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ajax is the abbreviation of asynchronous javascript and XML. The Chinese translation is asynchronous javascript and XML. This technology can request additional data from the server without unloading the page, which will bring a better user experience. Those who are interested in ajax Friends, let’s take a look at the XHR object, the first article in the ajax series.
Previous words
Ajax is the abbreviation of asynchronous javascript and XML , Chinese translation is asynchronous javascript and XML. This technology can request additional data from the server without unloading the page, which will bring a better user experience. Although XML is in the name, ajax communication has nothing to do with the data format. The following will introduce the content of ajax in detail
Creation
var xhr; if(window.XMLHttpRequest){ xhr = new XMLHttpRequest(); }else{ xhr = new ActiveXObject('Microsoft.XMLHTTP'); }
##Send a request
open()When using an XHR object, the first method to be called is open(), which accepts 3 parameters: the type of request to be sent (" get", "post", etc.), the requested URL and a Boolean value indicating whether to send the request asynchronously
xhr.open("get","example.php", false);
[Note]URL is relative to the current page where the code is executed, and can only send requests to URLs in the same domain using the same port and protocol. If the URL is any different from the page that initiated the request, a security error will occur.
The send() method receives a parameter, which is to be sent as the request body. The data. After calling the send() method, the request is dispatched to the server
xhr.open("get", "example.txt", false); xhr.send(null);
Receive response After receiving the response, the response data will automatically fill in the properties of the XHR object, mainly including the following four properties
responseText: The text returned as the response body
responseXML: If the content type of the response is 'text/xml' or 'application/xml', this attribute will save the XML DOM document of the response data
status: The HTTP status of the response
statusText: Description of HTTP status
After receiving the response, the first step is to check the status attribute to confirm that the response has been returned successfully. Generally speaking, HTTP status code 200 can be used as a sign of success. At this point, the content of the responseText attribute is ready, and responseXML can also be accessed if the content type is correct. In addition, a status code of 304 means that the requested resource has not been modified, and the cached version in the browser can be used directly; of course, it also means that the response is valid
Regardless of the content type, the content of the response body will be saved to the responseText attribute, and for non-XML data, the value of the responseXML attribute will be null
if((xhr.status >=200 && xhr.status < 300) || xhr.status == 304){ alert(xhr.responseText); }else{ alert('request was unsuccessful:' + xhr.status); }
Asynchronous If you need to receive an asynchronous response, you need to detect the readyState attribute of the XHR object, which represents the current active stage of the request/response process. Possible values for this attribute are as follows:
0 (UNSENT): Uninitialized. The open() method has not been called
1(OPENED): Start. The open() method has been called, but the send() method has not been called
2(HEADERS_RECEIVED): Send. The send() method has been called and the header information
3(LOADING): received. Partial response body information has been received
4 (DONE): Complete. All response data has been received and can be used on the client
Whenever the readyState attribute value changes from one value to another, a readystatechange event will be triggered. This event can be used to detect the value of readyState after each state change. Typically, we are interested in the stage with a readyState value of 4 because all data is ready
[Note] The onreadystatechange event handler must be specified before calling open() to ensure cross-browser compatibility. Otherwise, it will not be able to receive the cases where the readyState attribute is 0 and 1
xhr.onreadystatechange = function(){ if(xhr.readyState === 4){ if(xhr.status == 200){ alert(xhr.responseText); } } }
The following is a small example to demonstrate the application of xhr objects in ajax
<button id="btn">获取信息</button> <p id="result"></p> <script> btn.onclick = function(){ //创建xhr对象 var xhr; if(window.XMLHttpRequest){ xhr = new XMLHttpRequest(); }else{ xhr = new ActiveXObject('Microsoft.XMLHTTP'); } //异步接受响应 xhr.onreadystatechange = function(){ if(xhr.readyState == 4){ if(xhr.status == 200){ //实际操作 result.innerHTML += xhr.responseText; } } } //发送请求 xhr.open('get','message.xml',true); xhr.send(); } </script>
//message.xml
<p>hello world</p>
Through the demonstration of examples, we found that the content of the ajax front-end itself is not difficult. However, since ajax involves some back-end and network knowledge, it is not easy to learn. Future blog posts will gradually introduce the key contents of ajax in depth
The above is the first article of the in-depth understanding of ajax series introduced by the editor to the XHR object. I hope it will be helpful to everyone!
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