What is the danger in including the same JavaScript library twice?
One of the webapps I'm working in is made up of many partial HTML files. If the partial requires a JavaScript library such as YUI, the YUI library is included in the partial.
When the partials are combined at runtime, the resulting HTML often includes the YUI library several times.
<html>
...
<script type="text/javascript" src="/js/yahoo/yahoo-min.js"></script>
...
<script type="text/javascript" src="/js/yahoo/yahoo-min.js"></script>
...
<script type="text/javascript" src="/js/yahoo/yahoo-min.js"></script>
...
</html>
I've seen strange behavior from including jQuery several times, especially when using AJAX. Why, specifically, is including the same JavaScript library more than once a bad idea? Why does it only sometimes cause problems?
Answer
Depending on the library, including it more than once could have undesired effects.
Think of it like this, if you have a script that binds a click event to a button, and you include that script twice, those actions will be ran twice when the button is clicked.
You could write a simple function that you call to load a script and have it keep track of files that have already been loaded. Or, I'm sure you could probably use a pre-existing JS loader such as LabJS and modify it.