I can't think of much efficient way to do this but one way you can do this is by adding a breakpoint at the start of the JavaScript file or at the block you want to add or delete changes to.
Then when you reload, the debugger will pause on that breakpoint, and you can make any changes you want to the source, save the file and then run the debugger through the modified code.
But here's a thing that you should know: next reload the changes will vanish but at least it let's you run some slightly modified JavaScript client side codes.