How do you handle alert in Selenium?

To handle alert pop-ups, we need to 1st switch control to alert then click on ok or cancel then move control back to main page.

String mainPage = driver.getWindowHandle();

Alert alt = driver.switchTo().alert(); // to move control to alert popup
alt.accept(); // to click on ok.
alt.dismiss(); // to click on cancel.

//Then move the control back to main web page
driver.switchTo().window(mainPage);