blocks|key|1110818|text|if+(!('foo'+in+obj))+{
++//+not+set.
}|type|code-block|depth|inlineStyleRanges|entityRanges|data|syntax|javascript|1110819|unstyled|entityMap^0|0^^$0|@$1|2|3|4|5|6|7|G|8|@]|9|@]|A|$B|C]]|$1|D|3|-4|5|E|7|H|8|@]|9|@]|A|$]]]|F|$]]

blocks|key|1325998|text|//
//++tring+to+reference+non-existing+variable+throws+ReferenceError+
//++before+test+function+is+even+executed
//
//++example,+if+you+do:
//++++
//+++++if+(+isset(+someVar+)+)+
//++++++++doStuff(+someVar+);
//+++
//++you+get+a+ReferenceError+(+if+there+is+no+someVar...+)+
//++and+isset+fn+doesn't+get+executed.
//
//++if+you+pass+variable+name+as+string,+ex.+isset(+'novar'+);,+
//++this+might+work:
//
function+isset+(+strVariableName+)+{+

++++try+{+
++++++++eval(+strVariableName+);
++++}+catch(+err+)+{+
++++++++if+(+err+instanceof+ReferenceError+)+
+++++++++++return+false;
++++}

++++return+true;

+}+
//
//|type|code-block|depth|inlineStyleRanges|entityRanges|data|syntax|javascript|1325999|unstyled|entityMap^0|0^^$0|@$1|2|3|4|5|6|7|G|8|@]|9|@]|A|$B|C]]|$1|D|3|-4|5|E|7|H|8|@]|9|@]|A|$]]]|F|$]]

<pre><code>//
// tring to reference non-existing variable throws ReferenceError 
// before test function is even executed
//
// example, if you do:
// 
// if ( isset( someVar ) ) 
// doStuff( someVar );
// 
// you get a ReferenceError ( if there is no someVar... ) 
// and isset fn doesn't get executed.
//
// if you pass variable name as string, ex. isset( 'novar' );, 
// this might work:
//
function isset ( strVariableName ) { 

 try { 
 eval( strVariableName );
 } catch( err ) { 
 if ( err instanceof ReferenceError ) 
 return false;
 }

 return true;

 } 
//
//
</code></pre>

blocks|key|1110866|text|这个简单的解决方案有效，但不适用于深度对象检查。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1110867|function+isset(str)+{
++++return+window[str]+!==+undefined;
}|code-block|syntax|javascript|1110868|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 simple solution works, but not for deep object check.

<pre><code>function isset(str) {
 return window[str] !== undefined;
}
</code></pre>

blocks|key|1326156|text|我总是使用这个泛型函数来防止原始变量以及数组和对象出现错误。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1326157|isset+=+function(obj)+{
++var+i,+max_i;
++if(obj+===+undefined)+return+false;
++for+(i+=+1,+max_i+=+arguments.length;+i+<+max_i;+i%2B%2B)+{
++++if+(obj[arguments[i]]+===+undefined)+{
++++++++return+false;
++++}
++++obj+=+obj[arguments[i]];
++}
++return+true;
};

console.log(isset(obj));+++++++++++++++++++//+returns+false
var+obj+=+'huhu';
console.log(isset(obj));+++++++++++++++++++//+returns+true
obj+=+{hallo:{hoi:'hoi'}};
console.log(isset(obj,+'niet'));+++++++++++//+returns+false
console.log(isset(obj,+'hallo'));++++++++++//+returns+true
console.log(isset(obj,+'hallo',+'hallo'));+//+returns+false
console.log(isset(obj,+'hallo',+'hoi'));+++//+returns+true|code-block|syntax|javascript|1326158|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 always use this generic function to prevent errrors on primitive variables as well as arrays and objects.

<pre><code>isset = function(obj) {
 var i, max_i;
 if(obj === undefined) return false;
 for (i = 1, max_i = arguments.length; i &lt; max_i; i++) {
 if (obj[arguments[i]] === undefined) {
 return false;
 }
 obj = obj[arguments[i]];
 }
 return true;
};

console.log(isset(obj)); // returns false
var obj = 'huhu';
console.log(isset(obj)); // returns true
obj = {hallo:{hoi:'hoi'}};
console.log(isset(obj, 'niet')); // returns false
console.log(isset(obj, 'hallo')); // returns true
console.log(isset(obj, 'hallo', 'hallo')); // returns false
console.log(isset(obj, 'hallo', 'hoi')); // returns true
</code></pre>

blocks|key|1110941|text|这个解决方案对我很有效。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1110942|function+isset(object){
++++return+(typeof+object+!=='undefined');
}|code-block|syntax|javascript|1110943|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 solution worked for me. 

<pre><code>function isset(object){
 return (typeof object !=='undefined');
}
</code></pre>

blocks|key|1111001|text|如果您使用的是underscorejs，我总是使用|type|unstyled|depth|inlineStyleRanges|entityRanges|offset|length|data|1111002|if+(!_.isUndefined(data)+&&+!_.isNull(data))+{
+++++//your+stuff
}|code-block|syntax|javascript|1111003|entityMap|0|LINK|mutability|MUTABLE|url|http://underscorejs.org/^0|7|C|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]]]]

If you are using <a href="http://underscorejs.org/" rel="noreferrer">underscorejs</a> I always use

<pre><code>if (!_.isUndefined(data) &amp;&amp; !_.isNull(data)) {
 //your stuff
}
</code></pre>

blocks|key|1326215|text|window.isset+=+function(v_var)+{
++++if(typeof(v_var)+==+'number'){+if(isNaN(v_var)){+return+false;+}}
++++if(typeof(v_var)+==+'undefined'+%7C%7C+v_var+===+null){+return+false;+++}+else+{+return+true;+}
};|type|code-block|depth|inlineStyleRanges|entityRanges|data|syntax|javascript|1326216|加上测试：|unstyled|1326217|https://gist.github.com/daylik/24acc318b6abdcdd63b46607513ae073|offset|length|1326218|entityMap|0|LINK|mutability|MUTABLE|url^0|0|0|0|1R|0|0^^$0|@$1|2|3|4|5|6|7|R|8|@]|9|@]|A|$B|C]]|$1|D|3|E|5|F|7|S|8|@]|9|@]|A|$]]|$1|G|3|H|5|F|7|T|8|@]|9|@$I|U|J|V|1|W]]|A|$]]|$1|K|3|-4|5|F|7|X|8|@]|9|@]|A|$]]]|L|$M|$5|N|O|P|A|$Q|H]]]]

<pre><code>window.isset = function(v_var) {
 if(typeof(v_var) == 'number'){ if(isNaN(v_var)){ return false; }}
 if(typeof(v_var) == 'undefined' || v_var === null){ return false; } else { return true; }
};
</code></pre>

plus Tests:

<a href="https://gist.github.com/daylik/24acc318b6abdcdd63b46607513ae073" rel="nofollow noreferrer">https://gist.github.com/daylik/24acc318b6abdcdd63b46607513ae073</a>

blocks|key|1326301|text|(typeof+SOMETHING)+!==+'undefined'|type|code-block|depth|inlineStyleRanges|entityRanges|data|syntax|javascript|1326302|在使用时，它太长了，不能写。但我们不能将typeof关键字打包到函数中，因为在调用函数之前会抛出一个错误，如下所示：|unstyled|offset|length|style|CODE|1326303|function+isdef($var)+{
++++return+(typeof+$var)+!==+'undefined';
}

isdef(SOMETHING);+/////+thrown+error:+SOMETHING+is+not+defined|1326304|所以我想出了一个办法：|1326305|function+isdef($type)+{
++++return+$type+!==+'undefined';
}

isdef(typeof+SOMETHING);|1326306|它既可以处理单个变量(根本不存在的变量)，也可以处理对象属性(不存在的属性)。只比PHP+isset多7个字符。|1326307|entityMap^0|0|K|6|0|0|0|0|19|5|0^^$0|@$1|2|3|4|5|6|7|U|8|@]|9|@]|A|$B|C]]|$1|D|3|E|5|F|7|V|8|@$G|W|H|X|I|J]]|9|@]|A|$]]|$1|K|3|L|5|6|7|Y|8|@]|9|@]|A|$B|C]]|$1|M|3|N|5|F|7|Z|8|@]|9|@]|A|$]]|$1|O|3|P|5|6|7|10|8|@]|9|@]|A|$B|C]]|$1|Q|3|R|5|F|7|11|8|@$G|12|H|13|I|J]]|9|@]|A|$]]|$1|S|3|-4|5|F|7|14|8|@]|9|@]|A|$]]]|T|$]]

<pre><code>(typeof SOMETHING) !== 'undefined'
</code></pre>

It's too long to write when used. But we can't package the <code>typeof</code> keyword into a function, because an error will thrown before the function is called, like this:

<pre><code>function isdef($var) {
 return (typeof $var) !== 'undefined';
}

isdef(SOMETHING); ///// thrown error: SOMETHING is not defined
</code></pre>

So I figured out a way:

<pre><code>function isdef($type) {
 return $type !== 'undefined';
}

isdef(typeof SOMETHING);
</code></pre>

It can work both with individual variables (variables that does not exist at all), or object properties (non-existent properties). And only 7 more characters than PHP <code>isset</code>.

blocks|key|1326196|text|function+isset(variable)+{
++++try+{
++++++++return+typeof+eval(variable)+!==+'undefined';
++++}+catch+(err)+{
++++++++return+false;
++++}
}|type|code-block|depth|inlineStyleRanges|entityRanges|data|syntax|javascript|1326197|unstyled|entityMap^0|0^^$0|@$1|2|3|4|5|6|7|G|8|@]|9|@]|A|$B|C]]|$1|D|3|-4|5|E|7|H|8|@]|9|@]|A|$]]]|F|$]]

<pre><code>function isset(variable) {
 try {
 return typeof eval(variable) !== 'undefined';
 } catch (err) {
 return false;
 }
}
</code></pre>

blocks|key|1326238|text|为了检查html块是否存在，我使用以下代码：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1326239|if+(typeof($('selector').html())+!=+'undefined')+{
++++//+$('selector')+is+existing
++++//+your+code+here
}|code-block|syntax|javascript|1326240|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|$]]

To check wether html block is existing or not, I'm using this code:

<pre><code>if (typeof($('selector').html()) != 'undefined') {
 // $('selector') is existing
 // your code here
}
</code></pre>

blocks|key|1110927|text|将对象路径作为字符串提供，然后您可以将此字符串分解为路径，并在每次迭代覆盖对象本身的同时在每一步解析hasOwnProperty。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|1110928|如果你是在ES6环境中编码，看看this+stackoverflow+Ques吧。|1110929|​|1110930|var+a;

a+=+{
++++b:+{
++++++++c:+'e'
++++}
};

function+isset+(obj,+path)+{
++++var+stone;

++++path+=+path+%7C%7C+'';

++++if+(path.indexOf('[')+!==+-1)+{
++++++++throw+new+Error('Unsupported+object+path+notation.');
++++}

++++
++++path+=+path.split('.');
++++
++++do+{
++++++++if+(obj+===+undefined)+{
++++++++++++return+false;
++++++++}

++++++++stone+=+path.shift();
++++++++
++++++++if+(!obj.hasOwnProperty(stone))+{
++++++++++++return+false;
++++++++}
++++++++
++++++++obj+=+obj[stone];
++++++++
++++}+while+(path.length);

++++return+true;
}

console.log(
++++isset(a,+'b')+==+true,
++++isset(a,+'b.c')+==+true,
++++isset(a,+'b.c.d')+==+false,
++++isset(a,+'b.c.d.e')+==+false,
++++isset(a,+'b.c.d.e.f')+==+false
);|code-block|syntax|javascript|1110931|1110932|entityMap|0|LINK|mutability|MUTABLE|url|https://stackoverflow.com/a/26990347/368691^0|1E|E|0|G|N|0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|X|8|@$9|Y|A|Z|B|C]]|D|@]|E|$]]|$1|F|3|G|5|6|7|10|8|@]|D|@$9|11|A|12|1|13]]|E|$]]|$1|H|3|I|5|6|7|14|8|@]|D|@]|E|$]]|$1|J|3|K|5|L|7|15|8|@]|D|@]|E|$M|N]]|$1|O|3|I|5|6|7|16|8|@]|D|@]|E|$]]|$1|P|3|-4|5|6|7|17|8|@]|D|@]|E|$]]]|Q|$R|$5|S|T|U|E|$V|W]]]]

Provide the object path as a string, then you can break this string into a path and resolve <code>hasOwnProperty</code> at each step while overwriting the object itself with each iteration.

If you are coding in ES6 environment, take a look at <a href="https://stackoverflow.com/a/26990347/368691">this stackoverflow Ques</a>.

<div class="snippet" data-lang="js" data-hide="false" data-console="false" data-babel="false">
<div class="snippet-code">
<pre class="snippet-code-js lang-js prettyprint-override"><code>var a;

a = {
 b: {
 c: 'e'
 }
};

function isset (obj, path) {
 var stone;

 path = path || '';

 if (path.indexOf('[') !== -1) {
 throw new Error('Unsupported object path notation.');
 }

 
 path = path.split('.');
 
 do {
 if (obj === undefined) {
 return false;
 }

 stone = path.shift();
 
 if (!obj.hasOwnProperty(stone)) {
 return false;
 }
 
 obj = obj[stone];
 
 } while (path.length);

 return true;
}

console.log(
 isset(a, 'b') == true,
 isset(a, 'b.c') == true,
 isset(a, 'b.c.d') == false,
 isset(a, 'b.c.d.e') == false,
 isset(a, 'b.c.d.e.f') == false
);</code></pre>
</div>
</div>

blocks|key|1326367|text|我使用了一个可以检查变量和对象的函数。使用jQuery非常方便|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1326368|++++function+_isset+(variable)+{
++++++++if(typeof(variable)+==+"undefined"+%7C%7C+variable+==+null)
++++++++++++return+false;
++++++++else
++++++++++++if(typeof(variable)+==+"object"+&&+!variable.length)+
++++++++++++++++return+false;
++++++++++++else
++++++++++++++++return+true;
++++};|code-block|syntax|javascript|1326369|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 use a function that can check variables and objects. very convenient to work with jQuery

<pre><code> function _isset (variable) {
 if(typeof(variable) == "undefined" || variable == null)
 return false;
 else
 if(typeof(variable) == "object" &amp;&amp; !variable.length) 
 return false;
 else
 return true;
 };
</code></pre>

blocks|key|1326566|text|尝试在Javascript中创建类似PHP的空函数的函数。希望这能对你有所帮助。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1326567|function+empty(str){
++try{
++++if(typeof+str==="string"){
++++++++str=str.trim();
++++}
++++return+!(str+!==+undefined+&&+str+!==+"undefined"+&&+str+!==+null+&&+str!==""+&&+str!==0+&&+str!==false);
++}catch(ex){
++++return+true;
++}
+}

console.log(empty(0))//true
console.log(empty(null))//true
console.log(empty("+"))//true
console.log(empty(""))//true
console.log(empty(undefined))//true
console.log(empty("undefined"))//true

var+tmp=1;
console.log(empty(tmp))//false

var+tmp="Test";
console.log(empty(tmp))//false

var+tmp="+Test+";
console.log(empty(tmp))//false

var+tmp={a:1,b:false,c:0};
console.log(empty(tmp.a))//false
console.log(empty(tmp.b))//true
console.log(empty(tmp.c))//true
console.log(empty(tmp.c))//true
console.log(empty(tmp.c.d))//true|code-block|syntax|javascript|1326568|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|$]]

Try to create function like empty function of PHP in Javascript.
May this helps.
<pre><code>function empty(str){
 try{
 if(typeof str===&quot;string&quot;){
 str=str.trim();
 }
 return !(str !== undefined &amp;&amp; str !== &quot;undefined&quot; &amp;&amp; str !== null &amp;&amp; str!==&quot;&quot; &amp;&amp; str!==0 &amp;&amp; str!==false);
 }catch(ex){
 return true;
 }
 }

console.log(empty(0))//true
console.log(empty(null))//true
console.log(empty(&quot; &quot;))//true
console.log(empty(&quot;&quot;))//true
console.log(empty(undefined))//true
console.log(empty(&quot;undefined&quot;))//true

var tmp=1;
console.log(empty(tmp))//false

var tmp=&quot;Test&quot;;
console.log(empty(tmp))//false

var tmp=&quot; Test &quot;;
console.log(empty(tmp))//false

var tmp={a:1,b:false,c:0};
console.log(empty(tmp.a))//false
console.log(empty(tmp.b))//true
console.log(empty(tmp.c))//true
console.log(empty(tmp.c))//true
console.log(empty(tmp.c.d))//true
</code></pre>

blocks|key|1111252|text|当我访问一个对象的更深层次的属性时，这对我来说真的是一个问题，所以我做了一个函数，如果存在，它将返回属性值，否则将返回false。你可以用它来节省时间，|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1111253|//Object+on+which+we+want+to+test
var+foo+=+{
++++bar:+{
++++++++bik:+{
++++++++++++baz:+'Hello+world'
++++++++}
++++}
};


/*
USE:+To+get+value+from+the+object+using+it+properties+supplied+(Deeper),
++++if+found+it+will+return+the+property+value+if+not+found+then+will+return+false

You+can+use+this+function+in+two+ways
WAY+-+1:
Passing+an+object+as+parameter+1+and+array+of+the+properties+as+parameter+2
EG:+getValueFromObject(foo,+['bar',+'bik',+'baz']);
WAY+-+2:+(This+will+work+only+if,+your+object+available+in+window+object)
Passing+an+STRING+as+parameter+1(Just+similarly+how+we+retrieve+value+form+object+using+it's+properties+-+difference+is+only+the+quote)
EG:+getValueFromObject('foo.bar.bik.baz');
*/
function+getValueFromObject(object,+properties)+{
++++if(typeof(object)+==+'string')+{++++++++++++//Here+we+extract+our+object+and+it's+properties+from+the+string
++++++++properties+=+object.split('.');
++++++++object+=+window[properties[0]];
++++++++if(typeof(object)+==+'undefined')+{
++++++++++++return+false;
++++++++}
++++++++properties.shift();
++++}
++++var+property+=+properties[0];
++++properties.shift();
++++if(object+!=+null+&&+typeof(object[property])+!=+'undefined')+{
++++++++if(typeof(object[property])+==+'object')+{
++++++++++++if(properties.length+!=+0)+{
++++++++++++++++return+getValueFromObject(object[property],+properties);++++//Recursive+call+to+the+function
++++++++++++}+else+{
++++++++++++++++return+object[property];
++++++++++++}
++++++++}+else+{
++++++++++++return+object[property];
++++++++}
++++}+else+{
++++++++return+false;
++++}
}
console.log(getValueFromObject('fooo.bar.bik.baz'));++++++++//false
console.log(getValueFromObject('foo.bar.bik.baz'));+++++++++//Hello+world
console.log(getValueFromObject('foo'));+++++++++++++++++++++//false
console.log(getValueFromObject('foo.bar.bik'));+++++++++++++//returns+an+object+{+baz:+'Hello+World'+}
console.log(getValueFromObject(foo,+['bar',+'bik']));+++++++//returns+an+object+{+baz:+'Hello+World'+}
console.log(getValueFromObject(foo,+['bar',+'bik',+'baz']));//Hello+world|code-block|syntax|javascript|1111254|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|$]]

It was really a problem for me when I was accessing a deeper property of an object so I made a function which will return the property value if exist otherwise it will return false. You may use it to save your time,

<pre><code>//Object on which we want to test
var foo = {
 bar: {
 bik: {
 baz: 'Hello world'
 }
 }
};


/*
USE: To get value from the object using it properties supplied (Deeper),
 if found it will return the property value if not found then will return false

You can use this function in two ways
WAY - 1:
Passing an object as parameter 1 and array of the properties as parameter 2
EG: getValueFromObject(foo, ['bar', 'bik', 'baz']);
WAY - 2: (This will work only if, your object available in window object)
Passing an STRING as parameter 1(Just similarly how we retrieve value form object using it's properties - difference is only the quote)
EG: getValueFromObject('foo.bar.bik.baz');
*/
function getValueFromObject(object, properties) {
 if(typeof(object) == 'string') { //Here we extract our object and it's properties from the string
 properties = object.split('.');
 object = window[properties[0]];
 if(typeof(object) == 'undefined') {
 return false;
 }
 properties.shift();
 }
 var property = properties[0];
 properties.shift();
 if(object != null &amp;&amp; typeof(object[property]) != 'undefined') {
 if(typeof(object[property]) == 'object') {
 if(properties.length != 0) {
 return getValueFromObject(object[property], properties); //Recursive call to the function
 } else {
 return object[property];
 }
 } else {
 return object[property];
 }
 } else {
 return false;
 }
}
console.log(getValueFromObject('fooo.bar.bik.baz')); //false
console.log(getValueFromObject('foo.bar.bik.baz')); //Hello world
console.log(getValueFromObject('foo')); //false
console.log(getValueFromObject('foo.bar.bik')); //returns an object { baz: 'Hello World' }
console.log(getValueFromObject(foo, ['bar', 'bik'])); //returns an object { baz: 'Hello World' }
console.log(getValueFromObject(foo, ['bar', 'bik', 'baz']));//Hello world
</code></pre>

blocks|key|1326478|text|如果要检查元素是否存在，只需使用以下代码：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1326479|if+(object)+{
++//if+isset,+return+true
}+else+{
++//else+return+false
}|code-block|syntax|javascript|1326480|示例如下：|1326481|​|1326482|function+switchDiv()+{
++++if+(document.querySelector("#divId"))+{
++++++++document.querySelector("#divId").remove();
++++}+else+{
++++++++var+newDiv+=+document.createElement("div");
++++++++newDiv.id+=+"divId";
++++++++document.querySelector("body").appendChild(newDiv);
++++}
}

document.querySelector("#btn").addEventListener("click",+switchDiv);|1326483|#divId+{
++++background:+red;
++++height:+100px;
++++width:+100px;
++++position:+relative;
++++
}|1326484|<body>
++<button+id="btn">Let's+Diiiv!</button>
</body>|1326485|1326486|entityMap^0|0|0|0|0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|T|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|U|8|@]|9|@]|A|$E|F]]|$1|G|3|H|5|6|7|V|8|@]|9|@]|A|$]]|$1|I|3|J|5|6|7|W|8|@]|9|@]|A|$]]|$1|K|3|L|5|D|7|X|8|@]|9|@]|A|$E|F]]|$1|M|3|N|5|D|7|Y|8|@]|9|@]|A|$E|F]]|$1|O|3|P|5|D|7|Z|8|@]|9|@]|A|$E|F]]|$1|Q|3|J|5|6|7|10|8|@]|9|@]|A|$]]|$1|R|3|-4|5|6|7|11|8|@]|9|@]|A|$]]]|S|$]]

If you want to check if an element exists, just use the following code:

<pre><code>if (object) {
 //if isset, return true
} else {
 //else return false
}
</code></pre>

This is sample:

<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 switchDiv() {
 if (document.querySelector("#divId")) {
 document.querySelector("#divId").remove();
 } else {
 var newDiv = document.createElement("div");
 newDiv.id = "divId";
 document.querySelector("body").appendChild(newDiv);
 }
}

document.querySelector("#btn").addEventListener("click", switchDiv);</code></pre>
<pre class="snippet-code-css lang-css prettyprint-override"><code>#divId {
 background: red;
 height: 100px;
 width: 100px;
 position: relative;
 
}</code></pre>
<pre class="snippet-code-html lang-html prettyprint-override"><code>&lt;body&gt;
 &lt;button id="btn"&gt;Let's Diiiv!&lt;/button&gt;
&lt;/body&gt;</code></pre>
</div>
</div>

blocks|key|1111453|text|最后，我用简单的方法解决了问题：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1111454|if+(obj+&&+obj.foo+&&+obj.foo='somethings'){
console.log('i,m+work+without+error')
}|code-block|syntax|javascript|1111455|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|$]]

finally i solved problem with easy solution :
<pre><code>if (obj &amp;&amp; obj.foo &amp;&amp; obj.foo='somethings'){
console.log('i,m work without error')
}
</code></pre>

blocks|key|1111361|text|javascript+isset|type|unstyled|depth|inlineStyleRanges|entityRanges|offset|length|data|1111362|let+test+=+{
++a:+{
++++b:+[0,+1]
++}
};

console.log(test.isset('a.b'))+++//+true
console.log(test.isset('a.b.1'))+//+true
console.log(test.isset('a.b.5'))+//+false
console.log(test.isset('a.c'))+++//+false
console.log('abv'.isset('0'))++++//+true|code-block|syntax|javascript|1111363|entityMap|0|LINK|mutability|MUTABLE|url|https://gist.github.com/AlekseyArh/53811ecab8daa83a310215aa47eec1c2^0|0|G|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="https://gist.github.com/AlekseyArh/53811ecab8daa83a310215aa47eec1c2" rel="nofollow noreferrer">javascript isset</a>

<pre><code>let test = {
 a: {
 b: [0, 1]
 }
};

console.log(test.isset('a.b')) // true
console.log(test.isset('a.b.1')) // true
console.log(test.isset('a.b.5')) // false
console.log(test.isset('a.c')) // false
console.log('abv'.isset('0')) // true
</code></pre>

blocks|key|1326620|text|https://jsfiddle.net/31b6mhtj/|type|unstyled|depth|inlineStyleRanges|entityRanges|offset|length|data|1326621|arr={nested:{nested2:{val:'isset'}}}
if(t=isset(arr,'nested=>nested2=>val')){
alert(t)
}
function+isset(obj,nested)+{
++++var+dub=obj
++++var+isset=false
++++if(typeof(obj)!="undefined"+&&+typeof(nested)!="undefined"){
++++++++var+arr=nested.split('=>');
++++++++for(var+k+in+arr){
++++++++++++var+key=arr[k];
++++++++++++if(typeof(dub[key])=="undefined"){
++++++++++++++++isset=false;
++++++++++++++++break;
++++++++++++}
++++++++++++dub=dub[key];
++++++isset=dub
++++++++}+++
++++}
++++
++++return+isset;
}|code-block|syntax|javascript|1326622|entityMap|0|LINK|mutability|MUTABLE|url^0|0|U|0|0|0^^$0|@$1|2|3|4|5|6|7|P|8|@]|9|@$A|Q|B|R|1|S]]|C|$]]|$1|D|3|E|5|F|7|T|8|@]|9|@]|C|$G|H]]|$1|I|3|-4|5|6|7|U|8|@]|9|@]|C|$]]]|J|$K|$5|L|M|N|C|$O|4]]]]

<a href="https://jsfiddle.net/31b6mhtj/" rel="nofollow noreferrer">https://jsfiddle.net/31b6mhtj/</a>
<pre><code>arr={nested:{nested2:{val:'isset'}}}
if(t=isset(arr,'nested=&gt;nested2=&gt;val')){
alert(t)
}
function isset(obj,nested) {
 var dub=obj
 var isset=false
 if(typeof(obj)!=&quot;undefined&quot; &amp;&amp; typeof(nested)!=&quot;undefined&quot;){
 var arr=nested.split('=&gt;');
 for(var k in arr){
 var key=arr[k];
 if(typeof(dub[key])==&quot;undefined&quot;){
 isset=false;
 break;
 }
 dub=dub[key];
 isset=dub
 } 
 }
 
 return isset;
}
</code></pre>

blocks|key|1326254|text|if+(var)+{
++//+This+is+the+most+concise+equivalent+of+Php's+isset().
}+|type|code-block|depth|inlineStyleRanges|entityRanges|data|syntax|javascript|1326255|unstyled|entityMap^0|0^^$0|@$1|2|3|4|5|6|7|G|8|@]|9|@]|A|$B|C]]|$1|D|3|-4|5|E|7|H|8|@]|9|@]|A|$]]]|F|$]]

<pre><code>if (var) {
 // This is the most concise equivalent of Php's isset().
} 
</code></pre>

blocks|key|1326506|text|++++isset('user.permissions.saveProject',+args);

++++function+isset(string,+context)+{
++++++++try+{
++++++++++++var+arr+=+string.split('.');
++++++++++++var+checkObj+=+context+%7C%7C+window;

++++++++++++for+(var+i+in+arr)+{
++++++++++++++++if+(checkObj[arr[i]]+===+undefined)+return+false;
++++++++++++++++checkObj+=+checkObj[arr[i]];
++++++++++++}

++++++++++++return+true;
++++++++}+catch+(e)+{
++++++++++++return+false;
++++++++}
++++}|type|code-block|depth|inlineStyleRanges|entityRanges|data|syntax|javascript|1326507|unstyled|entityMap^0|0^^$0|@$1|2|3|4|5|6|7|G|8|@]|9|@]|A|$B|C]]|$1|D|3|-4|5|E|7|H|8|@]|9|@]|A|$]]]|F|$]]

<pre class="lang-js prettyprint-override"><code> isset('user.permissions.saveProject', args);

 function isset(string, context) {
 try {
 var arr = string.split('.');
 var checkObj = context || window;

 for (var i in arr) {
 if (checkObj[arr[i]] === undefined) return false;
 checkObj = checkObj[arr[i]];
 }

 return true;
 } catch (e) {
 return false;
 }
 }
</code></pre>

blocks|key|1326659|text|try+{
++const+value+=+array.foo.object.value;
++//+isset+true
}+catch+(err)+{
++//+isset+false
}|type|code-block|depth|inlineStyleRanges|entityRanges|data|syntax|javascript|1326660|unstyled|entityMap^0|0^^$0|@$1|2|3|4|5|6|7|G|8|@]|9|@]|A|$B|C]]|$1|D|3|-4|5|E|7|H|8|@]|9|@]|A|$]]]|F|$]]

<pre><code>try {
 const value = array.foo.object.value;
 // isset true
} catch (err) {
 // isset false
}
</code></pre>

In PHP you can do <code>if(isset($array['foo'])) { ... }</code>. In JavaScript you often use <code>if(array.foo) { ... }</code> to do the same, but this is not exactly the same statement. The condition will also evaluate to false if <code>array.foo</code> does exists but is <code>false</code> or <code>0</code> (and probably other values as well).

What is the perfect equivalent of PHP's <code>isset</code> in JavaScript?

In a broader sense, a general, complete guide on JavaScript's handling of variables that don't exist, variables without a value, etc. would be convenient.

JavaScript isset() equivalent

jQuery

在PHP中，你可以做if(isset($array['foo'])) { ... }。在JavaScript中，您经常使用if(array.foo) { ... }来做同样的事情，但这并不是完全相同的语句。如果array.foo确实存在，但为false或0 (也可能是其他值)，则该条件的计算结果也将为false。在JavaScript中，什么是PHP的isset的完美等价物？在更广泛的意义上，一个

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