Return after all "on" events are called inside a JS promise

Question

In Node.js, using NodeGit. I'm using a function similar to this inside a POST Express.js route. This route should get the commits between endCommit and startCommit:

router.post('/getLog', function(req, res, next) {

    var logData = [];



    Git.Repository.open(path.join(repositories.mine.path))

        .then(function(repo) {

            return {

                    endCommit: [ First commit object ], //Just a random commit object that has a "sha" inside

                    startCommit: [ Second commit object ]  //Just a random commit object that has a "sha" inside

                };

        })

        .then(function(range) {

            // History returns an event.



            var history = range.endCommit.history(Git.Revwalk.SORT.Time);



            // History emits "commit" event for each commit in the branch's history

            history.on("commit", function(commit) {

                logData.push({

                    commit: commit.sha(),

                    message: commit.message()

                });



                if (commit.sha() == range.startCommit.sha()) {

                    console.log("---LOG CREATED---");

                    history.end();

                }

            })



            history.start();

        })

        .done(function() {

            console.log("---RETURNING---");



            return res.json({ logData: logData });

        });

});

However, since

history.on("commit", ...)

is not a Promise the

.done()

function is called first. In the log I see:

---RETURNING---

---LOG CREATED---

How can I return only after the log has been created?

I've came across issues like this in the past however in this particular case I don't know how could I promisify the history object because it is based on events.

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Answer 1

You can wrap event handling in a promise that should be resolved once you are done, and return it:

.then(function(range) {
  // History returns an event.

  var history = range.endCommit.history(Git.Revwalk.SORT.Time);

  var commitPromise = new Promise(function(resolve, reject) {
    // History emits "commit" event for each commit in the branch's history
    history.on("commit", function(commit) {
      logData.push({
        commit: commit.sha(),
        message: commit.message()
      });

      if (commit.sha() == range.startCommit.sha()) {
        console.log("---LOG CREATED---");
        history.end();

        // resolve the promise
        resolve();
      }
    })
  });

  history.start();

  return commitPromise;
})

I assume you have Promise global. It's up to you choosing particular promise implementation - you may want to use bluebird for example.

Return after all "on" events are called inside a JS promise

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