Function fieldKey

docs/app/js/sanddance-app.js:148209–148211

(field4)

    return new DataScope(scope, input, output, values, aggr);
};
function fieldKey(field4) {
    return (0, _vegaUtil.isString)(field4) ? field4 : null;
}
function addSortField(scope, p, sort) {
    const as = aggrField(sort.op, sort.field);
    let s;