I am currently developing an ASP.NET User Control. My control has dependency on jQuery, so in order for my control to operate correctly and be robust, I needed to check that jQuery has/will be loaded.
First let's define the two states of jQuery:
- loaded: jQuery is referenced in the current page by way of
<script></script>
tags. - ready: jQuery is loaded and the DOM is ready for manipulation.
A non-denominational web search led me to several articles that suggested one of the following constructs:
if(jQuery) alert('jQuery is loaded.');
if(typeof(jQuery) != 'undefined') alert('jQuery is loaded.');
That makes sense; however, these simply don't work — at least not in all browsers. In actual practice, these tell us if jQuery is ready, not loaded.
Instead, I determined that the most reliable means of checking if jQuery is loaded is to check if the $
function is defined:
if(typeof($) == 'function') alert('jQuery is loaded.');
Note: this will yield a false positive in the unlikely event that the $
token has been defined by something other than jQuery as a token.
I like to use this anywhere that the standard jQuery $(document).ready()
construct is used:
if(typeof($) != 'function') alert('This component requires jQuery.');
else $(function() { alert('The DOM is ready for jQuery manipulation.'); });
Yvan Rodrigues has 30 years of experience in information systems and software development for the industry. He is Senior Concept Designer at
Robotic Assistance Devices
He is a Certified Technician (C.Tech.), a professional designation granted by the Institute of Engineering Technology of Ontario (IETO).
Yvan draws on experience as owner of
Red Cell Innovation Inc.,
Mabel's Labels Inc. as Manager of Systems and Development, the
University of Waterloo as Information Systems Manager, and
OTTO Motors as Senior Systems Engineer and Senior Concept Designer.
Yvan is currently focused on design of embedded systems.