Taking over development of a website

…and also moving it to a CMS.

My question is am I able to use some of the javascript the original developer wrote in order to keep some of the features?

Legally speaking? I guess that depends on what the contract says between the client and the original developer?

There’s no way to answer this without knowing the exact relationship between the developer and whom the website is owned by, and if there is any contract between them. Were they an employee, web design company, offshore company, etc…

It isn’t a question of whether you are able to, the original developer’s contract was with your client, so it’s your client’s problem. In the end, if your client asks you to edit their site, you are free to edit it and use what’s already there. If there’s any legal issues from the original developer, your client will have to deal with it. Just make sure you have a contract with the appropriate liability and indemnity clauses in.