Function fieldKey

docs/tests/v2/es6/js/sanddance.js:135622–135624

(field)

};

function fieldKey(field) {
  return (0, _vegaUtil.isString)(field) ? field : null;
}

function addSortField(scope, p, sort) {
  var as = (0, _util.aggrField)(sort.op, sort.field),