Promises causing memory leaks

promises.js

function bad() {
  var promise = somethingAsync();
  
  promise.finally(function () {
    logSomeThings();
  });

  promise = promise.then(function (result) {
    return transform(result);
  });

  return promise;
}


function good() {
  var promise = somethingAsync();
  
  promise = promise.finally(function () {  // <-- only changed line
    logSomeThings();
  });

  promise = promise.then(function (result) {
    return transform(result);
  });

  return promise;
}

This is using Q. The somethingAsync method essentially returns Q.defer().promise

I am not sure either, but the difference is that the original promise would not be retained. The same could be accomplished, I suspect, by writing promise = null in the handler. Does this pattern leak?

function maybe() {
    return somethingAsync()
    .finally(logSomeThings)
    .then(transform);
}

That should be equivalent, is the common usage, and doesn’t retain any intermediates. If that leaks, it’s bad news. (please ping on Twitter again; gist will not inform me if you reply).

@kriskowal: having some difficulty repro'ing the error right now (the original leak was very slow and was only visible under production load -- fun!). I'll trying boiling it down to the raw test case above and update tomorrow.