ajax异步回调

       function ajax(url, fnSucc, fnFail) {
            var oAjax = new XMLHttpRequest();
            oAjax.open('GET', url, true);
            oAjax.send();
            oAjax.onload = function () {
                if (oAjax.readyState == 4 && oAjax.status == 200) {
                    fnSucc(oAjax.responseText)
                } else {
                    fnFail(oAjax.status);
                }
            };
        }
        window.onload = function () {
            let oBtn = document.getElementById('btn1');

            oBtn.onclick = function () {
                ajax('a.txt', function (str) {
                    alert(str);
                }, function (str) {
                    alert(str);
                });
            };
        };

promise异步封装例子

    function ajax(url, fnSucc, fnFail) {
            var oAjax = new XMLHttpRequest();
            oAjax.open('GET', url, true);
            oAjax.send();
            oAjax.onload = function () {
                if (oAjax.readyState == 4 && oAjax.status == 200) {
                    fnSucc(oAjax.responseText)
                } else {
                    fnFail(oAjax.status);
                }
            };
        }
        window.onload = function () {
            let oBtn = document.getElementById('btn1');
            let oBox = document.getElementById('box1');

            oBtn.onclick = function () {
                let p1 = new Promise(function (resolve, reject) {
                    ajax('a.txt', function (str) {
                        resolve(str);
                    }, function (str) {
                        reject(str);
                    });
                });

                p1.then(function (str) {
                    oBox.innerHTML = str;
                }, function (str) {
                    oBox.innerHTML = str;
                });

            };
        };