blocks|key|946613|text|function+isEmpty(value){
++return+(value+==+null+%7C%7C+value.length+===+0);
}|type|code-block|depth|inlineStyleRanges|entityRanges|data|syntax|javascript|946614|这将返回true|unstyled|946615|undefined++//+Because+undefined+==+null

null

[]

""|946616|和零参数函数，因为函数的length是它所采用的声明参数的数量。|offset|length|style|CODE|946617|要禁止后一种类型，您可能只需要检查空字符串|946618|function+isEmpty(value){
++return+(value+==+null+%7C%7C+value+===+'');
}|946619|entityMap^0|0|0|0|C|6|0|0|0^^$0|@$1|2|3|4|5|6|7|U|8|@]|9|@]|A|$B|C]]|$1|D|3|E|5|F|7|V|8|@]|9|@]|A|$]]|$1|G|3|H|5|6|7|W|8|@]|9|@]|A|$B|C]]|$1|I|3|J|5|F|7|X|8|@$K|Y|L|Z|M|N]]|9|@]|A|$]]|$1|O|3|P|5|F|7|10|8|@]|9|@]|A|$]]|$1|Q|3|R|5|6|7|11|8|@]|9|@]|A|$B|C]]|$1|S|3|-4|5|F|7|12|8|@]|9|@]|A|$]]]|T|$]]

<pre><code>function isEmpty(value){
 return (value == null || value.length === 0);
}
</code></pre>

This will return true for

<pre><code>undefined // Because undefined == null

null

[]

""
</code></pre>

and zero argument functions since a function's <code>length</code> is the number of declared parameters it takes.

To disallow the latter category, you might want to just check for blank strings

<pre><code>function isEmpty(value){
 return (value == null || value === '');
}
</code></pre>

blocks|key|946662|text|用于检查value是否未定义或为空的详细方法为：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|946663|return+value+===+undefined+%7C%7C+value+===+null;|code-block|syntax|javascript|946664|您也可以使用know+all+the+rules运算符，但这需要一个运算符|offset|length|946665|return+value+==+null;+//+also+returns+true+if+value+is+undefined|946666|entityMap|0|LINK|mutability|MUTABLE|url|http://www.ecma-international.org/ecma-262/5.1/#sec-11.9.3^0|0|0|6|I|0|0|0^^$0|@$1|2|3|4|5|6|7|U|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|V|8|@]|9|@]|A|$E|F]]|$1|G|3|H|5|6|7|W|8|@]|9|@$I|X|J|Y|1|Z]]|A|$]]|$1|K|3|L|5|D|7|10|8|@]|9|@]|A|$E|F]]|$1|M|3|-4|5|6|7|11|8|@]|9|@]|A|$]]]|N|$O|$5|P|Q|R|A|$S|T]]]]

The verbose method to check if value is undefined or null is:

<pre><code>return value === undefined || value === null;
</code></pre>

You can also use the <code>==</code> operator but this expects one to <a href="http://www.ecma-international.org/ecma-262/5.1/#sec-11.9.3" rel="noreferrer">know all the rules</a>:

<pre><code>return value == null; // also returns true if value is undefined
</code></pre>

blocks|key|730633|text|你做得有点过头了。要检查变量是否没有赋值，只需检查undefined和null。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|730634|function+isEmpty(value){
++++return+(typeof+value+===+"undefined"+%7C%7C+value+===+null);
}|code-block|syntax|javascript|730635|这里假设0、""和对象(甚至是空对象和数组)都是有效的“值”。|offset|length|style|CODE|730636|entityMap^0|0|0|4|1|6|2|0^^$0|@$1|2|3|4|5|6|7|O|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|P|8|@]|9|@]|A|$E|F]]|$1|G|3|H|5|6|7|Q|8|@$I|R|J|S|K|L]|$I|T|J|U|K|L]]|9|@]|A|$]]|$1|M|3|-4|5|6|7|V|8|@]|9|@]|A|$]]]|N|$]]

You are a bit overdoing it. To check if a variable is not given a value, you would only need to check against undefined and null.

<pre><code>function isEmpty(value){
 return (typeof value === "undefined" || value === null);
}
</code></pre>

This is assuming <code>0</code>, <code>""</code>, and objects(even empty object and array) are valid "values".

blocks|key|730691|text|如果变量还没有声明，你将不能使用函数测试未定义的，因为你会得到一个错误。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|730692|if+(foo)+{}
function+(bar)+{}(foo)|code-block|syntax|javascript|730693|如果没有声明foo，这两种方法都会产生错误。|730694|如果要测试变量是否已声明，可以使用|730695|typeof+foo+!=+"undefined"|730696|如果您想测试foo是否已经声明，并且它有一个值，您可以使用|730697|if+(typeof+foo+!=+"undefined"+&&+foo)+{
++++//code+here
}|730698|entityMap^0|0|0|0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|S|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|T|8|@]|9|@]|A|$E|F]]|$1|G|3|H|5|6|7|U|8|@]|9|@]|A|$]]|$1|I|3|J|5|6|7|V|8|@]|9|@]|A|$]]|$1|K|3|L|5|D|7|W|8|@]|9|@]|A|$E|F]]|$1|M|3|N|5|6|7|X|8|@]|9|@]|A|$]]|$1|O|3|P|5|D|7|Y|8|@]|9|@]|A|$E|F]]|$1|Q|3|-4|5|6|7|Z|8|@]|9|@]|A|$]]]|R|$]]

If the variable hasn't been declared, you wont be able to test for undefined using a function because you will get an error. 

<pre><code>if (foo) {}
function (bar) {}(foo)
</code></pre>

Both will generate an error if foo has not been declared.

If you want to test if a variable has been declared you can use

<pre><code>typeof foo != "undefined"
</code></pre>

if you want to test if foo has been declared and it has a value you can use

<pre><code>if (typeof foo != "undefined" &amp;&amp; foo) {
 //code here
}
</code></pre>

blocks|key|730736|text|如果value为空、未定义等或为空(即只包含空格)，则返回true：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|730737|function+stringIsEmpty(value)+{

++++return+value+?+value.trim().length+==+0+:+true;

}|code-block|syntax|javascript|730738|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|$]]

Here's mine - returns true if value is null, undefined, etc or blank (ie contains only blank spaces):

<pre><code>function stringIsEmpty(value) {

 return value ? value.trim().length == 0 : true;

}
</code></pre>

blocks|key|946821|text|第一个评分最高的答案是错误的。如果值是未定义的，它将在现代浏览器中抛出异常。你必须使用：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|946822|if+(typeof(value)+!==+"undefined"+&&+value)|code-block|syntax|javascript|946823|或|946824|if+(typeof+value++!==+"undefined"+&&+value)|946825|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|$]]

The first answer with best rating is wrong. If value is undefined it will throw an exception in modern browsers. You have to use: 

<pre><code>if (typeof(value) !== "undefined" &amp;&amp; value)
</code></pre>

or 

<pre><code>if (typeof value !== "undefined" &amp;&amp; value)
</code></pre>

blocks|key|730843|text|您可能会发现以下函数很有用：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|730844|function+typeOf(obj)+{
++return+{}.toString.call(obj).split('+')[1].slice(0,+-1).toLowerCase();
}|code-block|syntax|javascript|730845|或在ES7中(如果有进一步的改进，请发表评论)|730846|function+typeOf(obj)+{
++const+{+toString+}+=+Object.prototype;
++const+stringified+=+obj::toString();
++const+type+=+stringified.split('+')[1].slice(0,+-1);

++return+type.toLowerCase();
}|730847|结果：|730848|typeOf();+//undefined
typeOf(null);+//null
typeOf(NaN);+//number
typeOf(5);+//number
typeOf({});+//object
typeOf([]);+//array
typeOf('');+//string
typeOf(function+()+{});+//function
typeOf(/a/)+//regexp
typeOf(new+Date())+//date
typeOf(new+WeakMap())+//weakmap
typeOf(new+Map())+//map|730849|“请注意，绑定运算符(::)既不是ES2016+(ES7)的一部分，也不是ECMAScript标准的任何后续版本。它目前是引入该语言的0阶段(稻草人)提案。”-+Simon+Kjellberg。作者希望添加他的支持这项美丽的提议，以接受皇家提升。|730850|entityMap^0|0|0|0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|S|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|T|8|@]|9|@]|A|$E|F]]|$1|G|3|H|5|6|7|U|8|@]|9|@]|A|$]]|$1|I|3|J|5|D|7|V|8|@]|9|@]|A|$E|F]]|$1|K|3|L|5|6|7|W|8|@]|9|@]|A|$]]|$1|M|3|N|5|D|7|X|8|@]|9|@]|A|$E|F]]|$1|O|3|P|5|6|7|Y|8|@]|9|@]|A|$]]|$1|Q|3|-4|5|6|7|Z|8|@]|9|@]|A|$]]]|R|$]]

You may find the following function useful: 

<pre><code>function typeOf(obj) {
 return {}.toString.call(obj).split(' ')[1].slice(0, -1).toLowerCase();
}
</code></pre>

Or in ES7 (comment if further improvements)

<pre><code>function typeOf(obj) {
 const { toString } = Object.prototype;
 const stringified = obj::toString();
 const type = stringified.split(' ')[1].slice(0, -1);

 return type.toLowerCase();
}
</code></pre>

Results:

<pre><code>typeOf(); //undefined
typeOf(null); //null
typeOf(NaN); //number
typeOf(5); //number
typeOf({}); //object
typeOf([]); //array
typeOf(''); //string
typeOf(function () {}); //function
typeOf(/a/) //regexp
typeOf(new Date()) //date
typeOf(new WeakMap()) //weakmap
typeOf(new Map()) //map
</code></pre>

"Note that the bind operator (::) is not part of ES2016 (ES7) nor any later edition of the ECMAScript standard at all. It's currently a stage 0 (strawman) proposal for being introduced to the language." – Simon Kjellberg. the author wishes to add his support for this beautiful proposal to receive royal ascension.

blocks|key|946896|text|此条件检查|type|unstyled|depth|inlineStyleRanges|entityRanges|data|946897|if+(!!foo)+{
++++//foo+is+defined
}|code-block|syntax|javascript|946898|就是你所需要的。|946899|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|$]]

This condition check

<pre><code>if (!!foo) {
 //foo is defined
}
</code></pre>

is all you need.

blocks|key|946922|text|如果你更喜欢普通的javascript，试试这个：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|946923|++/**
+++*+Checks+if+`value`+is+empty.+Arrays,+strings,+or+`arguments`+objects+with+a
+++*+length+of+`0`+and+objects+with+no+own+enumerable+properties+are+considered
+++*+"empty".
+++*
+++*+@static
+++*+@memberOf+_
+++*+@category+Objects
+++*+@param+{Array%7CObject%7Cstring}+value+The+value+to+inspect.
+++*+@returns+{boolean}+Returns+`true`+if+the+`value`+is+empty,+else+`false`.
+++*+@example
+++*
+++*+_.isEmpty([1,+2,+3]);
+++*+//+=>+false
+++*
+++*+_.isEmpty([]);
+++*+//+=>+true
+++*
+++*+_.isEmpty({});
+++*+//+=>+true
+++*
+++*+_.isEmpty('');
+++*+//+=>+true
+++*/

function+isEmpty(value)+{
++++if+(!value)+{
++++++return+true;
++++}
++++if+(isArray(value)+%7C%7C+isString(value))+{
++++++return+!value.length;
++++}
++++for+(var+key+in+value)+{
++++++if+(hasOwnProperty.call(value,+key))+{
++++++++return+false;
++++++}
++++}
++++return+true;
++}|code-block|syntax|javascript|946924|否则，如果您已经在使用下划线或斜线，请尝试：|946925|_.isEmpty(value)|946926|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|$]]

If you prefer plain javascript try this:

<pre><code> /**
 * Checks if `value` is empty. Arrays, strings, or `arguments` objects with a
 * length of `0` and objects with no own enumerable properties are considered
 * "empty".
 *
 * @static
 * @memberOf _
 * @category Objects
 * @param {Array|Object|string} value The value to inspect.
 * @returns {boolean} Returns `true` if the `value` is empty, else `false`.
 * @example
 *
 * _.isEmpty([1, 2, 3]);
 * // =&gt; false
 *
 * _.isEmpty([]);
 * // =&gt; true
 *
 * _.isEmpty({});
 * // =&gt; true
 *
 * _.isEmpty('');
 * // =&gt; true
 */

function isEmpty(value) {
 if (!value) {
 return true;
 }
 if (isArray(value) || isString(value)) {
 return !value.length;
 }
 for (var key in value) {
 if (hasOwnProperty.call(value, key)) {
 return false;
 }
 }
 return true;
 }
</code></pre>

Otherwise, if you are already using underscore or lodash, try:

<pre><code>_.isEmpty(value)
</code></pre>

blocks|key|946963|text|我认为使用？运算符稍微干净一些。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|946964|var+?+function_if_exists()+:+function_if_doesnt_exist();|code-block|syntax|javascript|946965|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 think using the ? operator is slightly cleaner.

<pre><code>var ? function_if_exists() : function_if_doesnt_exist();
</code></pre>

blocks|key|730983|text|这将检查不确定嵌套的变量是否未定义|type|unstyled|depth|inlineStyleRanges|entityRanges|data|730984|function+Undef(str)+
{
++var+ary+=+str.split('.');
++var+w+=+window;
++for+(i+in+ary)+{
++++try++++++{+if+(typeof(w+=+w[ary[i]])+===+"undefined")+return+true;+}
++++catch(e)+{+return+true;+}
++}
++return+false;
}

if+(!Undef("google.translate.TranslateElement"))+{|code-block|syntax|javascript|730985|上面的代码检查谷歌翻译函数TranslateElement是否存在。这相当于：|730986|if+(!(typeof+google+===+"undefined"+
+%7C%7C+typeof+google.translate+===+"undefined"+
+%7C%7C+typeof+google.translate.TranslateElement+===+"undefined"))+{|730987|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|$]]

This will check if variable of indeterminate nesting is undefined

<pre><code>function Undef(str) 
{
 var ary = str.split('.');
 var w = window;
 for (i in ary) {
 try { if (typeof(w = w[ary[i]]) === "undefined") return true; }
 catch(e) { return true; }
 }
 return false;
}

if (!Undef("google.translate.TranslateElement")) {
</code></pre>

The above checks if the Google translate function TranslateElement exists. This is equivalent to:

<pre><code>if (!(typeof google === "undefined" 
 || typeof google.translate === "undefined" 
 || typeof google.translate.TranslateElement === "undefined")) {
</code></pre>

blocks|key|947048|text|对于我的情况，我尝试使用if+null，''，!variable，但它不起作用。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|947049|要从html字段获取文本，请参阅下面的代码|947050|var+status=$(this).text();+//for+example+(for+my+case)|code-block|syntax|javascript|947051|如果状态变量中没有值(没有文本)，我试图将值'novalue‘设置为状态变量。|947052|以下代码起作用了。|947053|if(status+==+false)
{
+++status='novalue';
}+|947054|当没有找到satus变量的文本时，上面的代码将'novalue‘赋值给状态变量|947055|entityMap^0|0|0|0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|S|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|T|8|@]|9|@]|A|$]]|$1|D|3|E|5|F|7|U|8|@]|9|@]|A|$G|H]]|$1|I|3|J|5|6|7|V|8|@]|9|@]|A|$]]|$1|K|3|L|5|6|7|W|8|@]|9|@]|A|$]]|$1|M|3|N|5|F|7|X|8|@]|9|@]|A|$G|H]]|$1|O|3|P|5|6|7|Y|8|@]|9|@]|A|$]]|$1|Q|3|-4|5|6|7|Z|8|@]|9|@]|A|$]]]|R|$]]

For my case I tried with if null,'', !variable, But it did not work.

See my code below to get the text from an html field

<pre><code>var status=$(this).text(); //for example (for my case)
</code></pre>

if there was no value(no text) in the status variable ,I was trying to set the value 'novalue' to status variable. 

the following code worked.

<pre><code>if(status == false)
{
 status='novalue';
} 
</code></pre>

when there was no text found for satus variable the above code assigned 'novalue' to the status variable

blocks|key|731088|text|虽然很老，但忘记了他们应该包装代码块，然后捕获错误，然后进行测试……|type|unstyled|depth|inlineStyleRanges|entityRanges|data|731089|function+checkup(+t+){
++try{
++++for(p+in+t){
++++++if(+p.hasOwnProperty(+t+)+){
++++++++return+true;
++++++}
++++}
++++return+false;
++}catch(e){
++++console.log("ERROR+:+"%2Be);
++++return+e;
++}
}|code-block|syntax|javascript|731090|所以你真的不必事先检查潜在的问题，你只需捕捉它，然后按你想要的方式处理它。|731091|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|$]]

Although an oldie, what forget is that they should wrap their code block and then catch the error and then test...

<pre><code>function checkup( t ){
 try{
 for(p in t){
 if( p.hasOwnProperty( t ) ){
 return true;
 }
 }
 return false;
 }catch(e){
 console.log("ERROR : "+e);
 return e;
 }
}
</code></pre>

So you really don't have to check for a potential problem before hand, you simply catch it and then deal with it how you want.

blocks|key|731161|text|function+isEmpty(val){
++++return+!val;
}|type|code-block|depth|inlineStyleRanges|entityRanges|data|syntax|javascript|731162|但是这个解决方案是过度设计的，如果你不想稍后为busines-model需求修改函数，那么直接在代码中使用它会更干净：|unstyled|731163|if(!val)...|731164|entityMap^0|0|0|0^^$0|@$1|2|3|4|5|6|7|K|8|@]|9|@]|A|$B|C]]|$1|D|3|E|5|F|7|L|8|@]|9|@]|A|$]]|$1|G|3|H|5|6|7|M|8|@]|9|@]|A|$B|C]]|$1|I|3|-4|5|F|7|N|8|@]|9|@]|A|$]]]|J|$]]

<pre><code>function isEmpty(val){
 return !val;
}
</code></pre>

but this solution is over-engineered, if you dont'want to modify the function later for busines-model needings, then is cleaner to use it directly in code:

<pre><code>if(!val)...
</code></pre>

blocks|key|731212|text|对于每个来这里有类似问题的人，下面的工作很棒，我在过去的几年里把它放在了我的图书馆里：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|731213|(function(g3,+$,+window,+document,+undefined){
+++g3.utils+=+g3.utils+%7C%7C+{};
/********************************Function+type()********************************
*+Returns+a+lowercase+string+representation+of+an+object's+constructor.
*+@module+{g3.utils}
*+@function+{g3.utils.type}
*+@public
*+@param+{Type}+'obj'+is+any+type+native,+host+or+custom.
*+@return+{String}+Returns+a+lowercase+string+representing+the+object's+
*+constructor+which+is+different+from+word+'object'+if+they+are+not+custom.
*+@reference+http://perfectionkills.com/instanceof-considered-harmful-or-how-to-write-a-robust-isarray/
*+http://stackoverflow.com/questions/3215046/differentiating-between-arrays-and-hashes-in-javascript
*+http://javascript.info/tutorial/type-detection
*******************************************************************************/
g3.utils.type+=+function+(obj){
+++if(obj+===+null)
++++++return+'null';
+++else+if(typeof+obj+===+'undefined')
++++++return+'undefined';
+++return+Object.prototype.toString.call(obj).match(/%5E\[object\s(.*)\]$/)[1].toLowerCase();
};
}(window.g3+=+window.g3+%7C%7C+{},+jQuery,+window,+document));|code-block|syntax|javascript|731214|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|$]]

For everyone coming here for having similar question, the following works great and I have it in my library the last years:

<pre><code>(function(g3, $, window, document, undefined){
 g3.utils = g3.utils || {};
/********************************Function type()********************************
* Returns a lowercase string representation of an object's constructor.
* @module {g3.utils}
* @function {g3.utils.type}
* @public
* @param {Type} 'obj' is any type native, host or custom.
* @return {String} Returns a lowercase string representing the object's 
* constructor which is different from word 'object' if they are not custom.
* @reference http://perfectionkills.com/instanceof-considered-harmful-or-how-to-write-a-robust-isarray/
* http://stackoverflow.com/questions/3215046/differentiating-between-arrays-and-hashes-in-javascript
* http://javascript.info/tutorial/type-detection
*******************************************************************************/
g3.utils.type = function (obj){
 if(obj === null)
 return 'null';
 else if(typeof obj === 'undefined')
 return 'undefined';
 return Object.prototype.toString.call(obj).match(/^\[object\s(.*)\]$/)[1].toLowerCase();
};
}(window.g3 = window.g3 || {}, jQuery, window, document));
</code></pre>

blocks|key|947238|text|这可能是有用的。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|947239|数组中的所有值都表示您想要的值(null、undefined或其他值)，您可以在其中搜索您想要的内容。|947240|var+variablesWhatILookFor+=+[null,+undefined,+''];
variablesWhatILookFor.indexOf(document.DocumentNumberLabel)+>+-1|code-block|syntax|javascript|947241|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|$]]

It may be usefull.

All values in array represent what you want to be (null, undefined or another things) and you search what you want in it.

<pre><code>var variablesWhatILookFor = [null, undefined, ''];
variablesWhatILookFor.indexOf(document.DocumentNumberLabel) &gt; -1
</code></pre>

blocks|key|947262|text|您可以直接使用相等运算符|type|unstyled|depth|inlineStyleRanges|entityRanges|data|947263|<script>
++++var+firstName;
++++var+lastName+=+null;
++++/*+Since+null+==+undefined+is+true,+the+following+statements+will+catch+both+null+and+undefined+*/
++++++++if(firstName+==+null){
++++++++++++alert('Variable+"firstName"+is+undefined.');
++++++++}++++
++++++++if(lastName+==+null){
+++++++++++alert('Variable+"lastName"+is+null.');
++++++++}
</script>|code-block|syntax|javascript|947264|邮箱:+demo+@+How+to+determine+if+variable+is+undefined+or+null+using+JavaScript|offset|length|947265|entityMap|0|LINK|mutability|MUTABLE|url|http://www.jquerypot.com/how-to-determine-if-variable-is-undefined-or-null-using-javascript/^0|0|0|B|1U|0|0^^$0|@$1|2|3|4|5|6|7|S|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|T|8|@]|9|@]|A|$E|F]]|$1|G|3|H|5|6|7|U|8|@]|9|@$I|V|J|W|1|X]]|A|$]]|$1|K|3|-4|5|6|7|Y|8|@]|9|@]|A|$]]]|L|$M|$5|N|O|P|A|$Q|R]]]]

You can directly use the equality operator

<pre><code>&lt;script&gt;
 var firstName;
 var lastName = null;
 /* Since null == undefined is true, the following statements will catch both null and undefined */
 if(firstName == null){
 alert('Variable "firstName" is undefined.');
 } 
 if(lastName == null){
 alert('Variable "lastName" is null.');
 }
&lt;/script&gt;
</code></pre>

demo @ <a href="http://www.jquerypot.com/how-to-determine-if-variable-is-undefined-or-null-using-javascript/" rel="nofollow noreferrer">How to determine if variable is undefined or null using JavaScript</a>

blocks|key|731325|text|function+isEmpty(obj)+{
++++if+(typeof+obj+==+'number')+return+false;
++++else+if+(typeof+obj+==+'string')+return+obj.length+==+0;
++++else+if+(Array.isArray(obj))+return+obj.length+==+0;
++++else+if+(typeof+obj+==+'object')+return+obj+==+null+%7C%7C+Object.keys(obj).length+==+0;
++++else+if+(typeof+obj+==+'boolean')+return+false;
++++else+return+!obj;
}|type|code-block|depth|inlineStyleRanges|entityRanges|data|syntax|javascript|731326|在使用trim处理空格字符串的ES6中：|unstyled|731327|const+isEmpty+=+value+=>+{
++++if+(typeof+value+===+'number')+return+false
++++else+if+(typeof+value+===+'string')+return+value.trim().length+===+0
++++else+if+(Array.isArray(value))+return+value.length+===+0
++++else+if+(typeof+value+===+'object')+return+value+==+null+%7C%7C+Object.keys(value).length+===+0
++++else+if+(typeof+value+===+'boolean')+return+false
++++else+return+!value
}|731328|entityMap^0|0|0|0^^$0|@$1|2|3|4|5|6|7|K|8|@]|9|@]|A|$B|C]]|$1|D|3|E|5|F|7|L|8|@]|9|@]|A|$]]|$1|G|3|H|5|6|7|M|8|@]|9|@]|A|$B|C]]|$1|I|3|-4|5|F|7|N|8|@]|9|@]|A|$]]]|J|$]]

<pre><code>function isEmpty(obj) {
 if (typeof obj == 'number') return false;
 else if (typeof obj == 'string') return obj.length == 0;
 else if (Array.isArray(obj)) return obj.length == 0;
 else if (typeof obj == 'object') return obj == null || Object.keys(obj).length == 0;
 else if (typeof obj == 'boolean') return false;
 else return !obj;
}
</code></pre>

In ES6 with trim to handle whitespace strings:

<pre><code>const isEmpty = value =&gt; {
 if (typeof value === 'number') return false
 else if (typeof value === 'string') return value.trim().length === 0
 else if (Array.isArray(value)) return value.length === 0
 else if (typeof value === 'object') return value == null || Object.keys(value).length === 0
 else if (typeof value === 'boolean') return false
 else return !value
}
</code></pre>

blocks|key|731380|text|try{

+++++let+vari+=+obj.propTest;+//+obj+may+be+don't+have+propTest+property

++++++++...
}+catch(NullException){
++++//+do+something+here
}|type|code-block|depth|inlineStyleRanges|entityRanges|data|syntax|javascript|731381|我认为使用try+catch可以避免任何null检查的错误，在Angular或JavaScript中也可以捕获null异常和进程。|unstyled|731382|entityMap^0|0|0^^$0|@$1|2|3|4|5|6|7|I|8|@]|9|@]|A|$B|C]]|$1|D|3|E|5|F|7|J|8|@]|9|@]|A|$]]|$1|G|3|-4|5|F|7|K|8|@]|9|@]|A|$]]]|H|$]]

<pre><code>try{

 let vari = obj.propTest; // obj may be don't have propTest property

 ...
} catch(NullException){
 // do something here
}
</code></pre>

I think using try catch will avoid any error of null check, also in Angular or JavaScript
Just catching null exception and process in it.

blocks|key|947408|text|return+val+%7C%7C+'Handle+empty+variable'|type|code-block|depth|inlineStyleRanges|entityRanges|data|syntax|javascript|947409|是在很多地方处理它的一种非常好和干净的方法，也可以用来赋值变量|unstyled|947410|const+res+=+val+%7C%7C+'default+value'|947411|entityMap^0|0|0|0^^$0|@$1|2|3|4|5|6|7|K|8|@]|9|@]|A|$B|C]]|$1|D|3|E|5|F|7|L|8|@]|9|@]|A|$]]|$1|G|3|H|5|6|7|M|8|@]|9|@]|A|$B|C]]|$1|I|3|-4|5|F|7|N|8|@]|9|@]|A|$]]]|J|$]]

<pre><code>return val || 'Handle empty variable'
</code></pre>

is a really nice and clean way to handle it in a lot of places, can also be used to assign variables 

<pre><code>const res = val || 'default value'
</code></pre>

blocks|key|731486|text|下面的内容对我很有效。请稍微修改一下，让它快一点|type|unstyled|depth|inlineStyleRanges|entityRanges|data|731487|function+isEmpty(obj)+{
++++if+(!obj)+return+true;
++++if+(typeof+obj+==+'number')+return+false;
++++else+if+(typeof+obj+==+'string')+return+obj.length+==+0;
++++else+if+(Array.isArray(obj))+return+obj.length+==+0;
++++else+if+(typeof+obj+==+'object')+return+obj+==+null+%7C%7C+Object.keys(obj).length+==+0;
++++else+if+(typeof+obj+==+'boolean')+return+false;
}|code-block|syntax|javascript|731488|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|$]]

Below worked for me. Please do a slight change to make this fast

<pre class="lang-js prettyprint-override"><code>function isEmpty(obj) {
 if (!obj) return true;
 if (typeof obj == 'number') return false;
 else if (typeof obj == 'string') return obj.length == 0;
 else if (Array.isArray(obj)) return obj.length == 0;
 else if (typeof obj == 'object') return obj == null || Object.keys(obj).length == 0;
 else if (typeof obj == 'boolean') return false;
}
</code></pre>

blocks|key|731554|text|当引用或函数可能未定义或为空时，可选的链运算符提供了一种简化通过连接的对象访问值的方法。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|731555|let+customer+=+{
++name:+"Carl",
++details:+{
++++age:+82,
++++location:+"Paradise+Falls"+//+detailed+address+is+unknown
++}
};
let+customerCity+=+customer.details?.address?.city;|code-block|syntax|javascript|731556|在可选链接之后，可以使用空合并运算符，以便在未找到任何值时构建默认值：|731557|let+customer+=+{
++name:+"Carl",
++details:+{+age:+82+}
};
const+customerCity+=+customer?.city+??+"Unknown+city";
console.log(customerCity);+//+Unknown+city|731558|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|$]]

The optional chaining operator provides a way to simplify accessing values through connected objects when it's possible that a reference or function may be undefined or null.

<pre><code>let customer = {
 name: "Carl",
 details: {
 age: 82,
 location: "Paradise Falls" // detailed address is unknown
 }
};
let customerCity = customer.details?.address?.city;
</code></pre>

The nullish coalescing operator may be used after optional chaining in order to build a default value when none was found:

<pre><code>let customer = {
 name: "Carl",
 details: { age: 82 }
};
const customerCity = customer?.city ?? "Unknown city";
console.log(customerCity); // Unknown city
</code></pre>

blocks|key|731596|text|你可以随时使用loadash函数，比如_.nil或_.undefined+.They都非常好用。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|731597|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|$]]

you can always use loadash functions ,like _.nil or _.undefined .They are pretty good to use.

blocks|key|731661|text|尝试使用Boolean()和isNaN()+(对于number类型)来检查变量是否有值。|type|unstyled|depth|inlineStyleRanges|entityRanges|offset|length|data|731662|​|731663|function+isEmpty(val)+{
++return+typeof+val+===+'number'+?+isNaN(val)+:+!Boolean(val);
}

var+emptyVals+=+[undefined,+null,+false,+NaN,+''];
emptyVals.forEach(v+=>+console.log(isEmpty(v)));|code-block|syntax|javascript|731664|731665|entityMap|0|LINK|mutability|MUTABLE|url|https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Boolean|1|https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/isNaN^0|4|9|0|E|7|1|0|0|0|0^^$0|@$1|2|3|4|5|6|7|V|8|@]|9|@$A|W|B|X|1|Y]|$A|Z|B|10|1|11]]|C|$]]|$1|D|3|E|5|6|7|12|8|@]|9|@]|C|$]]|$1|F|3|G|5|H|7|13|8|@]|9|@]|C|$I|J]]|$1|K|3|E|5|6|7|14|8|@]|9|@]|C|$]]|$1|L|3|-4|5|6|7|15|8|@]|9|@]|C|$]]]|M|$N|$5|O|P|Q|C|$R|S]]|T|$5|O|P|Q|C|$R|U]]]]

Try <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Boolean" rel="nofollow noreferrer">Boolean()</a> and <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/isNaN" rel="nofollow noreferrer">isNaN()</a> (for number type) to check a variable has a value or not.
<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 isEmpty(val) {
 return typeof val === 'number' ? isNaN(val) : !Boolean(val);
}

var emptyVals = [undefined, null, false, NaN, ''];
emptyVals.forEach(v =&gt; console.log(isEmpty(v)));</code></pre>
</div>
</div>

blocks|key|947633|text|将仅为undefined和null返回false|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|947634|return+value+??+false|947635|entityMap^0|3|9|D|4|J|5|0|0|L|0^^$0|@$1|2|3|4|5|6|7|J|8|@$9|K|A|L|B|C]|$9|M|A|N|B|C]|$9|O|A|P|B|C]]|D|@]|E|$]]|$1|F|3|G|5|6|7|Q|8|@$9|R|A|S|B|C]]|D|@]|E|$]]|$1|H|3|-4|5|6|7|T|8|@]|D|@]|E|$]]]|I|$]]

Will return <code>false</code> only for <code>undefined</code> and <code>null</code>:
<code>return value ?? false</code>

blocks|key|731719|text|您可以使用空合并操作符??检查null和undefined的值。请参阅MDN+Docs|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|731720|null+??+'default+string';+//+returns+"default+string"

0+??+42;++//+returns+0

(null+%7C%7C+undefined)+??+"foo";+//+returns+"foo"|code-block|syntax|javascript|731721|entityMap|0|LINK|mutability|MUTABLE|url|https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Nullish_coalescing_operator^0|B|2|F|4|K|9|Z|8|0|0|0^^$0|@$1|2|3|4|5|6|7|S|8|@$9|T|A|U|B|C]|$9|V|A|W|B|C]|$9|X|A|Y|B|C]]|D|@$9|Z|A|10|1|11]]|E|$]]|$1|F|3|G|5|H|7|12|8|@]|D|@]|E|$I|J]]|$1|K|3|-4|5|6|7|13|8|@]|D|@]|E|$]]]|L|$M|$5|N|O|P|E|$Q|R]]]]

You could use the nullish coalescing operator <code>??</code> to check for <code>null</code> and <code>undefined</code> values. See the <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Nullish_coalescing_operator" rel="nofollow noreferrer">MDN Docs</a>
<pre class="lang-js prettyprint-override"><code>null ?? 'default string'; // returns &quot;default string&quot;

0 ?? 42; // returns 0

(null || undefined) ?? &quot;foo&quot;; // returns &quot;foo&quot;
</code></pre>

blocks|key|947667|text|这是我检查数据是否为空的解决方案。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|947668|const+_isEmpty+=+(data)+=>+{
++return+(
++++//+this+way+we+can+also+check+for+undefined+values.+null==undefined+is+true
++++data+==+null+%7C%7C
++++data+==+""+%7C%7C
++++(Array.isArray(data)+&&+data.length+===+0)+%7C%7C
++++//+we+want+{}+to+be+false.+we+cannot+use+!!+because+!!{}+turns+to+be+true
++++//+!!{}=true+and+!!{name:"yilmaz"}=true.+!!+does+not+work+ofr+objects
++++(data.constructor+===+Object+&&+Object.keys(data).length+===+0)

++);
};|code-block|syntax|javascript|947669|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 is my solution to check if data is empty or not.
<pre><code>const _isEmpty = (data) =&gt; {
 return (
 // this way we can also check for undefined values. null==undefined is true
 data == null ||
 data == &quot;&quot; ||
 (Array.isArray(data) &amp;&amp; data.length === 0) ||
 // we want {} to be false. we cannot use !! because !!{} turns to be true
 // !!{}=true and !!{name:&quot;yilmaz&quot;}=true. !! does not work ofr objects
 (data.constructor === Object &amp;&amp; Object.keys(data).length === 0)

 );
};
</code></pre>

Is there a universal JavaScript function that checks that a variable has a value and ensures that it's not <code>undefined</code> or <code>null</code>? I've got this code, but I'm not sure if it covers all cases:

<pre><code>function isEmpty(val){
 return (val === undefined || val == null || val.length &lt;= 0) ? true : false;
}
</code></pre>

Is there a standard function to check for null, undefined, or blank variables in JavaScript?

jQuery

有没有一个通用的JavaScript函数可以检查变量是否有值，并确保它不是undefined或null？我有这段代码，但我不确定它是否涵盖了所有情况：function isEmpty(val){ return (val === undefined || val == null || val.length <= 0...

问在JavaScript中是否有一个标准函数来检查null、未定义或空白变量？
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问在JavaScript中是否有一个标准函数来检查null、未定义或空白变量？
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