Some small cleanup items, see #153

js/common/model/BallSystem.js

@@ -105,8 +105,7 @@ class BallSystem {
     // @public (read-only) {AxonArray.<Ball>} - an array of the balls currently within the system. Balls
     //                                                **must** be from prepopulatedBalls. Its length should match the
     //                                                numberOfBallsProperty's value.
-    this.balls = new AxonArray( { elementOptions: { valueType: Ball } } );
-    //REVIEW: I can't find this elementOptions referenced, it looks like it used to exist? Can this be removed?
+    this.balls = new AxonArray();
 
     // Observe when the number of Balls is manipulated by the user and, if so, add or remove the correct number of Balls
     // to match the numberOfBallsProperty's value. The same Balls are in the system with the same number of Balls value.

js/common/model/CenterOfMass.js

@@ -23,6 +23,8 @@ import collisionLab from '../../collisionLab.js';
 import Ball from './Ball.js';
 import CollisionLabPath from './CollisionLabPath.js';
 
+const scratchVector = new Vector2( 0, 0 );
+
 class CenterOfMass {
 
   /**
@@ -75,7 +77,7 @@ class CenterOfMass {
     // This DerivedProperty is never disposed and persists for the lifetime of the sim.
     this.velocityProperty = new DerivedProperty(
       [ ...ballMassProperties, ...ballVelocityProperties, balls.lengthProperty ],
-      () => this.computeVelocity( balls ), { //REVIEW: computeVelocity doesn't take parameters anymore?
+      () => this.computeVelocity(), {
         valueType: Vector2
       } );
 
@@ -136,13 +138,7 @@ class CenterOfMass {
    * @returns {number} - in kg.
    */
   computeTotalBallSystemMass() {
-    //REVIEW: if helpful, return _.sumBy( this.balls, ball => ball.mass );
-    let totalMass = 0;
-
-    this.balls.forEach( ball => {
-      totalMass += ball.mass;
-    } );
-    return totalMass;
+    return _.sumBy( this.balls, ball => ball.mass );
   }
 
   /**
@@ -157,10 +153,7 @@ class CenterOfMass {
     // Determine the total first moment (mass * position) of the system.
     const totalFirstMoment = Vector2.ZERO.copy();
     this.balls.forEach( ball => {
-      //REVIEW: I see effort to prevent GC here, if that's needed then you can have a scratch vector declared in the
-      //REVIEW: top-level scope such that this could be totalFirstMoment.add( scratchVector.set( ball.position ).multiply( ball.mass ) )
-      //REVIEW: so that the .times() wouldn't create garbage.
-      totalFirstMoment.add( ball.position.times( ball.mass ) );
+      totalFirstMoment.add( scratchVector.set( ball.position ).multiply( ball.mass ) );
     } );
 
     // The position of the center of mass is the total first moment divided by the total mass.