function sendAjax(url) {
    return new Promise((resolve,reject) => {
        // 1. 创建对象
        const x = new XMLHttpRequest();
        // 2. 初始化
        x.open('GET',url);
        // 3. 发送
        x.send();
        // 4. 事件绑定
        x.onreadystatechange = function() {
            if (x.readyState === 4) {
                if (x.status >= 200 && x.status <= 300) {
                    resolve(x.response);
                } else {
                    reject(x.status);
                };
            };
        };
    });
};

async function readAjax() {
    let result = await sendAjax('https://api.apiopen.top/getJoke');
    console.log(result);
}

readAjax();