发送http请求

见http://www.informit.com/articles/article.aspx?p=669066&seqNum=2

，附内容：

XMLHttp.open("GET", "phrasebook.txt");
XMLHttp.onreadystatechange = handlerFunction;
XMLHttp.send(null);

Sending an HTTP request to a server using XMLHttpRequest consists of the following steps:

1. Provide the URL and the HTTP verb to use.
2. Define a callback function when results arrive.
3. Send the request.

Step 1 can be taken with the open() method of the XMLHttpRequest object. This does not—unlike what the method name suggests—actually open an HTTP connection, but just initializes the object. You provide the HTTP verb to use (usuallyGET or POST) and the URL.

Step 2, the callback function, is provided to the onreadystatechange property of the object. Whenever thereadyState property ofXMLHttpRequest changes, this callback function is called. Finally, thesend() method sends the HTTP request.

In the callback function, the readyState value 4 represents the state of the object we want: call completed. In that case, theresponseText property contains the data returned from the server.

Here is a fully working example, sending a GET request to the server (a file calledphrasebook.txt with simple text content) and evaluating the response of that call:

Sending a GET Request (xmlhttpget.html)

<script language="JavaScript"
  type="text/javascript" src="xmlhttp.js"></script>
<script language="JavaScript"
 type="text/javascript">
var XMLHttp = getXMLHttp();
XMLHttp.open("GET", "phrasebook.txt");
XMLHttp.onreadystatechange = handlerFunction;
XMLHttp.send(null);

function handlerFunction() {
  if (XMLHttp.readyState == 4) {
    window.alert("Returned data: " +
                 XMLHttp.responseText);
  }
}
</script>

Understating the States of XMLHttpRequest

All in all, the XMLHttpRequest object supports five states, as shown in Table 11.1. Depending on the implementation, up to all of these states happen during the execution of an AJAX script. That is the reason it is so important to always query the state before trying to access other XMLHttpRequest data.