How to use the datalib.keys function in datalib

function properties(model, mark, spec) {
  var config = model.config(),
      code = "",
      names = dl.keys(spec),
      exprs = [], // parsed expressions injected in the generated code
      i, len, name, ref, vars = {},
      deps = {
        signals: {},
        scales:  {},
        data:    {},
        fields:  {},
        nested:  [],
        _nRefs:  {},  // Temp stash to de-dupe nested refs.
        reflow:  false
      };

  code += "var o = trans ? {} : item, d=0, exprs=this.exprs, set=this.tpl.set, tmpl=signals||{}, t;\n" +
          // Stash for dl.template
          "tmpl.datum  = item.datum;\n" +
          "tmpl.group  = group;\n" +

field = lookup(ref);
    return {
      data: getParent(field).name,
      field: field._name
    };
  }

  // More than one ref
  for (i = 0, len = ref.length; i < len; ++i) {
    field = lookup(ref[i]);
    data = getParent(field);
    sets[data._id] = sets[data._id] || (sets[data._id] = []);
    sets[data._id].push(field);
  }

  keys = dl.keys(sets);
  if (keys.length === 1) {
    ref = {
      data: data.name,
      field: sets[data._id].map(fmap)
    };
  } else {
    ref = {fields: []};
    for (i = 0, len = keys.length; i < len; ++i) {
      data = lookup(keys[i]);
      ref.fields.push({
        data: data.name,
        field: sets[data._id].map(fmap)
      });
    }
  }
}

function nsProperties(propset) {
    dl.keys(propset).forEach(function(k) {
      var p = propset[k];
      if (p.signal) p.signal = ns(p.signal, sg);
      else if (p.rule) {
        p.rule.forEach(function(r) {
          if (r.signal) r.signal = ns(r.signal, sg);
          if (r.predicate) nsOperand(r);
        });
      }
    });
  }

prototype.data = function(data) {
  var v = this;
  if (!arguments.length) return v._model.values();
  else if (dl.isString(data)) return streaming.call(v, data);
  else if (dl.isObject(data)) {
    dl.keys(data).forEach(function(k) {
      var api = streaming.call(v, k);
      data[k](api);
    });
  }
  return this;
};

prototype.values = function(type, names, hash) {
  var data = (type === Deps.SIGNALS ? this._signals : this._data),
      n = (names !== undefined ? names : dl.keys(data)),
      vals, i;

  if (Array.isArray(n)) {
    vals = hash || {};
    for (i=0; i

prototype.values = function(type, names, hash) {
  var data = (type === Deps.SIGNALS ? this._signals : this._data),
      n = (names !== undefined ? names : dl.keys(data)),
      vals, i;

  if (Array.isArray(n)) {
    vals = hash || {};
    for (i=0; i

function inject() {
    if (dl.keys(mk).length > 0) injectMarks(spec.marks);
    spec.signals = dl.array(spec.signals);
    spec.predicates = dl.array(spec.predicates);
    spec.signals.unshift.apply(spec.signals, signals);
    spec.predicates.unshift.apply(spec.predicates, predicates);
    defFactory();
  }