diff --git a/core/toolbox/toolbox.ts b/core/toolbox/toolbox.ts
index efd5381b4..12465813c 100644
--- a/core/toolbox/toolbox.ts
+++ b/core/toolbox/toolbox.ts
@@ -70,9 +70,6 @@ export class Toolbox
   /** Whether the Toolbox is visible. */
   protected isVisible_ = false;
 
-  /** The list of items in the toolbox. */
-  protected contents_: IToolboxItem[] = [];
-
   /** The width of the toolbox. */
   protected width_ = 0;
 
@@ -82,7 +79,10 @@ export class Toolbox
 
   /** The flyout for the toolbox. */
   private flyout_: IFlyout | null = null;
-  protected contentMap_: {[key: string]: IToolboxItem};
+
+  /** Map from ID to the corresponding toolbox item. */
+  protected contents = new Map<string, IToolboxItem>();
+
   toolboxPosition: toolbox.Position;
 
   /** The currently selected item. */
@@ -118,9 +118,6 @@ export class Toolbox
     /** Is RTL vs LTR. */
     this.RTL = workspace.options.RTL;
 
-    /** A map from toolbox item IDs to toolbox items. */
-    this.contentMap_ = Object.create(null);
-
     /** Position of the toolbox and flyout relative to the workspace. */
     this.toolboxPosition = workspace.options.toolboxPosition;
   }
@@ -367,14 +364,8 @@ export class Toolbox
    */
   render(toolboxDef: toolbox.ToolboxInfo) {
     this.toolboxDef_ = toolboxDef;
-    for (let i = 0; i < this.contents_.length; i++) {
-      const toolboxItem = this.contents_[i];
-      if (toolboxItem) {
-        toolboxItem.dispose();
-      }
-    }
-    this.contents_ = [];
-    this.contentMap_ = Object.create(null);
+    this.contents.forEach((item) => item.dispose());
+    this.contents.clear();
     this.renderContents_(toolboxDef['contents']);
     this.position();
     this.handleToolboxItemResize();
@@ -445,8 +436,7 @@ export class Toolbox
    * @param toolboxItem The item in the toolbox.
    */
   protected addToolboxItem_(toolboxItem: IToolboxItem) {
-    this.contents_.push(toolboxItem);
-    this.contentMap_[toolboxItem.getId()] = toolboxItem;
+    this.contents.set(toolboxItem.getId(), toolboxItem);
     if (toolboxItem.isCollapsible()) {
       const collapsibleItem = toolboxItem as ICollapsibleToolboxItem;
       const childToolboxItems = collapsibleItem.getChildToolboxItems();
@@ -463,7 +453,7 @@ export class Toolbox
    * @returns The list of items in the toolbox.
    */
   getToolboxItems(): IToolboxItem[] {
-    return this.contents_;
+    return [...this.contents.values()];
   }
 
   /**
@@ -618,7 +608,7 @@ export class Toolbox
    * @returns The toolbox item with the given ID, or null if no item exists.
    */
   getToolboxItemById(id: string): IToolboxItem | null {
-    return this.contentMap_[id] || null;
+    return this.contents.get(id) || null;
   }
 
   /**
@@ -765,14 +755,13 @@ export class Toolbox
    * @internal
    */
   refreshTheme() {
-    for (let i = 0; i < this.contents_.length; i++) {
-      const child = this.contents_[i];
+    this.contents.forEach((child) => {
       // TODO(#6097): Fix types or add refreshTheme to IToolboxItem.
       const childAsCategory = child as ToolboxCategory;
       if (childAsCategory.refreshTheme) {
         childAsCategory.refreshTheme();
       }
-    }
+    });
   }
 
   /**
@@ -923,11 +912,9 @@ export class Toolbox
    * @param position The position of the item to select.
    */
   selectItemByPosition(position: number) {
-    if (position > -1 && position < this.contents_.length) {
-      const item = this.contents_[position];
-      if (item.isSelectable()) {
-        this.setSelectedItem(item);
-      }
+    const item = this.getToolboxItems()[position];
+    if (item) {
+      this.setSelectedItem(item);
     }
   }
 
@@ -1034,11 +1021,12 @@ export class Toolbox
       return false;
     }
 
-    let nextItemIdx = this.contents_.indexOf(this.selectedItem_) + 1;
-    if (nextItemIdx > -1 && nextItemIdx < this.contents_.length) {
-      let nextItem = this.contents_[nextItemIdx];
+    const items = [...this.contents.values()];
+    let nextItemIdx = items.indexOf(this.selectedItem_) + 1;
+    if (nextItemIdx > -1 && nextItemIdx < items.length) {
+      let nextItem = items[nextItemIdx];
       while (nextItem && !nextItem.isSelectable()) {
-        nextItem = this.contents_[++nextItemIdx];
+        nextItem = items[++nextItemIdx];
       }
       if (nextItem && nextItem.isSelectable()) {
         this.setSelectedItem(nextItem);
@@ -1058,11 +1046,12 @@ export class Toolbox
       return false;
     }
 
-    let prevItemIdx = this.contents_.indexOf(this.selectedItem_) - 1;
-    if (prevItemIdx > -1 && prevItemIdx < this.contents_.length) {
-      let prevItem = this.contents_[prevItemIdx];
+    const items = [...this.contents.values()];
+    let prevItemIdx = items.indexOf(this.selectedItem_) - 1;
+    if (prevItemIdx > -1 && prevItemIdx < items.length) {
+      let prevItem = items[prevItemIdx];
       while (prevItem && !prevItem.isSelectable()) {
-        prevItem = this.contents_[--prevItemIdx];
+        prevItem = items[--prevItemIdx];
       }
       if (prevItem && prevItem.isSelectable()) {
         this.setSelectedItem(prevItem);
@@ -1076,16 +1065,13 @@ export class Toolbox
   dispose() {
     this.workspace_.getComponentManager().removeComponent('toolbox');
     this.flyout_!.dispose();
-    for (let i = 0; i < this.contents_.length; i++) {
-      const toolboxItem = this.contents_[i];
-      toolboxItem.dispose();
-    }
+    this.contents.forEach((item) => item.dispose());
 
     for (let j = 0; j < this.boundEvents_.length; j++) {
       browserEvents.unbind(this.boundEvents_[j]);
     }
     this.boundEvents_ = [];
-    this.contents_ = [];
+    this.contents.clear();
 
     if (this.HtmlDiv) {
       this.workspace_.getThemeManager().unsubscribe(this.HtmlDiv);
diff --git a/tests/mocha/test_helpers/toolbox_definitions.js b/tests/mocha/test_helpers/toolbox_definitions.js
index 2f767ed60..f05c29620 100644
--- a/tests/mocha/test_helpers/toolbox_definitions.js
+++ b/tests/mocha/test_helpers/toolbox_definitions.js
@@ -243,9 +243,8 @@ export function getBasicToolbox() {
 }
 
 export function getCollapsibleItem(toolbox) {
-  const contents = toolbox.contents_;
-  for (let i = 0; i < contents.length; i++) {
-    const item = contents[i];
+  const contents = toolbox.contents.values();
+  for (const item of contents) {
     if (item.isCollapsible()) {
       return item;
     }
@@ -253,9 +252,8 @@ export function getCollapsibleItem(toolbox) {
 }
 
 export function getNonCollapsibleItem(toolbox) {
-  const contents = toolbox.contents_;
-  for (let i = 0; i < contents.length; i++) {
-    const item = contents[i];
+  const contents = toolbox.contents.values();
+  for (const item of contents) {
     if (!item.isCollapsible()) {
       return item;
     }
diff --git a/tests/mocha/toolbox_test.js b/tests/mocha/toolbox_test.js
index 1cb8df979..c6a1c726d 100644
--- a/tests/mocha/toolbox_test.js
+++ b/tests/mocha/toolbox_test.js
@@ -98,7 +98,7 @@ suite('Toolbox', function () {
           {'kind': 'category', 'contents': []},
         ],
       });
-      assert.lengthOf(this.toolbox.contents_, 2);
+      assert.equal(this.toolbox.contents.size, 2);
       sinon.assert.called(positionStub);
     });
     // TODO: Uncomment once implemented.
@@ -153,7 +153,7 @@ suite('Toolbox', function () {
         ],
       };
       this.toolbox.render(jsonDef);
-      assert.lengthOf(this.toolbox.contents_, 1);
+      assert.equal(this.toolbox.contents.size, 1);
     });
     test('multiple icon classes can be applied', function () {
       const jsonDef = {
@@ -176,7 +176,7 @@ suite('Toolbox', function () {
       assert.doesNotThrow(() => {
         this.toolbox.render(jsonDef);
       });
-      assert.lengthOf(this.toolbox.contents_, 1);
+      assert.equal(this.toolbox.contents.size, 1);
     });
   });
 
@@ -204,7 +204,7 @@ suite('Toolbox', function () {
       const evt = {
         'target': categoryXml,
       };
-      const item = this.toolbox.contentMap_[categoryXml.getAttribute('id')];
+      const item = this.toolbox.contents.get(categoryXml.getAttribute('id'));
       const setSelectedSpy = sinon.spy(this.toolbox, 'setSelectedItem');
       const onClickSpy = sinon.spy(item, 'onClick');
       this.toolbox.onClick_(evt);
@@ -356,14 +356,16 @@ suite('Toolbox', function () {
         assert.isFalse(handled);
       });
       test('Next item is selectable -> Should select next item', function () {
-        const item = this.toolbox.contents_[0];
+        const items = [...this.toolbox.contents.values()];
+        const item = items[0];
         this.toolbox.selectedItem_ = item;
         const handled = this.toolbox.selectNext_();
         assert.isTrue(handled);
-        assert.equal(this.toolbox.selectedItem_, this.toolbox.contents_[1]);
+        assert.equal(this.toolbox.selectedItem_, items[1]);
       });
       test('Selected item is last item -> Should not handle event', function () {
-        const item = this.toolbox.contents_[this.toolbox.contents_.length - 1];
+        const items = [...this.toolbox.contents.values()];
+        const item = items.at(-1);
         this.toolbox.selectedItem_ = item;
         const handled = this.toolbox.selectNext_();
         assert.isFalse(handled);
@@ -387,15 +389,16 @@ suite('Toolbox', function () {
         assert.isFalse(handled);
       });
       test('Selected item is first item -> Should not handle event', function () {
-        const item = this.toolbox.contents_[0];
+        const item = [...this.toolbox.contents.values()][0];
         this.toolbox.selectedItem_ = item;
         const handled = this.toolbox.selectPrevious_();
         assert.isFalse(handled);
         assert.equal(this.toolbox.selectedItem_, item);
       });
       test('Previous item is selectable -> Should select previous item', function () {
-        const item = this.toolbox.contents_[1];
-        const prevItem = this.toolbox.contents_[0];
+        const items = [...this.toolbox.contents.values()];
+        const item = items[1];
+        const prevItem = items[0];
         this.toolbox.selectedItem_ = item;
         const handled = this.toolbox.selectPrevious_();
         assert.isTrue(handled);
@@ -404,9 +407,10 @@ suite('Toolbox', function () {
       test('Previous item is collapsed -> Should skip over children of the previous item', function () {
         const childItem = getChildItem(this.toolbox);
         const parentItem = childItem.getParent();
-        const parentIdx = this.toolbox.contents_.indexOf(parentItem);
+        const items = [...this.toolbox.contents.values()];
+        const parentIdx = items.indexOf(parentItem);
         // Gets the item after the parent.
-        const item = this.toolbox.contents_[parentIdx + 1];
+        const item = items[parentIdx + 1];
         this.toolbox.selectedItem_ = item;
         const handled = this.toolbox.selectPrevious_();
         assert.isTrue(handled);
@@ -728,9 +732,10 @@ suite('Toolbox', function () {
     });
     test('Child categories visible if all ancestors expanded', function () {
       this.toolbox.render(getDeeplyNestedJSON());
-      const outerCategory = this.toolbox.contents_[0];
-      const middleCategory = this.toolbox.contents_[1];
-      const innerCategory = this.toolbox.contents_[2];
+      const items = [...this.toolbox.contents.values()];
+      const outerCategory = items[0];
+      const middleCategory = items[1];
+      const innerCategory = items[2];
 
       outerCategory.toggleExpanded();
       middleCategory.toggleExpanded();
@@ -743,8 +748,9 @@ suite('Toolbox', function () {
     });
     test('Child categories not visible if any ancestor not expanded', function () {
       this.toolbox.render(getDeeplyNestedJSON());
-      const middleCategory = this.toolbox.contents_[1];
-      const innerCategory = this.toolbox.contents_[2];
+      const items = [...this.toolbox.contents.values()];
+      const middleCategory = items[1];
+      const innerCategory = items[2];
 
       // Don't expand the outermost category
       // Even though the direct parent of inner is expanded, it shouldn't be visible