为什么我们在 XMLHttpRequest 中写 send() 之前要写 onload() 函数-前端问题

Why do we write onload() function before we write send() in XMLHttpRequest(为什么我们在 XMLHttpRequest 中写 send() 之前要写 onload() 函数)

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问题描述

我是 XMLHttpRequest 的新手.我不明白为什么我们要在 send() 函数之前编写 onload() 函数.onload() 函数处理我们收到的响应，然后 send() 函数向服务器发送请求.所以根据我的理解，onload() 必须在 send() 函数之后编写.有人可以帮助理解这一点.

I am new to XMLHttpRequest. I dont understand why do we write onload() function before send() function. onload() function process the response what we receive and send() function sends a request to server. So onload() has to be written after send() function as per my understanding. Can somebody help to understand this.

var xmlhttp = new XMLHttpRequest(),
  method = 'GET',
  url = 'https://developer.mozilla.org/';

xmlhttp.open(method, url, true);
xmlhttp.onload = function () {
  // Do something with the retrieved data
};
xmlhttp.send();

推荐答案

我不明白为什么我们要在 send() 函数之前写 onload() 函数.

I dont understand why do we write onload() function before send() function.

以便在发送请求之前加载处理程序，因为发送请求将导致调用处理程序(如果成功).

So that the load handler is in place before the request is sent, since sending the request will result in calling the handler (if successful).

onload() 函数处理我们收到的响应，send() 函数向服务器发送请求.所以根据我的理解，onload() 必须写在 send() 函数之后.

onload() function process the response what we receive and send() function sends a request to server. So onload() has to be written after send() function as per my understanding.

它在 send 被调用(由 XHR 基础架构)(或可能在其期间)之后被调用.当您将它分配给 onload 时，您并不是在调用它.您只是定义它，以便当 XHR 需要调用它时它就在那里.

It's called after send is called (by the XHR infrastructure) (or potentially during). When you assign it to onload, you're not calling it. You're just defining it so that it's there when XHR needs to call it.

会发生什么:

您创建 XHR.
您为其 load 事件注册一个处理程序(在您的情况下，通过将函数分配给 onload).
你调用 send.

You create the XHR.
You register a handler for its load event (in your case, by assigning a function to onload).
You call send.

浏览器启动(并可能完成)请求

当请求完成时，如果成功，浏览器的 XHR 处理会触发 load 事件.这会查找任何当前注册的 load 处理程序，并将对这些处理程序的调用(如果有)排队.这些调用会在主 JavaScript 线程可用于运行它们时立即运行.

When the request finishes, if it's successful, the browser's XHR handling triggers the load event. That looks for any currently-registered handlers for load and queues calls to those handlers, if any. Those calls are run as soon as the main JavaScript thread is available to run them.

很多时候，你会以错误的方式逃脱惩罚，因为在请求完成时，你已经将 load 处理程序放在那里；但不是总是.load 是一个事件.如果可以立即满足请求(例如，从缓存中)，浏览器可以在 send 期间触发 load，并查看是否有任何 load 处理程序在调用 send 期间，如果没有，则不对任何回调的调用进行排队.稍后当您附加处理程序时，该事件已经被触发(当没有附加处理程序时).

Very often, you'd get away with doing it the wrong way around because by the time the request completes, you'll have put the load handler there; but not always. load is an event. If the request can be satisfied immediately (for instance, from cache), the browser could fire load during send, and look to see if there's any load handler during the call to send, and if there isn't, not queue a call to any callback. Later when you attach a handler, the event has already been fired (when none were attached).

所以你必须在调用 send 之前附加处理程序.

So you have to attach the handler before calling send.

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