A brief introduction to ajax

1.ajax native

ajax includes the following steps: 1. Create an AJAX object; 2. Issue an HTTP request; 3. Receive data returned by the server; 4. Update web page data. To summarize, in one sentence, ajax issues an HTTP request through the native XMLHttpRequest object, and then processes the data returned by the server.

Steps:

var xhr = createXHR();//创建对象 
xhr.open(“方式”,”地址”,”标志位”);//初始化请求 
xhr.setRequestHeader(“”,””);//设置http头信息 
xhr.onreadystatechange =function(){}//指定回调函数 
xhr.send();//发送请求

/   //1.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
              ;
            }
        }
    }
    //发送请求
    xhr.open('get',url,true);
    xhr.send();

2.jquery encapsulates ajax

		$.ajax({
			type : "get",
			url : '',
			dataType : "json",
			data:{},
			success : function(data){
				
			},error:function(){
			    console.log('fail');
			}
		});

3.jsonp cross-domain principle

For security reasons , the browser prohibits ajaxcross-domain data acquisition

Step by step demonstration ofscriptsrc Attribute loadingjsFile method to obtain data

Explain the mechanism of cross-domain data acquisition

(1) Dynamic creation scriptTag

(2)Define callback function

(3) Return function call

(4) Pass parameters or global variables

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