如果我们不单击页面，则不会触发window.onbeforeunload事件

Question

window.onbeforeunload event is not working for the first time, when we 
reload the url no alert coming while closing the browser tab. 

但是当我们触摸页面上的任何元素并关闭选项卡时，它就会起作用。

$(document).ready(function(){
    window.onbeforeunload = function (event) {
    var message = 'Important: Please click on \'Save\' button to leave this page.';
    if (typeof event == 'undefined') {
        event = window.event;
    }
    if (event) {
        event.returnValue = message;
    }
    return message;
};


    $(function () {
    $("a").click(function () {
        window.onbeforeunload = null;
    });

        });

  });

Sample Code here : https://jsfiddle.net/oe9L0fdb/

Answer 1

这是某些浏览器上的预期行为。如here所述：

To combat unwanted pop-ups, some browsers don't display prompts created in beforeunload event handlers unless the page has been interacted with. Moreover, some don't display them at all.

实质上，这是为了防止网页被滥用。