Javascript to alert on window web closure

Here is a code to alert the user on window closure or tab closure in web browser.

function setConfirmUnload(on) {

            window.onbeforeunload = (on) ? unloadMessage : null;

        }

        function unloadMessage() {

            return "Are you sure you want to leave this page without saving your details? Please click the save button at the bottom of the page.";

        }

        window.onload = function () {

            setConfirmUnload(true);

        }

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