Is there any way to only sometimes offer a prompt on the beforeunload event in javascript?

Question

I have a page with some data and I have a function to check if it has been modified. If it has, I want to offer a confirm prompt when the page is unloaded. I am curious as to whether or not this can be done in the beforeunload function or if I am forced to enable/disable the event listener whenever the data is changed, which would be rather inefficient.

Answer 1

I am not sure this is what you want but from what I understand you need something like this

window.onload = function() {
    window.addEventListener("beforeunload", function (e) {
        const msg = 'Save Changes Before Leaving';

        (e || window.event).returnValue = msg;
        return msg;
    });
};

if you want to check something before asking use if before the (e || window.event).returnValue = msg; return msg; (e || window.event).returnValue = msg; return msg;