Manipulate auto execution of Javascript since Burp can only see HTTP Requests/responses

There a javascript that is executing in my browser and it is generating session token. (This was a design requirement to the dev team Sesion token is generated on the client side – don’t ask me why lol)

I want to be able to modify the javascript variables during execution (just as if I was using Debug in Netbeans for instance)

I though I’d use burp suite but it only catches request (not the building of a request by Js)

What can I do to do that ?

Also, I thought I’d use browser debugger but strangely, none of the loaded JS seems to be generating the session token. One of the JS just do that and I see it later in burp interceptor.

Any help here ?