What does '+variable+' mean?
<p>What does this: '+variable+' mean?</p>

<p>I use jQuery append method to add some DOM element:</p>

<pre><code>$('#story_pages').append('&lt;div&gt;&lt;button value="'+window_value+'" onclick= "reload_to_canvas(this.value)" &gt;&lt;img id="w'+window_value+'", src=white_data_URL, width="100px", height="100px"/&gt; &lt;/button&gt;&lt;/div&gt;');
</code></pre>

<p>Actually, I don't why this works... 
If I just write</p>

<pre><code>value= window_value 
img id= w+window_value
</code></pre>

<p>,and use the selector
    <strong>$('#(w+window_value)')</strong> 
to find the element. It must fail. 
<br>
What is this operator? '+variable+'
<br>
And why should we use the weird notation "'+variable+'"?
<br>
When should we use this notation?</p>
