var xhr1 = makeXHR("foo.json",
                   function(txt) { foo = txt; success() },
                   function(err) { failure() });  // missing argument `err`?
var xhr2 = makeXHR("bar.json",
                   function(txt) { bar = txt; success() },
                   function(e) { failure(e) });

var xhr1 = makeXHR("foo.json",
                   function(txt) { foo = txt; success() },
                   failure);
var xhr2 = makeXHR("bar.json",
                   function(txt) { bar = txt; success() },
                   failure);