want to click a button through javascript for testing purpose. i am able to click the button which is on first page, but dont have idea how to click button on second page which comes after clicking button on first page.
You just need a reference to that button on the second page -- so you repeat the process from the first page. Probably, you did something like this (this code is from my app using a tab bar controller):
// Now tap the add button
var navBar = mainWindow.navigationBar();
navBar.buttons()["Add"].tap();
// Now the app loads a new page
// Get the nav bar again (it would have changed after the tap above)
navBar = mainWindow.navigationBar();
So, the answer is easy -- just call back to the same functions, they will return whatever's on screen at the moment.