blocks|key|509636|text|if+(typeof+image_array+!==+'undefined'+&&+image_array.length+>+0)+{
++++//+the+array+is+defined+and+has+at+least+one+element
}|type|code-block|depth|inlineStyleRanges|entityRanges|data|syntax|javascript|509637|您的问题可能是由于隐式全局变量和变量提升的混合造成的。确保在声明变量时使用var：|unstyled|offset|length|style|CODE|509638|<?php+echo+"var+image_array+=+".json_encode($images);?>
//+add+var++%5E%5E%5E+here|509639|然后确保以后不会意外地重新声明该变量：|509640|else+{
++++...
++++image_array+=+[];+//+no+var+here
}|509641|entityMap^0|0|11|3|0|0|0|0^^$0|@$1|2|3|4|5|6|7|S|8|@]|9|@]|A|$B|C]]|$1|D|3|E|5|F|7|T|8|@$G|U|H|V|I|J]]|9|@]|A|$]]|$1|K|3|L|5|6|7|W|8|@]|9|@]|A|$B|C]]|$1|M|3|N|5|F|7|X|8|@]|9|@]|A|$]]|$1|O|3|P|5|6|7|Y|8|@]|9|@]|A|$B|C]]|$1|Q|3|-4|5|F|7|Z|8|@]|9|@]|A|$]]]|R|$]]

<pre><code>if (typeof image_array !== 'undefined' &amp;&amp; image_array.length &gt; 0) {
 // the array is defined and has at least one element
}
</code></pre>

Your problem may be happening due to a mix of implicit global variables and variable hoisting. Make sure you use <code>var</code> whenever declaring a variable:

<pre><code>&lt;?php echo "var image_array = ".json_encode($images);?&gt;
// add var ^^^ here
</code></pre>

And then make sure you never accidently redeclare that variable later:

<pre><code>else {
 ...
 image_array = []; // no var here
}
</code></pre>

blocks|key|293250|text|(ECMA+5.1)怎么样：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|293251|if(Array.isArray(image_array)+&&+image_array.length){
++//+array+exists+and+is+not+empty
}|code-block|syntax|javascript|293252|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|$]]

How about (ECMA 5.1): 

<pre><code>if(Array.isArray(image_array) &amp;&amp; image_array.length){
 // array exists and is not empty
}
</code></pre>

blocks|key|293632|text|optional+chaining|type|unstyled|depth|inlineStyleRanges|entityRanges|offset|length|data|293633|随着可选链接提案达到阶段4并得到更广泛的支持，有一种非常优雅的方法可以做到这一点。|293634|if(image_array?.length){

++//+image_array+is+defined+and+has+at+least+one+element

}|code-block|syntax|javascript|293635|entityMap|0|LINK|mutability|MUTABLE|url|https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Optional_chaining^0|0|H|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]]]]

<h1><a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Optional_chaining" rel="noreferrer">optional chaining</a></h1>

As optional chaining proposal reached stage 4 and is getting wider support, there is a very elegant way to do this

<pre class="lang-js prettyprint-override"><code>if(image_array?.length){

 // image_array is defined and has at least one element

}
</code></pre>

blocks|key|292918|text|您应该使用：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|292919|++if+(image_array+!==+undefined+&&+image_array.length+>+0)|code-block|syntax|javascript|292920|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 should use: 

<pre><code> if (image_array !== undefined &amp;&amp; image_array.length &gt; 0)
</code></pre>

blocks|key|509443|text|如果您想测试是否已经定义了image数组变量，可以这样做|type|unstyled|depth|inlineStyleRanges|entityRanges|data|509444|if(typeof+image_array+===+'undefined')+{
++++//+it+is+not+defined+yet
}+else+if+(image_array.length+>+0)+{
++++//+you+have+a+greater+than+zero+length+array
}|code-block|syntax|javascript|509445|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 test whether the image array variable had been defined you can do it like this

<pre><code>if(typeof image_array === 'undefined') {
 // it is not defined yet
} else if (image_array.length &gt; 0) {
 // you have a greater than zero length array
}
</code></pre>

blocks|key|293200|text|您可以使用jQuery的isEmptyObject()来检查数组是否包含元素。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|293201|var+testArray=[1,2,3,4,5];+
var+testArray1=[];
console.log(jQuery.isEmptyObject(testArray));+//false
console.log(jQuery.isEmptyObject(testArray1));+//true+|code-block|syntax|javascript|293202|来源：https://api.jquery.com/jQuery.isEmptyObject/|293203|entityMap|0|LINK|mutability|MUTABLE|url|https://api.jquery.com/jQuery.isEmptyObject/^0|C|F|0|0|3|18|0|0^^$0|@$1|2|3|4|5|6|7|U|8|@$9|V|A|W|B|C]]|D|@]|E|$]]|$1|F|3|G|5|H|7|X|8|@]|D|@]|E|$I|J]]|$1|K|3|L|5|6|7|Y|8|@]|D|@$9|Z|A|10|1|11]]|E|$]]|$1|M|3|-4|5|6|7|12|8|@]|D|@]|E|$]]]|N|$O|$5|P|Q|R|E|$S|T]]]]

You can use jQuery's <code>isEmptyObject()</code> to check whether the array contains elements or not.

<pre><code>var testArray=[1,2,3,4,5]; 
var testArray1=[];
console.log(jQuery.isEmptyObject(testArray)); //false
console.log(jQuery.isEmptyObject(testArray1)); //true 
</code></pre>

Source: <a href="https://api.jquery.com/jQuery.isEmptyObject/" rel="nofollow noreferrer">https://api.jquery.com/jQuery.isEmptyObject/</a>

blocks|key|293502|text|JavaScript|type|unstyled|depth|inlineStyleRanges|entityRanges|data|293503|(+typeof(myArray)+!==+'undefined'+&&+Array.isArray(myArray)+&&+myArray.length+>+0+)|code-block|syntax|javascript|293504|下划线(&R)|293505|(+_.isArray(myArray)+&&+myArray.length+>+0+)|293506|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|$]]

JavaScript

<pre><code>( typeof(myArray) !== 'undefined' &amp;&amp; Array.isArray(myArray) &amp;&amp; myArray.length &gt; 0 )
</code></pre>

Lodash &amp; Underscore

<pre><code>( _.isArray(myArray) &amp;&amp; myArray.length &gt; 0 )
</code></pre>

blocks|key|293359|text|一种简单的方法，如果不存在则不会导致异常，并转换为布尔值：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|293360|!!array|code-block|syntax|javascript|293361|示例：|293362|if+(!!arr)+{
++//+array+exists
}|293363|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|$]]

A simple way that doesn't result in exceptions if not exist and convert to boolean:

<pre><code>!!array
</code></pre>

Example:

<pre><code>if (!!arr) {
 // array exists
}
</code></pre>

blocks|key|293343|text|使用undescore或lodash|type|unstyled|depth|inlineStyleRanges|entityRanges|offset|length|data|293344|_.isArray(image_array)+&&+!_.isEmpty(image_array)|style|CODE|293345|entityMap|0|LINK|mutability|MUTABLE|url|http://underscorejs.org/#isArray|1|https://lodash.com/docs/4.17.4#isArray^0|2|9|0|C|6|1|0|0|1D|0^^$0|@$1|2|3|4|5|6|7|R|8|@]|9|@$A|S|B|T|1|U]|$A|V|B|W|1|X]]|C|$]]|$1|D|3|E|5|6|7|Y|8|@$A|Z|B|10|F|G]]|9|@]|C|$]]|$1|H|3|-4|5|6|7|11|8|@]|9|@]|C|$]]]|I|$J|$5|K|L|M|C|$N|O]]|P|$5|K|L|M|C|$N|Q]]]]

Using <a href="http://underscorejs.org/#isArray" rel="nofollow noreferrer">undescore</a> or <a href="https://lodash.com/docs/4.17.4#isArray" rel="nofollow noreferrer">lodash</a>:

<code>_.isArray(image_array) &amp;&amp; !_.isEmpty(image_array)</code>

blocks|key|293388|text|这个怎么样？检查未定义数组的长度可能会抛出异常。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|293389|if(image_array){
//array+exists
++++if(image_array.length){
++++//array+has+length+greater+than+zero
++++}
}|code-block|syntax|javascript|293390|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|$]]

How about this ? checking for length of undefined array may throw exception.

<pre><code>if(image_array){
//array exists
 if(image_array.length){
 //array has length greater than zero
 }
}
</code></pre>

blocks|key|510351|text|最好是像这样检查：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|510352|++++let+someArray:+string[]+=+[];
++++let+hasAny1:+boolean+=+!!someArray+&&+!!someArray.length;
++++let+hasAny2:+boolean+=+!!someArray+&&+someArray.length+>+0;+//or+like+this
++++console.log("And+now+on+empty......",+hasAny1,+hasAny2);|code-block|syntax|javascript|510353|请参阅完整示例列表：|510354|📷|atomic|offset|length|510355|510356|​|510357|entityMap|0|IMAGE|mutability|IMMUTABLE|imageUrl|https://ask.qcloudimg.com/http-save/yehe-900000/dd930ef2e7848c0821156ef0cfa52ab0.png|imageAlt|1|https://ask.qcloudimg.com/http-save/yehe-900000/1a11d6c05de8bc785b314fdbbfe527fa.png^0|0|0|0|0|1|0|0|0|1|1|0|0^^$0|@$1|2|3|4|5|6|7|11|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|12|8|@]|9|@]|A|$E|F]]|$1|G|3|H|5|6|7|13|8|@]|9|@]|A|$]]|$1|I|3|J|5|K|7|14|8|@]|9|@$L|15|M|16|1|17]]|A|$]]|$1|N|3|J|5|K|7|18|8|@]|9|@$L|19|M|1A|1|1B]]|A|$]]|$1|O|3|P|5|6|7|1C|8|@]|9|@]|A|$]]|$1|Q|3|-4|5|6|7|1D|8|@]|9|@]|A|$]]]|R|$S|$5|T|U|V|A|$W|X|Y|-4]]|Z|$5|T|U|V|A|$W|10|Y|-4]]]]

The best is to check like:
<pre><code> let someArray: string[] = [];
 let hasAny1: boolean = !!someArray &amp;&amp; !!someArray.length;
 let hasAny2: boolean = !!someArray &amp;&amp; someArray.length &gt; 0; //or like this
 console.log(&quot;And now on empty......&quot;, hasAny1, hasAny2);
</code></pre>
See full samples list:
<a href="https://i.stack.imgur.com/rjhpw.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/rjhpw.png" alt="code sample" /></a>
<a href="https://i.stack.imgur.com/ocxjg.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/ocxjg.png" alt="results" /></a>

blocks|key|293329|text|我在Javascript中经常遇到这个问题。对我来说，最好的方法是在检查长度之前进行非常广泛的检查。我在这个问答中看到了一些其他的解决方案，但我希望能够检查null或undefined或任何其他错误的值。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|293330|if(!array+%7C%7C+array.length+==+0){
++++console.log("Array+is+either+empty+or+does+not+exist")
}|code-block|syntax|javascript|293331|这将首先检查undefined、null或其他false值。如果其中任何一个为真，它将完成布尔值，因为这是一个OR。然后，可以检查更有风险的array.length检查，如果数组未定义，可能会导致错误。如果array为undefined或null，则永远不会达到此目的，因此条件的顺序非常重要。|293332|entityMap^0|26|4|2B|9|0|0|6|9|G|4|1J|2|1Y|C|2V|5|31|9|3B|4|0^^$0|@$1|2|3|4|5|6|7|O|8|@$9|P|A|Q|B|C]|$9|R|A|S|B|C]]|D|@]|E|$]]|$1|F|3|G|5|H|7|T|8|@]|D|@]|E|$I|J]]|$1|K|3|L|5|6|7|U|8|@$9|V|A|W|B|C]|$9|X|A|Y|B|C]|$9|Z|A|10|B|C]|$9|11|A|12|B|C]|$9|13|A|14|B|C]|$9|15|A|16|B|C]|$9|17|A|18|B|C]]|D|@]|E|$]]|$1|M|3|-4|5|6|7|19|8|@]|D|@]|E|$]]]|N|$]]

I come across this issue quite a lot in Javascript. For me the best way to do it is to put a very broad check before checking for length. I saw some other solutions in this Q&amp;A, but I wanted to be able to check for either <code>null</code> or <code>undefined</code> or any other false value.

<pre><code>if(!array || array.length == 0){
 console.log("Array is either empty or does not exist")
}
</code></pre>

This will first check for <code>undefined</code>, <code>null</code>, or other false values. If any of those are true, it will complete the boolean as this is an <code>OR</code>. Then the more risky check of <code>array.length</code>, which could error us if array is undefined, can be checked. This will never be reached if <code>array</code> is <code>undefined</code> or <code>null</code>, so the ordering of conditions is very important.

blocks|key|509781|text|你应该这样做|type|unstyled|depth|inlineStyleRanges|entityRanges|data|509782|++++if+(!image_array)+{
++++++//+image_array+defined+but+not+assigned+automatically+coerces+to+false
++++}+else+if+(!(0+in+image_array))+{
++++++//+empty+array
++++++//+doSomething
++++}|code-block|syntax|javascript|509783|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 should do this

<pre><code> if (!image_array) {
 // image_array defined but not assigned automatically coerces to false
 } else if (!(0 in image_array)) {
 // empty array
 // doSomething
 }
</code></pre>

blocks|key|510222|text|您的字符串可能不是数组，而是某个具有length属性的对象(如image_array+)-尝试|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|510223|if(image_array+instanceof+Array+&&+image_array.length)|code-block|syntax|javascript|510224|​|510225|function+test(image_array)+{
++if(image_array+instanceof+Array+&&+image_array.length)+{+
++++console.log(image_array,'-+it+is+not+empty+array!')
++}+else+{
++++console.log(image_array,'-+it+is+empty+array+or+not+array+at+all!')
++}
}

test({length:5});
test('undefined');
test([]);
test(["abc"]);|510226|510227|entityMap^0|I|6|V|B|0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|R|8|@$9|S|A|T|B|C]|$9|U|A|V|B|C]]|D|@]|E|$]]|$1|F|3|G|5|H|7|W|8|@]|D|@]|E|$I|J]]|$1|K|3|L|5|6|7|X|8|@]|D|@]|E|$]]|$1|M|3|N|5|H|7|Y|8|@]|D|@]|E|$I|J]]|$1|O|3|L|5|6|7|Z|8|@]|D|@]|E|$]]|$1|P|3|-4|5|6|7|10|8|@]|D|@]|E|$]]]|Q|$]]

Probably your <code>image_array</code> is not array but some OBJECT with <code>length</code> property (like string) - try
<pre><code>if(image_array instanceof Array &amp;&amp; image_array.length)
</code></pre>
<div class="snippet" data-lang="js" data-hide="true" data-console="true" data-babel="false">
<div class="snippet-code snippet-currently-hidden">
<pre class="snippet-code-js lang-js prettyprint-override"><code>function test(image_array) {
 if(image_array instanceof Array &amp;&amp; image_array.length) { 
 console.log(image_array,'- it is not empty array!')
 } else {
 console.log(image_array,'- it is empty array or not array at all!')
 }
}

test({length:5});
test('undefined');
test([]);
test(["abc"]);</code></pre>
</div>
</div>

blocks|key|293898|text|在我的例子中，array_.length总是返回0，即使它里面有值。可能是因为有非默认索引。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|293899|因此，为了检查是否定义了数组，我们使用typeof+_array+!==+'undefined'，然后检查它是否包含任何日期，我只是将它与一个空数组_array+!==+[]进行比较|293900|entityMap^0|7|D|0|J|T|22|D|0^^$0|@$1|2|3|4|5|6|7|J|8|@$9|K|A|L|B|C]]|D|@]|E|$]]|$1|F|3|G|5|6|7|M|8|@$9|N|A|O|B|C]|$9|P|A|Q|B|C]]|D|@]|E|$]]|$1|H|3|-4|5|6|7|R|8|@]|D|@]|E|$]]]|I|$]]

In my case, <code>array_.length</code> always returned 0, even if it had values inside. Probably, because of non-default indexes.
So to check if array is defined we use <code>typeof _array !== 'undefined'</code>
And then to check if it contains any date i just simply compare it to an empty array <code>_array !== []</code>

blocks|key|293957|text|我发现的工作方式(来自另一种语言)是创建一个简单的函数来测试。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|293958|创建一个检查数组大小的函数，并传递lenght+by参数。|293959|isEmpty(size){
++++++++if(size==0)+{
++++++++++++return+true;
++++++++}+else++{
++++++++++++return+false;
++++++++}
++++}

//then+check
if(isEmpty(yourArray.length)==true){
++++++++++++//its+empty
++++++++}+else+{
++++++++++++//not+empty
++++++++}|code-block|syntax|javascript|293960|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|$]]

the way I found to work (comming from another language) is to make a simple function to test.
create a function that check the size of the array and pass the lenght by parameter.
<pre><code>isEmpty(size){
 if(size==0) {
 return true;
 } else {
 return false;
 }
 }

//then check
if(isEmpty(yourArray.length)==true){
 //its empty
 } else {
 //not empty
 }
</code></pre>

blocks|key|293564|text|以ts为单位|type|unstyled|depth|inlineStyleRanges|entityRanges|data|293565|+isArray(obj:+any)+
{
++++return+Array.isArray(obj)
++}|code-block|syntax|javascript|293566|在html中|293567|(照片==未定义%7C%7C！(isArray(照片)+&&+photos.length+>+0)|293568|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]]|$1|G|3|H|5|6|7|O|8|@]|9|@]|A|$]]|$1|I|3|J|5|6|7|P|8|@]|9|@]|A|$]]|$1|K|3|-4|5|6|7|Q|8|@]|9|@]|A|$]]]|L|$]]

in ts

<pre><code> isArray(obj: any) 
{
 return Array.isArray(obj)
 }
</code></pre>

in html

(photos == undefined || !(isArray(photos) &amp;&amp; photos.length > 0) )

blocks|key|292880|text|当您创建image_array时，它为空，因此您的image_array.length为0|type|unstyled|depth|inlineStyleRanges|entityRanges|data|292881|如下面的评论所述，我根据此question's+answer编辑我的答案)：|offset|length|292882|var+image_array+=+[]|code-block|syntax|javascript|292883|else括号中的任何内容都不会更改之前在代码中定义的image_array|292884|entityMap|0|LINK|mutability|MUTABLE|url|https://stackoverflow.com/questions/500431/javascript-variable-scope^0|0|D|H|0|0|0|0^^$0|@$1|2|3|4|5|6|7|U|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|V|8|@]|9|@$D|W|E|X|1|Y]]|A|$]]|$1|F|3|G|5|H|7|Z|8|@]|9|@]|A|$I|J]]|$1|K|3|L|5|6|7|10|8|@]|9|@]|A|$]]|$1|M|3|-4|5|6|7|11|8|@]|9|@]|A|$]]]|N|$O|$5|P|Q|R|A|$S|T]]]]

When you create your image_array, it's empty, therefore your image_array.length is 0

As stated in the comment below, i edit my answer based on this <a href="https://stackoverflow.com/questions/500431/javascript-variable-scope">question's answer</a>) :

<pre><code>var image_array = []
</code></pre>

inside the else brackets doesn't change anything to the image_array defined before in the code

When the page is loading for the first time, I need to check if there is an image in <code>image_array</code> and load the last image.

Otherwise, I disable the preview buttons, alert the user to push new image button and create an empty array to put the images;

The problem is that <code>image_array</code> in the <code>else</code> fires all time. If an array exists - it just overrides it, but alert doesn't work.

<pre><code>if(image_array.length &gt; 0)
 $('#images').append('&lt;img src="'+image_array[image_array.length-1]+'" class="images" id="1" /&gt;');
else{
 $('#prev_image').attr('disabled', 'true');
 $('#next_image').attr('disabled', 'true');
 alert('Please get new image');
 var image_array = [];
}
</code></pre>

UPDATE
Before loading html, I have something like this:

<pre><code>&lt;?php if(count($images) != 0): ?&gt;
&lt;script type="text/javascript"&gt;
 &lt;?php echo "image_array = ".json_encode($images);?&gt;
&lt;/script&gt;
&lt;?php endif; ?&gt;
</code></pre>

Check if an array is empty or exists

当页面第一次加载时，我需要检查image_array中是否有图像，并加载最后一个图像。否则，我禁用预览按钮，提醒用户按下新图像按钮，并创建一个空数组来放置图像；问题是else中的image_array总是会触发。如果数组存在-它只是覆盖它，但是alert不起作用。if(image_array.length > 0) $('#images').append('<img src="'+image

问检查数组是否为空或存在
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回答 18

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回答 18

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