How to pop up an alert box when the browser's refresh button is clicked?

Question

If the user refreshes the page in question it will add another record to the database, so I want to warn the user through an alert box if they really want to refresh the pag

Answer 1

There isn't a way to tie it to just the refresh action, but you may want to look into window.onbeforeunload. This will allow you to run a function that returns a string just before the page is unloaded. If this string is not empty, then a popup confirmation dialog, containing this string and some other boilerplate text provided from the browser.

For example:

window.onbeforeunload = function () {
    if (someConditionThatIndicatesIShouldConfirm) {
        return "If you reload this page, your previous action will be repeated";
    } else {
        //Don't return anything
    }
}

Also, if the current page was loaded via a POST operation, then the browser should already display a confirmation box when the user tries to refresh it. As a general rule, any action that changes the state of the data on the server should be done through a POST request, rather than a GET.