ps  需要提前引入<script src="https://unpkg.com/axios/dist/axios.min.js"></script>
// Generator实例，维护多个ajax依次执行的队列
function ajax(url){
    axios.get(url).then(res => userGen.next(res.data))
}
function *steps(){
    console.log('fetching users');
    const users = yield ajax('https://api.github.com/users');
    console.log(users);

    console.log('fetching firstUser');
    const firstUser = yield ajax(`https://api.github.com/users/${users[0].login}`);
    console.log(firstUser);

    console.log('fetching followers');
    const followers = yield ajax(firstUser.followers_url);
    console.log(followers);
}
const userGen = steps();
userGen.next();