queryRange_promise.js

var data = Promise.resolve(queryRange(start, end));

data.then(function(values) {
  console.log(values);
});

function queryRange(start, end) {
  var data = []; 
  var start = parseInt(start, 10);
  var end = parseInt(end, 10);
  var length = (end - start) + 1;
  var localforagePromises = []; 

  for (i = 0; i != length; i++) {
    var key = (start + i).toString(); // keys are '47513199', '47513200', '47513201', etc.
    var localforagePromise = localforage.getItem(key).then(function(value) { return value }); 
    var localforageResolve = Promise.resolve(localforagePromise);
    localforagePromises.push(localforageResolve);
  }
  
  var dataPromise = Promise.all(localforagePromises).then(function(values) { return values });

  return dataPromise;
}