I wanted to know if it's possible to use a sandBox to test my code in SharePoint online ? This way, I can test my code, and my colleagues can continues to work on sites.
Thank's !
Not for any compiled code. I.e. anything that requires a DLL. You can still deploy Sandbox Solutions that are all HTML, CSS and JavaScript (with a few exceptions).
Note that many of the Sandbox Solutions will not work with the "modern UI".