方法一：通常的讲,我们可以设置一个flag变量,然后在各自的ajax的成功回调内去维护这个变量数量，当满足条件时,我们来触发后续函数

let flag = 0;
// ajax为异步操作，结合Promise使⽤可以轻松实现异步操作队列
function ajax1(num) {return new Promise(resolve => {setTimeout(() => {flag++;console.log('第1个异步请求'+flag); // 输出处理结果if (flag === 3) {donefn();}resolve();}, 3000);});
}function ajax2() {return new Promise(resolve => {setTimeout(() => {flag++console.log('第2个异步请求'+flag); // 输出处理结果if (flag === 3) {donefn();}resolve();}, 2000);});
}function ajax3() {return new Promise(resolve => {setTimeout(() => {flag++console.log('第3个异步请求'+flag); // 输出处理结果if (flag === 3) {donefn();}resolve();}, 1000);});
}function donefn() {console.log('执行完成！');
}ajax1();
ajax2();
ajax3();

方法二：Promise.all([promise1,promise2]).then(function(){})

// ajax为异步操作，结合Promise使⽤可以轻松实现异步操作队列
function ajax1(num) {return new Promise(resolve => {setTimeout(() => {console.log('第1个异步请求'); // 输出处理结果resolve();}, 3000);});
}function ajax2() {return new Promise(resolve => {setTimeout(() => {console.log('第2个异步请求'); // 输出处理结果resolve();}, 2000);});
}function ajax3() {return new Promise(resolve => {setTimeout(() => {console.log('第3个异步请求'); // 输出处理结果resolve();}, 1000);});
}function donefn() {console.log('执行完成！');
}Promise.all([ajax1(),ajax2(),ajax3()]).then(function(){console.log('执行完成！')})

方法三：使⽤Generator

// 使⽤Generator顺序执⾏三次异步操作
function* r(num) {yield ajax1(num);yield ajax2();yield ajax3();
}// ajax为异步操作，结合Promise使⽤可以轻松实现异步操作队列
function ajax1(num) {return new Promise(resolve => {setTimeout(() => {console.log('第1个异步请求'); // 输出处理结果resolve()}, 1000);});
}function ajax2() {return new Promise(resolve => {setTimeout(() => {console.log('第2个异步请求'); // 输出处理结果resolve(); // 操作成功}, 1000);});
}function ajax3() {return new Promise(resolve => {setTimeout(() => {console.log('第3个异步请求'); // 输出处理结果resolve(); // 操作成功}, 1000);});
}// 不使⽤递归函数调⽤
let it = r();// 修改为可处理Promise的next
function next(data) {let { value, done } = it.next(data); // 启动if (!done) {value.then(() => {next();});} else {console.log('执行完毕！');}
}next();