diff --git a/js/quadrilateral/model/QuadrilateralShapeModel.ts b/js/quadrilateral/model/QuadrilateralShapeModel.ts
index d8b89526..01923440 100644
--- a/js/quadrilateral/model/QuadrilateralShapeModel.ts
+++ b/js/quadrilateral/model/QuadrilateralShapeModel.ts
@@ -58,14 +58,14 @@ export default class QuadrilateralShapeModel {
   public readonly vertexB: QuadrilateralVertex;
   public readonly vertexC: QuadrilateralVertex;
   public readonly vertexD: QuadrilateralVertex;
-  public readonly vertices: QuadrilateralVertex[];
+  public readonly vertices: readonly QuadrilateralVertex[];
 
   // Sides of the quadrilateral.
   public readonly sideAB: QuadrilateralSide;
   public readonly sideBC: QuadrilateralSide;
   public readonly sideCD: QuadrilateralSide;
   public readonly sideDA: QuadrilateralSide;
-  public readonly sides: QuadrilateralSide[];
+  public readonly sides: readonly QuadrilateralSide[];
 
   // Available space for the Vertices to move.
   private readonly modelBounds: Bounds2;
@@ -73,7 +73,7 @@ export default class QuadrilateralShapeModel {
   // Monitors angles of the shape to determine when pairs of opposite sides are parallel.
   public readonly sideABSideCDParallelSideChecker: ParallelSideChecker;
   public readonly sideBCSideDAParallelSideChecker: ParallelSideChecker;
-  public readonly parallelSideCheckers: ParallelSideChecker[];
+  public readonly parallelSideCheckers: readonly ParallelSideChecker[];
 
   // Whether the quadrilateral is a parallelogram. This Property is true when both ParallelSideCheckers report
   // parallel sides.
diff --git a/js/quadrilateral/view/QuadrilateralDescriber.ts b/js/quadrilateral/view/QuadrilateralDescriber.ts
index f549988a..259274cc 100644
--- a/js/quadrilateral/view/QuadrilateralDescriber.ts
+++ b/js/quadrilateral/view/QuadrilateralDescriber.ts
@@ -141,7 +141,7 @@ export default class QuadrilateralDescriber {
   public readonly sideBCDescriber: SideDescriber;
   public readonly sideCDDescriber: SideDescriber;
   public readonly sideDADescriber: SideDescriber;
-  private readonly sideDescribers: SideDescriber[];
+  private readonly sideDescribers: readonly SideDescriber[];
 
   public readonly vertexADescriber: VertexDescriber;
   public readonly vertexBDescriber: VertexDescriber;
diff --git a/js/quadrilateral/view/QuadrilateralNode.ts b/js/quadrilateral/view/QuadrilateralNode.ts
index ba57120f..d7c46955 100644
--- a/js/quadrilateral/view/QuadrilateralNode.ts
+++ b/js/quadrilateral/view/QuadrilateralNode.ts
@@ -51,8 +51,8 @@ export default class QuadrilateralNode extends Voicing( Node ) {
   private readonly scratchShapeModel: QuadrilateralShapeModel;
   private readonly modelViewTransform: ModelViewTransform2;
 
-  private readonly vertexNodes: QuadrilateralVertexNode[];
-  private readonly sideNodes: QuadrilateralSideNode[];
+  private readonly vertexNodes: readonly QuadrilateralVertexNode[];
+  private readonly sideNodes: readonly QuadrilateralSideNode[];
 
   private remainingTimeForShapeChangeFill: number;
   private activeFill: TPaint | null;
diff --git a/js/quadrilateral/view/sound/TracksSoundView.ts b/js/quadrilateral/view/sound/TracksSoundView.ts
index c8c5c988..7f5303c2 100644
--- a/js/quadrilateral/view/sound/TracksSoundView.ts
+++ b/js/quadrilateral/view/sound/TracksSoundView.ts
@@ -55,7 +55,7 @@ export default class TracksSoundView extends SoundGenerator {
 
   // Array of all SoundGenerators from the provided tracks. They will play and loop forever in the background. Depending
   // on input and state of the quadrilateral shape, their output level will change.
-  public readonly soundClips: SoundClip[];
+  public readonly soundClips: readonly SoundClip[];
 
   // A map that goes from the index of the sound file to play to its desired output leve. Subclasses will populate this.
   public readonly indexToOutputLevelMap = new Map<number, number>();