Ajax(JavaScript)

function success(text) {    var textarea = document.getElementById('test-response-text');    textarea.value = text;}function fail(code) {    var textarea = document.getElementById('test-response-text');    textarea.value = 'Error code: ' + code;}var request = new XMLHttpRequest(); // 新建XMLHttpRequest对象request.onreadystatechange = function () { // 状态发生变化时，函数被回调    if (request.readyState === 4) { // 成功完成        // 判断响应结果:        if (request.status === 200) {            // 成功，通过responseText拿到响应的文本:            return success(request.responseText);        } else {            // 失败，根据响应码判断失败原因:            return fail(request.status);        }    } else {        // HTTP请求还在继续...    }}// 发送请求:request.open('GET', '/api/categories');request.send();alert('请求已发送，请等待响应...');