为什么 onbeforeunload 总是显示一个对话框

Question

I am trying to have my javascript pop a dialog if there is data that hasn't been written yet, but it is always showing the confirmation dialog, even if there is nothing to be written.

I have reduced the problem to this minimal failing case:

window.addEventListener("beforeunload", (e) => {
    e.returnValue = null;
    return null;
});

With this in my code, the windows always ask for confirmation (both Chrome and Firefox). Changing the null's to undefined's doesn't change anything.

Can someone please lend me a clue?

Answer 1

A confirmation dialog will be displayed if event.returnValue is set to a non-empty. And undefined and null aren't empty in Javascript. If I understand what you're trying to do, you should use a conditional and only set/return something if the conditional is true. In other words:

window.addEventListener("beforeunload", (e) => {
    if (dataWaitingToBeSent) {
        e.returnValue = null;
        return null;
    }
});

References: beforeUnload event , null primitive , empty statement

Answer 2

Try using return; in order to avoid triggering the confirmation prompt.

In case if you return '',null,false it may trigger the prompt.

If you want to disable the dialog

window.addEventListener("beforeunload", (e) => {
    .....
return;
});

Hope this helps ...!

Working fiddle on jquery: https://jsfiddle.net/KishorVelayutham/wfd2xsxk/10/

return ; will stop prompting the dialog.