js 原生ajax实现

<!DOCTYPE html>
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8"/>
    <title></title>
    <meta charset="utf-8" />
</head>
<body>
    <input type="text" id="txt1" value="1212" />

    <script>
        var obj = document.getElementById('txt1');
        console.log(obj.value);

        var xmlHttp;
        if (window.XMLHttpRequest) {
            xmlHttp = new XMLHttpRequest();
        } else {
            xmlHttp = new ActiveXObject("Microsoft.XMLHTTP");
        }

        xmlHttp.onreadystatechange = function () {
            if (xmlHttp.readyState == 4 && xmlHttp.status == 200) {
                console.log(1);
                obj.value = xmlHttp.responseText;
            }
        };
       
        xmlHttp.open('get', 'json1.json', true);   // true 异步 false 同步        url:   aaa.asp?a=1&b=2          
        xmlHttp.send();   //send('11111'). 当post的时候提交 '11111'
        console.log(2);

        //xmlhttp.open("POST", "ajax_test.asp", true);
        //xmlhttp.setRequestHeader("Content-type", "application/x-www-form-urlencoded");
        //xmlhttp.send("fname=Bill&lname=Gates");

        //当为true，打出顺序为  2  1
        //当为false，打出顺序为 1  2
    </script>
</body>
</html>
教程地址：http://www.w3school.com.cn/ajax/index.asp
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