Use ES6 class syntax

lib/cartodb/models/dataview/histogram.js

@@ -307,7 +307,11 @@ Time series:
     }
  }
  */
-function Histogram(query, options, queries) {
+
+module.exports = class Histogram extends BaseWidget {
+    constructor (query, options, queries) {
+        super();
+
         if (!_.isString(options.column)) {
             throw new Error('Histogram expects `column` in widget options');
         }
@@ -320,14 +324,9 @@ function Histogram(query, options, queries) {
         this.offset = options.offset;
 
         this._columnType = null;
-}
+    }
 
-Histogram.prototype = new BaseWidget();
-Histogram.prototype.constructor = Histogram;
-
-module.exports = Histogram;
-
-Histogram.prototype.sql = function(psql, override, callback) {
+    sql (psql, override, callback) {
         var self = this;
 
         if (!callback) {
@@ -350,13 +349,13 @@ Histogram.prototype.sql = function(psql, override, callback) {
         }
 
         this._buildQuery(psql, override, callback);
-};
+    }
 
-Histogram.prototype.isDateHistogram = function (override) {
+    isDateHistogram (override) {
         return this._columnType === 'date' && (this.aggregation !== undefined || override.aggregation !== undefined);
-};
+    }
 
-Histogram.prototype._buildQuery = function (psql, override, callback) {
+    _buildQuery (psql, override, callback) {
         var filteredQuery, basicsQuery, binsQuery;
         var _column = this.column;
         var _query = this.query;
@@ -452,28 +451,17 @@ Histogram.prototype._buildQuery = function (psql, override, callback) {
         debug(histogramSql);
 
         return callback(null, histogramSql);
-};
+    }
 
-Histogram.prototype._shouldOverride = function (override) {
+    _shouldOverride (override) {
         return override && _.has(override, 'start') && _.has(override, 'end') && _.has(override, 'bins');
-};
+    }
 
-Histogram.prototype._shouldOverrideBins = function (override) {
+    _shouldOverrideBins (override) {
         return override && _.has(override, 'bins');
-};
+    }
 
-var DATE_AGGREGATIONS = {
-    'auto': true,
-    'minute': true,
-    'hour': true,
-    'day': true,
-    'week': true,
-    'month': true,
-    'quarter': true,
-    'year': true
-};
-
-Histogram.prototype._buildDateHistogramQuery = function (psql, override, callback) {
+    _buildDateHistogramQuery (psql, override, callback) {
         var _column = this.column;
         var _query = this.query;
         var _aggregation = override && override.aggregation ? override.aggregation : this.aggregation;
@@ -549,9 +537,9 @@ Histogram.prototype._buildDateHistogramQuery = function (psql, override, callbac
         debug(histogramSql);
 
         return callback(null, histogramSql);
-};
+    }
 
-Histogram.prototype.getAutomaticAggregation = function (psql, callback) {
+    getAutomaticAggregation (psql, callback) {
         var dateIntervalQuery = dateIntervalQueryTpl({
             query: this.query,
             column: this.column
@@ -589,9 +577,9 @@ Histogram.prototype.getAutomaticAggregation = function (psql, callback) {
 
             callback(null, aggregation.name);
         });
-};
+    }
 
-Histogram.prototype.format = function(result, override) {
+    format (result, override) {
         override = override || {};
         var buckets = [];
 
@@ -649,6 +637,30 @@ Histogram.prototype.format = function(result, override) {
             avg: avg,
             bins: buckets
         };
+    }
+
+    getType () {
+        return TYPE;
+    }
+
+    toString () {
+        return JSON.stringify({
+            _type: TYPE,
+            _column: this.column,
+            _query: this.query
+        });
+    }
+};
+
+var DATE_AGGREGATIONS = {
+    'auto': true,
+    'minute': true,
+    'hour': true,
+    'day': true,
+    'week': true,
+    'month': true,
+    'quarter': true,
+    'year': true
 };
 
 function getAggregation(override, aggregation) {
@@ -720,15 +732,3 @@ function populateBinStart(override, firstRow) {
 
     return binStart;
 }
-
-Histogram.prototype.getType = function() {
-    return TYPE;
-};
-
-Histogram.prototype.toString = function() {
-    return JSON.stringify({
-        _type: TYPE,
-        _column: this.column,
-        _query: this.query
-    });
-};