Refactor to extract lookupBelow and findFollowingMapboxLayerId functions

Author: jonmmease

src/plots/mapbox/layers.js

@@ -85,6 +85,10 @@ proto.needsNewLayer = function(opts) {
     );
 };
 
+proto.lookupBelow = function() {
+    return this.subplot.belowLookup['layout-' + this.index];
+};
+
 proto.updateImage = function(opts) {
     var map = this.subplot.map;
     map.getSource(this.idSource).updateImage({
@@ -107,29 +111,29 @@ proto.updateSource = function(opts) {
     map.addSource(this.idSource, sourceOpts);
 };
 
-proto.updateLayer = function(opts) {
-    var subplot = this.subplot;
-    var convertedOpts = convertOpts(opts);
-
-    var below = this.subplot.belowLookup['layout-' + this.index];
-    var _below;
-
+proto.findFollowingMapboxLayerId = function(below) {
     if(below === 'traces') {
-        var mapLayers = subplot.getMapLayers();
+        var mapLayers = this.subplot.getMapLayers();
 
         // find id of first plotly trace layer
         for(var i = 0; i < mapLayers.length; i++) {
             var layerId = mapLayers[i].id;
             if(typeof layerId === 'string' &&
                 layerId.indexOf(constants.traceLayerPrefix) === 0
             ) {
-                _below = layerId;
+                below = layerId;
                 break;
             }
         }
-    } else {
-        _below = below;
     }
+    return below;
+};
+
+proto.updateLayer = function(opts) {
+    var subplot = this.subplot;
+    var convertedOpts = convertOpts(opts);
+    var below = this.lookupBelow();
+    var _below = this.findFollowingMapboxLayerId(below);
 
     this.removeLayer();