onReadyJS

I want to be able to do a set of actions on the page using Javascript before taking screenshot for comparison.
there is onReady script available which I think is designed for the same purpose.
but there's not a working example available which uses onReady script to perform browser operations
I've tried to do the same but it doesn't perform any action on the browser and I checked that it's still performing actions on server rather than on browser

need help of a working example of onReadyJS file that performs some browser interactions e.g. logging in etc