blocks|key|871231|text|试试这个表达式：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|871232|typeof(variable)+!=+"undefined"+&&+variable+!==+null|code-block|syntax|javascript|871233|如果变量被定义并且不为null，这将是正确的，这等同于PHP的isset的工作方式。|871234|你可以这样使用它：|871235|if(typeof(variable)+!=+"undefined"+&&+variable+!==+null)+{
++++bla();
}|871236|entityMap^0|0|0|0|0|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]]|$1|G|3|H|5|6|7|Q|8|@]|9|@]|A|$]]|$1|I|3|J|5|6|7|R|8|@]|9|@]|A|$]]|$1|K|3|L|5|D|7|S|8|@]|9|@]|A|$E|F]]|$1|M|3|-4|5|6|7|T|8|@]|9|@]|A|$]]]|N|$]]

Try this expression:

<pre><code>typeof(variable) != "undefined" &amp;&amp; variable !== null
</code></pre>

This will be true if the variable is defined and not null, which is the equivalent of how PHP's isset works.

You can use it like this:

<pre><code>if(typeof(variable) != "undefined" &amp;&amp; variable !== null) {
 bla();
}
</code></pre>

blocks|key|871294|text|JavaScript+isset()+on+PHP+JS|type|unstyled|depth|inlineStyleRanges|entityRanges|offset|length|data|871295|function+isset+()+{
++++//+discuss+at:+http://phpjs.org/functions/isset
++++//+%2B+++original+by:+Kevin+van+++++Zonneveld+(http://kevin.vanzonneveld.net)
++++//+%2B+++improved+by:+FremyCompany
++++//+%2B+++improved+by:+Onno+Marsman
++++//+%2B+++improved+by:+Rafał+Kukawski
++++//+*+++++example+1:+isset(+undefined,+true);
++++//+*+++++returns+1:+false
++++//+*+++++example+2:+isset(+'Kevin+van+Zonneveld'+);
++++//+*+++++returns+2:+true
++++var+a+=+arguments,
++++++++l+=+a.length,
++++++++i+=+0,
++++++++undef;

++++if+(l+===+0)+{
++++++++throw+new+Error('Empty+isset');
++++}

++++while+(i+!==+l)+{
++++++++if+(a[i]+===+undef+%7C%7C+a[i]+===+null)+{
++++++++++++return+false;
++++++++}
++++++++i%2B%2B;
++++}
++++return+true;
}|code-block|syntax|javascript|871296|entityMap|0|LINK|mutability|MUTABLE|url|http://phpjs.org/functions/isset:454^0|0|S|0|0|0^^$0|@$1|2|3|4|5|6|7|Q|8|@]|9|@$A|R|B|S|1|T]]|C|$]]|$1|D|3|E|5|F|7|U|8|@]|9|@]|C|$G|H]]|$1|I|3|-4|5|6|7|V|8|@]|9|@]|C|$]]]|J|$K|$5|L|M|N|C|$O|P]]]]

<a href="http://phpjs.org/functions/isset:454" rel="noreferrer">JavaScript isset() on PHP JS</a>

<pre><code>function isset () {
 // discuss at: http://phpjs.org/functions/isset
 // + original by: Kevin van Zonneveld (http://kevin.vanzonneveld.net)
 // + improved by: FremyCompany
 // + improved by: Onno Marsman
 // + improved by: Rafał Kukawski
 // * example 1: isset( undefined, true);
 // * returns 1: false
 // * example 2: isset( 'Kevin van Zonneveld' );
 // * returns 2: true
 var a = arguments,
 l = a.length,
 i = 0,
 undef;

 if (l === 0) {
 throw new Error('Empty isset');
 }

 while (i !== l) {
 if (a[i] === undef || a[i] === null) {
 return false;
 }
 i++;
 }
 return true;
}
</code></pre>

blocks|key|871188|text|typeof将达到我认为的目的|type|unstyled|depth|inlineStyleRanges|entityRanges|data|871189|if(typeof+foo+!=+"undefined"){}|code-block|syntax|javascript|871190|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|$]]

typeof will serve the purpose I think

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

blocks|key|1086073|text|如果你想检查一个属性是否存在：hasOwnProperty是最好的选择|type|unstyled|depth|inlineStyleRanges|entityRanges|offset|length|data|1086074|由于大多数对象都是其他对象的属性(最终指向window对象)，因此这可以很好地检查值是否已被声明。|style|CODE|1086075|entityMap|0|LINK|mutability|MUTABLE|url|https://developer.mozilla.org/en/JavaScript/Reference/Global_Objects/Object/hasOwnProperty^0|F|E|0|0|L|6|0^^$0|@$1|2|3|4|5|6|7|P|8|@]|9|@$A|Q|B|R|1|S]]|C|$]]|$1|D|3|E|5|6|7|T|8|@$A|U|B|V|F|G]]|9|@]|C|$]]|$1|H|3|-4|5|6|7|W|8|@]|9|@]|C|$]]]|I|$J|$5|K|L|M|C|$N|O]]]]

If you want to check if a property exists: <a href="https://developer.mozilla.org/en/JavaScript/Reference/Global_Objects/Object/hasOwnProperty" rel="nofollow">hasOwnProperty</a> is the way to go

And since most objects are properties of some other object (eventually leading to the <code>window</code> object) this can work well for checking if values have been declared.

blocks|key|1085937|text|不是很自然，不..。然而，在谷歌上搜索一下，就会得到这样的结果：http://phpjs.org/functions/isset:454|type|unstyled|depth|inlineStyleRanges|entityRanges|offset|length|data|1085938|entityMap|0|LINK|mutability|MUTABLE|url|http://phpjs.org/functions/isset:454^0|W|10|0|0^^$0|@$1|2|3|4|5|6|7|L|8|@]|9|@$A|M|B|N|1|O]]|C|$]]|$1|D|3|-4|5|6|7|P|8|@]|9|@]|C|$]]]|E|$F|$5|G|H|I|C|$J|K]]]]

Not naturally, no... However, a googling of the thing gave this: <a href="http://phpjs.org/functions/isset:454" rel="nofollow">http://phpjs.org/functions/isset:454</a>

blocks|key|1085961|text|http://phpjs.org/functions/isset:454|type|unstyled|depth|inlineStyleRanges|entityRanges|offset|length|data|1085962|phpjs项目是一个值得信赖的来源。这里有很多与js等价的php函数。我已经使用了很长一段时间，到目前为止没有发现任何问题。|1085963|entityMap|0|LINK|mutability|MUTABLE|url^0|0|10|0|0|0^^$0|@$1|2|3|4|5|6|7|M|8|@]|9|@$A|N|B|O|1|P]]|C|$]]|$1|D|3|E|5|6|7|Q|8|@]|9|@]|C|$]]|$1|F|3|-4|5|6|7|R|8|@]|9|@]|C|$]]]|G|$H|$5|I|J|K|C|$L|4]]]]

<a href="http://phpjs.org/functions/isset:454" rel="nofollow">http://phpjs.org/functions/isset:454</a>

phpjs project is a trusted source. Lots of js equivalent php functions available there. I have been using since a long time and found no issues so far.

blocks|key|871361|text|问题是，将未定义的变量传递给函数会导致错误。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|871362|这意味着在将typeof作为参数传递之前，必须先运行typeof。|871363|我找到的最干净的方法是这样的：|871364|function+isset(v){
++++if(v+===+'undefined'){
++++++++return+false;
++++}
++++return+true;
}|code-block|syntax|javascript|871365|用法：|871366|if(isset(typeof(varname))){
++alert('is+set');
}+else+{
++alert('not+set');
}|871367|现在代码更加紧凑，可读性也更好了。|871368|如果您尝试从未实例化的变量调用变量，这仍然会给出一个错误，例如：|871369|isset(typeof(undefVar.subkey))|871370|因此，在尝试运行此操作之前，您需要确保定义了对象：|871371|undefVar+=+isset(typeof(undefVar))?undefVar:{};|871372|entityMap^0|0|0|0|0|0|0|0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|10|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|11|8|@]|9|@]|A|$]]|$1|D|3|E|5|6|7|12|8|@]|9|@]|A|$]]|$1|F|3|G|5|H|7|13|8|@]|9|@]|A|$I|J]]|$1|K|3|L|5|6|7|14|8|@]|9|@]|A|$]]|$1|M|3|N|5|H|7|15|8|@]|9|@]|A|$I|J]]|$1|O|3|P|5|6|7|16|8|@]|9|@]|A|$]]|$1|Q|3|R|5|6|7|17|8|@]|9|@]|A|$]]|$1|S|3|T|5|H|7|18|8|@]|9|@]|A|$I|J]]|$1|U|3|V|5|6|7|19|8|@]|9|@]|A|$]]|$1|W|3|X|5|H|7|1A|8|@]|9|@]|A|$I|J]]|$1|Y|3|-4|5|6|7|1B|8|@]|9|@]|A|$]]]|Z|$]]

The problem is that passing an undefined variable to a function causes an error.

This means you have to run typeof before passing it as an argument.

The cleanest way I found to do this is like so:

<pre><code>function isset(v){
 if(v === 'undefined'){
 return false;
 }
 return true;
}
</code></pre>

Usage:

<pre><code>if(isset(typeof(varname))){
 alert('is set');
} else {
 alert('not set');
}
</code></pre>

Now the code is much more compact and readable.

This will still give an error if you try to call a variable from a non instantiated variable like:

<pre><code>isset(typeof(undefVar.subkey))
</code></pre>

thus before trying to run this you need to make sure the object is defined:

<pre><code>undefVar = isset(typeof(undefVar))?undefVar:{};
</code></pre>

blocks|key|1086187|text|这里:)|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1086188|function+isSet(iVal){
+return+(iVal!==""+&&+iVal!=null+&&+iVal!==undefined+&&+typeof(iVal)+!=+"undefined")+?+1+:+0;
}+//+Returns+1+if+set,+0+false|code-block|syntax|javascript|1086189|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 :)

<pre><code>function isSet(iVal){
 return (iVal!=="" &amp;&amp; iVal!=null &amp;&amp; iVal!==undefined &amp;&amp; typeof(iVal) != "undefined") ? 1 : 0;
} // Returns 1 if set, 0 false
</code></pre>

blocks|key|871383|text|您只需：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|871384|if(variable%7C%7Cvariable===0){
++++//Yes+it+is+set
++++//do+something
}
else+{
++++//No+it+is+not+set
++++//Or+its+null
++++//do+something+else+
}|code-block|syntax|javascript|871385|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|$]]

You can just:

<pre><code>if(variable||variable===0){
 //Yes it is set
 //do something
}
else {
 //No it is not set
 //Or its null
 //do something else 
}
</code></pre>

Is there something in javascript/jQuery to check whether variable is set/available or not? In php, we use <code>isset($variable)</code> to check something like this.

thanks.

is there something like isset of php in javascript/jQuery?

jQuery

javascript/jQuery中有没有什么东西可以用来检查变量是否被设置/可用？在php中，我们使用isset($variable)来检查类似的东西。谢谢。

问在javascript/jQuery中有没有像isset这样的php？
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问在javascript/jQuery中有没有像isset这样的php？
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