How to use promise to avoid callback hell?

This question already has an answer here:

Aren't promises just callbacks? 7 answers

---

Try whit this:

function getPost(id) {
  return Post
    .findOne({id: id})
    .then( post => {
      return post;
    });
}

using Q module

function getCommentsAndLinks(post) {
  return Q.all([
    Comment.find({id: post.id}),
    Links.find({id: post.id})
  ])
  .done( results => {
    let comments = results[0];
    let links = results[1];
    return [post, comments, links];
  })
  .catch( err => {
  // handle err
  })

on controller

getPost(postId)
.then(getCommentsAndLinks)
.then( results => {
  let post = results[0];
  let comments = results[1];
  let links = results[2];
  // more code here
})
.catch( err => {
// handle err
}) 

but i suggest you not save string of IDS, save the instance of object, so you can use populate to get all data of comments and links, something like this:

Post
.findOne({id: id})
.populate('comments')
.populate('links')
.then( post => {
    // here have the post with data of comments and links
});

---

You are nesting the callbacks. You don't need to do this. If you return a promise from .then then any .then you chain to it will be resolved when that promise gets resolved:

promise.then(post => Comments.find({id: post.id})
  .then(comments => Links.find({id: post.id})
  .then(links => {});

The comments query does not depend on links so you can actually do both queries at once:

promise.then(post => {
  return Promise.all([
     post,
     Comments.find({id: post.id}),
     Links.find({id: post.id}),
  ]);
}).then(data => res.json({
  post: data[0],
  comments: data[1],
  links: data[2],
});

If you use a library like bluebird you can also use something like the spread operator to make the names more transparent.

---

I would also look into using co for generator-based control flow as I think this is even clearer:

co(function* () {
  const post = yield Posts.findOne({id});
  const [comments, links] = yield [
    Comments.find({id: post.id}),
    Links.find({id: post.id}),
  ];

  res.json({post, comments, links});
});

---

The advantage of using promises is you can chain them, so your code can be reduced to:

let post, comments;
Posts.findOne({id: id}).exec().then(_post => {
    post = _post;
    return Comments.find({id: post.id}).exec();
  }).then(_comments => {
    comments = _comments;
    return Links.find({id: post.id}).exec();
  }).then(links => res.json({post, comment, links}))
  .catch(error => res.error(error.message));

you would notice that I needed only a single catch block.

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