如何避免在多层嵌套的回调函数中陷入回调地狱？

// 使用Promise链来避免回调地狱
function fetchUserData(userId) {
    return fetch(`/users/${userId}`)
        .then(response => response.json())
        .then(user => {
            return fetch(`/posts?userId=${user.id}`)
                .then(response => response.json())
                .then(posts => {
                    user.posts = posts;
                    return user;
                });
        });
}

// 使用async/await进一步简化
async function fetchUserData(userId) {
    const response = await fetch(`/users/${userId}`);
    const user = await response.json();
    const postsResponse = await fetch(`/posts?userId=${user.id}`);
    const posts = await postsResponse.json();
    user.posts = posts;
    return user;
}

// 使用事件驱动编程
const EventEmitter = require('events');
const eventEmitter = new EventEmitter();

eventEmitter.on('userFetched', (user) => {
    fetch(`/posts?userId=${user.id}`)
        .then(response => response.json())
        .then(posts => {
            user.posts = posts;
            eventEmitter.emit('userWithPosts', user);
        });
});

eventEmitter.on('userWithPosts', (user) => {
    console.log(user);
});

fetch('/users/1')
    .then(response => response.json())
    .then(user => eventEmitter.emit('userFetched', user));

// 使用生成器函数
function* fetchUserData(userId) {
    const response = yield fetch(`/users/${userId}`);
    const user = yield response.json();
    const postsResponse = yield fetch(`/posts?userId=${user.id}`);
    const posts = yield postsResponse.json();
    user.posts = posts;
    return user;
}

// 执行生成器函数
const iterator = fetchUserData(1);
let result = iterator.next();

while (!result.done) {
    result.value.then(data => {
        result = iterator.next(data);
    });
}

// 使用RxJS
import { from } from 'rxjs';
import { switchMap, map } from 'rxjs/operators';

function fetchUserData(userId) {
    return from(fetch(`/users/${userId}`)).pipe(
        switchMap(response => response.json()),
        switchMap(user => from(fetch(`/posts?userId=${user.id}`)).pipe(
            switchMap(response => response.json()),
            map(posts => {
                user.posts = posts;
                return user;
            })
        ))
    );
}

fetchUserData(1).subscribe(user => console.log(user));