How to use the JavaScript code in Construct 3?

Hi, I've realized that it's already possible to insert JavaScript code in Contruct 3.

I would like to know if it is possible to manipulate objects and use variables with this code, and if possible, is the common JavaScript code used or is there any variant?

Yes, it's possible. Refer to the scripting documentation and the examples in the 'Scripting' section of the Start Page.