ajax执行流程1

/*
 第一步创建ajax对象：
	new XMLHttpRequest();
第二步填写请求信息:
	xhr.open('method',url,Asynchronous)
	method(请求方式):
		1、get通常用来获取数据，偶尔也会携带一些数据
			(1)发送的内容显示在浏览器地址栏上。
			(2)因为浏览器地址栏长度有限制，而get是通过url向服务器发送数据的，所以发送的数据大小会被限制。
		2、post通常用来向服务器发送数据，偶尔也会携带一些数据到客户端。
			(1)显示在请求头信息。
			(2)post传输数据理论来说是没有大小限制的,post的大小限制来自于服务器。
	url：请求文件的地址
	Asynchronous(是否异步):
		异步：不会等待数据请求结束继续执行下边的代码，当数据请求成功的时候，在回去进行处理。
		同步：必须等待前边的数据请求结束才可以执行下一行代码，可能会造成页面假死。
第三步监控请求结果
	ajax对象下边有一个onload在数据拿到之后就会触发
	xhr.responseText表示请求到的文本内容。
	xhr.responseXML表示请求到的XML。
第四步发送请求
	xhr.send(data)
	data发送给服务器的数据

 * */
document.onclick=function(){
	var xhr=new XMLHttpRequest();
	xhr.open('get','1.txt',true);
	
	xhr.onload=function(){
		alert(xhr.responseText);
	}
	xhr.send();
}

例子

	document.onclick = function(){
		var xhr = new XMLHttpRequest();

		xhr.open('get','php/data.xml',true);
		xhr.onload = function(){
			var entrys = xhr.responseXML.getElementsByTagName('entry');

			for(var i=0;i<entrys.length;i++){
				var names = entrys[i].getElementsByTagName('name')[0].innerHTML;
				console.log(names);
			}
		};
		xhr.send();
	};

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