TrackballControls: Fix undefined constant.

examples/js/controls/TrackballControls.js

@@ -31,7 +31,7 @@ THREE.TrackballControls = function ( object, domElement ) {
 
 	this.keys = [ 65 /*A*/, 83 /*S*/, 68 /*D*/ ];
 
-	this.mouseButtons = { LEFT: THREE.MOUSE.ROTATE, MIDDLE: THREE.MOUSE.ZOOM, RIGHT: THREE.MOUSE.PAN };
+	this.mouseButtons = { LEFT: THREE.MOUSE.ROTATE, MIDDLE: THREE.MOUSE.DOLLY, RIGHT: THREE.MOUSE.PAN };
 
 	// internals
 

examples/jsm/controls/TrackballControls.js

@@ -39,7 +39,7 @@ var TrackballControls = function ( object, domElement ) {
 
 	this.keys = [ 65 /*A*/, 83 /*S*/, 68 /*D*/ ];
 
-	this.mouseButtons = { LEFT: MOUSE.ROTATE, MIDDLE: MOUSE.ZOOM, RIGHT: MOUSE.PAN };
+	this.mouseButtons = { LEFT: MOUSE.ROTATE, MIDDLE: MOUSE.DOLLY, RIGHT: MOUSE.PAN };
 
 	// internals