Create an empty promise

Question

I have a function in a chain of promises that may or may not do something. E.g.

  getYear().then(function(results){
    if(results.is1999) return party();
    else return Q.fcall(function(){/*do nothing here*/});
  }).then(sleep)

Where getYear, party, and sleep all return promises. Is there a more concise way to write the else statement? That is, do nothing, but still return a chainable promise?

Answer 1

Yes. Q(value) returns a promise for the value (it also unwraps the value if value is a promise).

  getYear().then(function(results){
    if(results.is1999) return party();
    else return Q(undefined);
  }).then(sleep)

Alternatively, you can get the exact same semantics by just not returning anything:

  getYear().then(function(results){
    if(results.is1999) return party();
  }).then(sleep)

If what you wanted was a promise that's never resolved, your best bet would be

  getYear().then(function(results){
    if(results.is1999) return party();
    else return Q.promise(function () {});
  }).then(sleep)

What you could do is re-use the same promise:

  var stop = Q.promise(function () {});
  getYear().then(function(results){
    if(results.is1999) return party();
    else return stop
  }).then(sleep)