compare promise vs boolean based async call duplication prevention

gistfile1.js

// async lock using promise
// this queues all calls
var pending = Q.when(true);
function callToServer() {
  pending = pending.then(function() {
    return callService("myService", params);
  });
}

// async avoid double call using flag
// this skips any calls made while the a call is pending
var running = false;
function callToServer() {
  if(!running) {
    running = true;
    return callService("myService", params)
      .then(function(result) {
        running = false;
        return result;
      });
  }
}

// I guess you could duplicate the skip behavior with a promise like this
var pending = Q.when(true);
function callToServer() {
  if(!pending.isPending()) {
    pending = callService("myService", params);
  }
}