Why do people have to use multiple versions of jQuery on the same page?

Question

I have noticed that sometimes people have to use multiple versions of jQuery in the same page (See question 1 and question 2). I assume people have to carry old versions of jQuery because some pieces of their code is based on an older version of jQuery. Obviously, this approach causes inefficiency. The ideal solution is to refactor the old code to use the newer jQuery API. I wonder if there are tools that automate the process of upgrading a piece of code to use a newer version of jQuery. I've never written programs in in either Javascript or jQuery. So, I'd like to hear from programmers experienced in these language about their opinion on this issue. In particular, I'd like to know the following.

1. How much of problem it is to have to load multiple versions of jQuery?
2. Have you ever had to load multiple versions of any other library in the same page?
3. Do you know of any refactoring tool that helps you migrate your code to use the updated API?
4. Do you think such a refactoring tool is useful? Are you willing to use it?

Please don't consider this question closed. Current responses have already gave me good insight about the process and tools available for upgrading jQuery code. But, please feel free to add your own experience with upgrading jQuery code.

Answer 1

your questions are all big. But from my personal experience except from very little problems in the first version which happened after migrating to later ones. Pretty much you should not run into any problems unless something is wrong with the code you have written and not jquery.

I have not seen any proper js developer who uses two different versions at the same time.

JQuery now itself offers a pretty nice api for unit testing called qunit. and if you start implementing test functions there you should be certain that your code works seamlessly.

I Use QUnit for heavy projects. which requires maybe alot of ajax requests etc. ofcourse it is not really worth it to use it for couple of animations etc.

I hope this information is helpful

Answer 2

Ignoring the fact that you can do this with JQuery, using 2 versions of any API is unusual and error prone. It is a rare practice, but at least with JQuery, it is done.

If I were evaluating a vendor's components for a new project and there was a requirement for an older library, I'd think twice before investing in that component. Vendors who supply tools that require old verions are one of:

1) Very cautious and stability minded (good)

2) Not up to speed (not too good)

3) Low on resources (bad)

4) Not motivated to update their tools because not enough people are demanding the tool (really bad)

Of course (5) could be that their tool is so good and they have such a big customer base they don't feel the need to update it. That is ok as far as it goes, and eventually goes back to one of the other issues.

I use enough 3rd party tools to know that sometimes, a tool does a job so well that I pick it anyway, but in general, I want a tool to be current for a new project. There are a lot of "fly by night" vendors out there that just aren't maintaining their products and you don't find it out until you ask for support or new features.