Pyramidal promises

Question

I have a situation in my Node (express) controller where I need to check whether a particular language has a parent language, if it does, I need to do a new DB (SQL) call to retrieve the parent language details.

The problem is that this is encouraging a pyramid style (not too far from callback-hell). What if next I need to do another DB call depending on something in the parent language? My code will rapidly become unreadable. I thought promises were meant to solve all this. Is there a better way?

Promise.all(promises)
    .then(function(results) {
        // check if the language has a macrolanguage, increases execution
        // time of api, as queries cannot run in parallel
        if (results[0].language.parent_language_id) {
            Languages.getParentLanguageDetails(results[0].language.parent_language_id)
            //embedded thens are bad style, should be cleaned up
                .then(function(newResult) {
                    results[results.length] = newResult
                    var data = u.extendResultsObject(results)
                    res.send(200, data) //success
                })
        } else {
            var data = u.extendResultsObject(results)
            res.send(200, data) //success
        }
    })

Answer 1

To unnest promises you just have to return a promise, then you can chain then calls, for example:

p1.then(function(data1) {
  p2(data1).then(function(data2) {
    ...
  })
})

Can be written as:

p1.then(function(data1) {
  return p2(data1)
}).then(function(data2) {
  ...
})

Your edit looks better, all I would do is remove the extra else, cache the id, and use push instead of results[results.length]:

Promise.all(promises)
  .then(function(results) {
    var id = results[0].language.parent_language_id
    if (id) {
      return Languages.getParentLanguageDetails(id)
        .then(function(res) {
          results.push(res)
          return results
        })
    }
    return results
  }).then(function(results) {
    var data = u.extendResultsObject(results)
    res.send(200, data)
  })

You can go a bit further, by realizing that:

results.push(res)
return result

could be re-written as:

return results.concat(res)

If res is an array then this might yield unexpected results though.