ajax

    <button class="btn">btn</button>
    <div id="div"></div>
    <script>
        var btn = document.getElementsByClassName('btn')[0]
        btn.onclick = function () {
            // 兼容处理 new ajax()
            var xmlhttp;
            if (window.XMLHttpRequest) {// code for IE7+, Firefox, Chrome, Opera, Safari
                xmlhttp = new XMLHttpRequest();
            }
            else {// code for IE6, IE5
                xmlhttp = new ActiveXObject("Microsoft.XMLHTTP");
            }
            console.log(1)
            // 交互 返回状态码
            xmlhttp.onreadystatechange = function () {
                if (xmlhttp.readyState == 4 && xmlhttp.status == 200) {
                    document.getElementById("div").innerHTML = xmlhttp.responseText;
                    console.log(xmlhttp)
                }
            }
            console.log(2)
            xmlhttp.open("GET", "https://www.easy-mock.com/mock/5d099a583f67b5437759b813/example/mock", true);
            console.log(3)
            xmlhttp.send();
        }