How do I get a synchronous response, or an equivalent, using AWS JavaScript SDK?

Question

I am reading the contents of a directory, and returning the filename and some metadata for each file. I need to save this in an object and then do some filtering before I append it to the page. The problem is that the function ends before the asynchronous call adds all the items to the object. Maybe there is something I don't get about asynchronous javascript, but it's a pain in the butt.

function getAr(dir) {
        var ar = {} || [];
        ar.files = [];
        ar.files.posts = [];
        ar.files.pages = [];

        ar.set = function(params) {
            s3.headObject(params, function(err, data) {
              if (err) console.log(err, err.stack);
              else     {
                  console.log(data);
                  if (params['Key'].match('/page/')) {
                    var size = ar['files']['pages'].length;
                    ar.files.pages[size] = {'id': data.Metadata.id, 'name': params['Key']};
                  }
                  else if (params['Key'].match('/post/')) {
                      var size = ar['files']['pages'].length;
                      ar.files.posts[size] = {'id': data.Metadata.id, 'name': params['Key']};
                  }
              }
            });
        };

        ar.ls = function(path) {
            var params = {
                Bucket: bucket,
                EncodingType: 'url',
                Prefix: path
            };

            s3.listObjects(params, function(err, data) {
                if (err) {console.log(err, err.stack);}
                else {
                    var file = null;
                    for (var num in data.Contents) {
                        params = {};
                        file = data.Contents[num]['Key'];
                        param = {
                          Bucket: bucket, 
                          Key: file
                        };
                        ar.set(params);
                    }
                    //I need to do some sorting of ar.files here, but the script continues on without waiting for the asynchronous calls to finish.
                    //console.log(ar.files);
                }
            });
            //ar.files returns null because the scripts haven't finished
            return ar.files;

        };

        return ar.ls(dir);

    }

var path = '/_admin/src/pages/';
console.log(getAr(path));