Reduce complexity by extracting validation condition to its function

lib/cartodb/backends/cluster.js

@@ -7,7 +7,7 @@ const AggregationMapConfig = require('../models/aggregation/aggregation-mapconfi
 
 module.exports = class ClusterBackend {
     // TODO: reduce complexity
-    // jshint maxcomplexity: 15
+    // jshint maxcomplexity: 13
     getClusterFeatures (mapConfigProvider, params, callback) {
         mapConfigProvider.getMapConfig((err, _mapConfig) => {
             if (err) {
@@ -77,9 +77,7 @@ module.exports = class ClusterBackend {
                 }
 
                 if (expressions !== undefined) {
-                    if (expressions === null ||
-                            Array.isArray(expressions) ||
-                            ['string', 'number', 'boolean'].includes(typeof expressions)) {
+                    if (!isValidExpression(expressions)) {
                         const error = new Error(
                             `Invalid aggregation input, expressions should be and object with valid functions`
                         );
@@ -265,3 +263,9 @@ function hasAggregationLayer (mapConfig, layerIndex) {
 function hasColumns (columns) {
     return Array.isArray(columns) && columns.length;
 }
+
+function isValidExpression (expressions) {
+    const invalidTypes = ['string', 'number', 'boolean'];
+
+    return expressions !== null && !Array.isArray(expressions) && !invalidTypes.includes(typeof expressions);
+}