JavaScript before leaving the page

Question

I want to make a confirmation before user leaving the page. If he says ok then it would redirect to new page or cancel to leave. I tried to make it with onunload

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Answer 1

This what I did to show the confirmation message just when I have unsaved data

window.onbeforeunload = function () {
            if (isDirty) {
                return "There are unsaved data.";
            }
            return undefined;
        }

returning "undefined" will disable the confirmation

Note: returning "null" will not work with IE

Also you can use "undefined" to disable the confirmation

window.onbeforeunload = undefined;