I’m contributing to a 3rd-party JavaScript application, let’s call it X. A website owner may place X in their own site, let’s call it Y, using a script tag, and X might render some content, respond to events, etc. Think of something like Disqus.
X may depend on libraries such as Handlebars, Underscore.js, etc. I face the problem of resolving these dependencies in a way that does not interfere with Y. If X pulls in Handlebars and Underscore.js for itself, and Y already includes these libraries, things will probably blow up due to there being two versions there which could cause undefined behaviour.
So what is the best way to deal with this kind of situation?
Since you cannot guarantee that any website owner will use the same dependency management system as X, why not just use conditional loading? Just check to see whether (e.g.) window.jQuery is present, and only load jQuery if it isn’t already defined. And so on and so forth for your properly-namespaced fourth-party components. If you ask your users to just load your libraries at the bottom of their page, you should be fine.
You could use a conditional loader like http://yepnopejs.com/ for clarity and speed.
Alternatively, could you require your users to insert the needed libraries themselves? You state your dependences (and provide code) at your distribution point, and provide a good-looking error state (e.g., a dialog box, a console.log, etc.) when other needed libraries aren’t on the page.