浏览器在“PageUnload”和新“PageLoad”之后继续执行Javascript

我们有以下AJAX throttler.这被实现为能够对一个页面执行许多(20)ajax请求而没有剩余时间超时仅仅因为前X个请求总共花费60秒.

RequestThrottler: {
    maximumConcurrentRequests: 3, //default to 3        
    requestQueue: new Array(),
    numberOfRequestCurrentlyProcessing: 0,

    addRequestToQueue: function (currentRequest) {
        var self = this;
        self.requestQueue.push(currentRequest);

        if (self.numberOfRequestCurrentlyProcessing < self.maximumConcurrentRequests) { self.sendNextRequest(); }
    },

    sendNextRequest: function () {
        var self = this;
        if (self.numberOfRequestCurrentlyProcessing >= self.maximumConcurrentRequests) { return; }
        if (self.requestQueue.length === 0) { return; }

        var currentRequest = self.requestQueue.pop();
        self.numberOfRequestCurrentlyProcessing++;
        AJAX.SendAjaxRequest(currentRequest.url, currentRequest.httpMethod, 
            function(data){
                self.numberOfRequestCurrentlyProcessing--;
                currentRequest.onSuccessCallback(data);
                self.sendNextRequest();
            }, 
            function(){
                self.numberOfRequestCurrentlyProcessing--;
                currentRequest.onErrorCallback();
                self.sendNextRequest();
            });
    },

    sendUpdateRequest: function (currentRequest) {
        var self = this;
        self.addRequestToQueue(currentRequest);
    }
}

但是,由于这些请求位于Javascript队列中,因此当用户尝试加载新页面时,开发人员工具会在新页面的NET区域中显示响应.我们的应用程序出于隐私原因进行了检查,不允许这种行为.这对浏览器来说是正常的,还是某种bug,或者我做错了什么？

解决方法:

一个干净的解决方案是监听window.onbeforeunload事件以中止任何尚未收到响应的ajax请求.

> https://developer.mozilla.org/en-US/docs/Web/API/XMLHttpRequest
> Abort Ajax requests using jQuery
> jQuery: Automatically abort AjaxRequests on Page Unload?

由于以下原因,应使用beforeunload事件而不是卸载：

1)beforeunload事件比unload事件更可靠：

The exact handling of the unload event has varied from version to
version of browsers. For example, some versions of Firefox trigger the
event when a link is followed, but not when the window is closed. In
practical usage, behavior should be tested on all supported browsers,
and contrasted with the proprietary beforeunload event.

资源：

> http://api.jquery.com/unload/
> jquery: unload or beforeunload?

2)可以取消beforeunload事件,而不能取消unload事件.如果您想在beforeunload事件发生时提示用户,这将为您提供灵活性.确认将询问用户是否希望继续导航到其他页面,或者他们是否想要取消,因为并非所有ajax请求都已完成.

window.addEventListener("beforeunload", function (e) {
  var confirmationMessage = "\o/";

  (e || window.event).returnValue = confirmationMessage;     // Gecko and Trident
  return confirmationMessage;                                // Gecko and WebKit
});

来源：

> https://developer.mozilla.org/en-US/docs/Web/Events/beforeunload
> https://developer.mozilla.org/en-US/docs/Web/Events/unload