Improved "fieldSorter"

sample.js

function fieldSorterOptimized(fields) {
  var dir = [], i, l = fields.length;
  fields = fields.map(function(o, i) {
    if (o[0] === "-") {
      dir[i] = -1;
      o = o.substring(1);
    } else {
      dir[i] = 1;
    }
    return o;
  });

  return function (a, b) {
    for (i = 0; i < l; i++) {
      var o = fields[i];
      if (a[o] > b[o]) return dir[i];
      if (a[o] < b[o]) return -(dir[i]);
    }
    return 0;
  };
}