blocks|key|1350783|text|关于|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1350784|function+isNumberRegex(value)+{++++++++
++++var+pattern+=+/%5E[-%2B]?\d*\.?\d*$/i;
++++var+match+=+value.match(pattern);
++++return+value.length+>+0+&&+match+!=+null;
}|code-block|syntax|javascript|1350785|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|$]]

What about 

<pre><code>function isNumberRegex(value) { 
 var pattern = /^[-+]?\d*\.?\d*$/i;
 var match = value.match(pattern);
 return value.length &gt; 0 &amp;&amp; match != null;
}
</code></pre>

blocks|key|1350940|text|isNAN(<argument>)是一个用来判断给定参数是否为非法数字的函数。isNaN将参数类型转换为Number类型。如果你想检查参数是否是数值的？请使用jQuery中的$.isNumeric()函数。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|1350941|也就是说，isNaN(foo)等同于isNaN(Number(foo))，由于显而易见的原因，它接受所有数字都是数字的字符串。对于ex。|1350942|isNaN(123)+//false
isNaN(-1.23)+//false
isNaN(5-2)+//false
isNaN(0)+//false
isNaN('123')+//false
isNaN('Hello')+//true
isNaN('2005/12/12')+//true
isNaN('')+//false
isNaN(true)+//false
isNaN(undefined)+//true
isNaN('NaN')+//true
isNaN(NaN)+//true
isNaN(0+/+0)+//true|code-block|syntax|javascript|1350943|entityMap^0|0|H|13|5|2F|D|0|0|0^^$0|@$1|2|3|4|5|6|7|O|8|@$9|P|A|Q|B|C]|$9|R|A|S|B|C]|$9|T|A|U|B|C]]|D|@]|E|$]]|$1|F|3|G|5|6|7|V|8|@]|D|@]|E|$]]|$1|H|3|I|5|J|7|W|8|@]|D|@]|E|$K|L]]|$1|M|3|-4|5|6|7|X|8|@]|D|@]|E|$]]]|N|$]]

<code>isNAN(&lt;argument&gt;)</code> is a function to tell whether given argument is illegal number.
<code>isNaN</code> typecasts the arguments into Number type. If you want to check if argument is Numeric or not? Please use <code>$.isNumeric()</code> function in jQuery.

That is, isNaN(foo) is equivalent to isNaN(Number(foo))
It accepts any strings having all numerals as numbers for obvious reasons. For ex.

<pre><code>isNaN(123) //false
isNaN(-1.23) //false
isNaN(5-2) //false
isNaN(0) //false
isNaN('123') //false
isNaN('Hello') //true
isNaN('2005/12/12') //true
isNaN('') //false
isNaN(true) //false
isNaN(undefined) //true
isNaN('NaN') //true
isNaN(NaN) //true
isNaN(0 / 0) //true
</code></pre>

blocks|key|1351060|text|我用这个|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1351061|​|1351062|++++function+isNotANumeric(val)+{
++++	if(val.trim+&&+val.trim()+==+"")+{
+++++++++return+true;
++++++}+else+{
++++++	+return+isNaN(parseFloat(val+*+1));
++++++}
++++}
++++
++++alert(isNotANumeric("100"));++//+false
++++alert(isNotANumeric("1a"));+++//+true
++++alert(isNotANumeric(""));+++++//+true
++++alert(isNotANumeric("+++"));++//+true|code-block|syntax|javascript|1351063|1351064|entityMap^0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|L|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|M|8|@]|9|@]|A|$]]|$1|D|3|E|5|F|7|N|8|@]|9|@]|A|$G|H]]|$1|I|3|C|5|6|7|O|8|@]|9|@]|A|$]]|$1|J|3|-4|5|6|7|P|8|@]|9|@]|A|$]]]|K|$]]

I use this

<div class="snippet" data-lang="js" data-hide="false" data-console="true" data-babel="false">
<div class="snippet-code">
<pre class="snippet-code-js lang-js prettyprint-override"><code> function isNotANumeric(val) {
 	if(val.trim &amp;&amp; val.trim() == "") {
 return true;
 } else {
 	 return isNaN(parseFloat(val * 1));
 }
 }
 
 alert(isNotANumeric("100")); // false
 alert(isNotANumeric("1a")); // true
 alert(isNotANumeric("")); // true
 alert(isNotANumeric(" ")); // true</code></pre>
</div>
</div>

blocks|key|1351190|text|当检查带有空格或"+"的某个字符串值是否为isNaN时，可能会尝试执行字符串验证，例如：|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|1351191|//+value+=+"123+"+if+(value.match(/\s/)+%7C%7C+isNaN(value))+{++++//+do+something+}|1351192|entityMap^0|8|3|L|5|0|0|27|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|@$9|P|A|Q|B|C]]|D|@]|E|$]]|$1|H|3|-4|5|6|7|R|8|@]|D|@]|E|$]]]|I|$]]

When checking if certain string value with whitespace or <code>" "</code>is <code>isNaN</code> maybe try to perform string validation, example :

<code>// value = "123 "
if (value.match(/\s/) || isNaN(value)) {
 // do something
}</code>

blocks|key|1351263|text|我发现有一个特定于Number类的方法很方便(因为执行转换的其他函数，如parseInt，对其中一些值有不同的输出)，并使用原型继承。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1351264|Object.assign(Number.prototype,+{
++isNumericallyValid(num)+{
++++if+(
++++++num+===+null
++++++%7C%7C+typeof+num+===+'boolean'
++++++%7C%7C+num+===+''
++++++%7C%7C+Number.isNaN(Number(num))
++++)+{
++++++return+false;
++++}
++++return+true;
++}
});|code-block|syntax|javascript|1351265|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|$]]

I find it convenient to have a method specific to the Number class (since other functions that do conversions like parseInt have different outputs for some of these values) and use prototypal inheritance.
<pre><code>Object.assign(Number.prototype, {
 isNumericallyValid(num) {
 if (
 num === null
 || typeof num === 'boolean'
 || num === ''
 || Number.isNaN(Number(num))
 ) {
 return false;
 }
 return true;
 }
});
</code></pre>

blocks|key|1351286|text|​|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1351287|let+isNotNumber+=+val+=>+isNaN(val)+%7C%7C+(val.trim+&&+val.trim()+===+'');

console.log(isNotNumber('+'));
console.log(isNotNumber('1'));
console.log(isNotNumber('123x'));
console.log(isNotNumber('x123'));
console.log(isNotNumber('0'));
console.log(isNotNumber(3));
console.log(isNotNumber('++++x'));
console.log(isNotNumber('1.23'));
console.log(isNotNumber('1.23.1.3'));

if(!isNotNumber(3)){
++console.log('This+is+a+number');
}|code-block|syntax|javascript|1351288|1351289|entityMap^0|0|0|0^^$0|@$1|2|3|4|5|6|7|J|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|K|8|@]|9|@]|A|$E|F]]|$1|G|3|4|5|6|7|L|8|@]|9|@]|A|$]]|$1|H|3|-4|5|6|7|M|8|@]|9|@]|A|$]]]|I|$]]

<div class="snippet" data-lang="js" data-hide="false" data-console="true" data-babel="false">
<div class="snippet-code">
<pre class="snippet-code-js lang-js prettyprint-override"><code>let isNotNumber = val =&gt; isNaN(val) || (val.trim &amp;&amp; val.trim() === '');

console.log(isNotNumber(' '));
console.log(isNotNumber('1'));
console.log(isNotNumber('123x'));
console.log(isNotNumber('x123'));
console.log(isNotNumber('0'));
console.log(isNotNumber(3));
console.log(isNotNumber(' x'));
console.log(isNotNumber('1.23'));
console.log(isNotNumber('1.23.1.3'));

if(!isNotNumber(3)){
 console.log('This is a number');
}</code></pre>
</div>
</div>

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I use the following.
<code>x=(isNaN(parseFloat(x)))? 0.00 : parseFloat(x);</code>

In JavaScript, why does <code>isNaN(" ")</code> evaluate to <code>false</code>, but <code>isNaN(" x")</code> evaluate to <code>true</code>?

I’m performing numerical operations on a text input field, and I’m checking if the field is <code>null</code>, <code>""</code>, or <code>NaN</code>. When someone types a handful of spaces into the field, my validation fails on all three, and I’m confused as to why it gets past the <code>isNaN</code> check.

Why does isNaN(" ") (string with spaces) equal false?

在JavaScript中，为什么isNaN("   ")的计算结果为false，而isNaN("  x")的计算结果为true我在一个文本输入字段上执行数值操作，并检查该字段是null、""还是NaN。当有人在字段中输入几个空格时，我对这三个字段的验证都会失败，我很困惑为什么它能通过isNaN检查。

问为什么isNaN(“") (带空格的字符串)等于false？
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回答 17

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问为什么isNaN(“") (带空格的字符串)等于false？EN

回答 17

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问为什么isNaN(“") (带空格的字符串)等于false？
EN