Use arrow function

lib/cartodb/models/dataview/histograms/date-histogram.js

@@ -187,7 +187,7 @@ module.exports = class DateHistogram extends BaseHistogram {
     }
 
     _buildQuery (psql, override, callback) {
-        if (!this.isValidAggregation(override)) {
+        if (!this._isValidAggregation(override)) {
             return callback(new Error('Invalid aggregation value. Valid ones: ' +
                 Object.keys(DATE_AGGREGATIONS).join(', ')
             ));
@@ -238,13 +238,8 @@ module.exports = class DateHistogram extends BaseHistogram {
 
             const aggegations = result.rows[0];
             const aggregation = Object.keys(aggegations)
-                .map(function (key) {
-                    return {
-                        name: key,
-                        value: aggegations[key]
-                    };
-                })
-                .reduce(function (closer, current) {
+                .map(key => ({ name: key, value: aggegations[key] }))
+                .reduce((closer, current) => {
                     if (current.value > MAX_INTERVAL_VALUE) {
                         return closer;
                     }