jQuery $('.classname').length fails, but $.find('.classname').length works, why?

问题

My JavaScript module fails to find a particular DOM element with a classname using $('.classname') even when the element exists.

$('.classname').length returns 0, where as $.find('.classname').length returns 1.

Website contains other JavaScript modules also.

Can anyone help me to find why this is happening?

My jQuery version is 1.7.1.

To reproduce this 1- Go to avc.com
2- Open console.
3- Paste the following code in console.

var head = document.getElementsByTagName("head")[0];  
var script = document.createElement("script");  
script.type = "text/javascript";  
script.src = "//ajax.googleapis.com/ajax/libs/jquery/1.7.1/jquery.min.js";  
script.async = true;  
head.appendChild(script);

4- Try
$('.pkg').length and $.find('.pkg').length.

Find the screen-shot from here http://i.imgur.com/4UDHz.png

One more point, if you try this in some other websites, It works correctly.

回答1:

You may have some conflicts with your other JavaScript modules. Look into jQuery noconflict

Many JavaScript libraries use $ as a function or variable name, just as jQuery does. In jQuery's case, $ is just an alias for jQuery, so all functionality is available without using $. If we need to use another JavaScript library alongside jQuery, we can return control of $ back to the other library with a call to $.noConflict()

Try using

jQuery('.classname').length

来源：https://stackoverflow.com/questions/9890878/jquery-classname-length-fails-but-find-classname-length-works-why