Monitor whether the user closes the browser's js code_javascript skills

<title>test</title>
[Ctrl A select all Note: If you need to introduce external Js, you need to refresh to execute

]

The effect is as follows:

When wonder.event.returnValue is empty, this selection prompt box will not appear.

But the question comes again: If I want the user to be able to save user-related data when he clicks the "OK" button to close the browser. So what should I do?

It seems that the only way is to use the onunload parameter.

Modify the above code as follows:

<title> </title>test
[Ctrl A select all Note:

If you need to introduce external Js, you need to refresh to execute ]