diff --git a/core/interfaces/i_autohideable.ts b/core/interfaces/i_autohideable.ts
index 41e761f57..1193023d2 100644
--- a/core/interfaces/i_autohideable.ts
+++ b/core/interfaces/i_autohideable.ts
@@ -23,5 +23,5 @@ export interface IAutoHideable extends IComponent {
 
 /** Returns true if the given object is autohideable. */
 export function isAutoHideable(obj: any): obj is IAutoHideable {
-  return obj.autoHide !== undefined;
+  return obj && typeof obj.autoHide === 'function';
 }
diff --git a/core/interfaces/i_comment_icon.ts b/core/interfaces/i_comment_icon.ts
index 05f86f40f..1ab5bead4 100644
--- a/core/interfaces/i_comment_icon.ts
+++ b/core/interfaces/i_comment_icon.ts
@@ -31,17 +31,17 @@ export interface ICommentIcon extends IIcon, IHasBubble, ISerializable {
 }
 
 /** Checks whether the given object is an ICommentIcon. */
-export function isCommentIcon(obj: object): obj is ICommentIcon {
+export function isCommentIcon(obj: any): obj is ICommentIcon {
   return (
     isIcon(obj) &&
     hasBubble(obj) &&
     isSerializable(obj) &&
-    (obj as any)['setText'] !== undefined &&
-    (obj as any)['getText'] !== undefined &&
-    (obj as any)['setBubbleSize'] !== undefined &&
-    (obj as any)['getBubbleSize'] !== undefined &&
-    (obj as any)['setBubbleLocation'] !== undefined &&
-    (obj as any)['getBubbleLocation'] !== undefined &&
+    typeof (obj as any).setText === 'function' &&
+    typeof (obj as any).getText === 'function' &&
+    typeof (obj as any).setBubbleSize === 'function' &&
+    typeof (obj as any).getBubbleSize === 'function' &&
+    typeof (obj as any).setBubbleLocation === 'function' &&
+    typeof (obj as any).getBubbleLocation === 'function' &&
     obj.getType() === IconType.COMMENT
   );
 }
diff --git a/core/interfaces/i_copyable.ts b/core/interfaces/i_copyable.ts
index 6c354926a..8d1853967 100644
--- a/core/interfaces/i_copyable.ts
+++ b/core/interfaces/i_copyable.ts
@@ -35,5 +35,5 @@ export type ICopyData = ICopyable.ICopyData;
 
 /** @returns true if the given object is an ICopyable. */
 export function isCopyable(obj: any): obj is ICopyable<ICopyData> {
-  return obj.toCopyData !== undefined;
+  return obj && typeof obj.toCopyData === 'function';
 }
diff --git a/core/interfaces/i_deletable.ts b/core/interfaces/i_deletable.ts
index 046770940..156e43ddc 100644
--- a/core/interfaces/i_deletable.ts
+++ b/core/interfaces/i_deletable.ts
@@ -27,8 +27,9 @@ export interface IDeletable {
 /** Returns whether the given object is an IDeletable. */
 export function isDeletable(obj: any): obj is IDeletable {
   return (
-    obj['isDeletable'] !== undefined &&
-    obj['dispose'] !== undefined &&
-    obj['setDeleteStyle'] !== undefined
+    obj &&
+    typeof obj.isDeletable === 'function' &&
+    typeof obj.dispose === 'function' &&
+    typeof obj.setDeleteStyle === 'function'
   );
 }
diff --git a/core/interfaces/i_draggable.ts b/core/interfaces/i_draggable.ts
index cb723e7b8..913038116 100644
--- a/core/interfaces/i_draggable.ts
+++ b/core/interfaces/i_draggable.ts
@@ -62,11 +62,12 @@ export interface IDragStrategy {
 /** Returns whether the given object is an IDraggable or not. */
 export function isDraggable(obj: any): obj is IDraggable {
   return (
-    obj.getRelativeToSurfaceXY !== undefined &&
-    obj.isMovable !== undefined &&
-    obj.startDrag !== undefined &&
-    obj.drag !== undefined &&
-    obj.endDrag !== undefined &&
-    obj.revertDrag !== undefined
+    obj &&
+    typeof obj.getRelativeToSurfaceXY === 'function' &&
+    typeof obj.isMovable === 'function' &&
+    typeof obj.startDrag === 'function' &&
+    typeof obj.drag === 'function' &&
+    typeof obj.endDrag === 'function' &&
+    typeof obj.revertDrag === 'function'
   );
 }
diff --git a/core/interfaces/i_focusable_node.ts b/core/interfaces/i_focusable_node.ts
index 00557168a..24833328d 100644
--- a/core/interfaces/i_focusable_node.ts
+++ b/core/interfaces/i_focusable_node.ts
@@ -102,16 +102,16 @@ export interface IFocusableNode {
  * Determines whether the provided object fulfills the contract of
  * IFocusableNode.
  *
- * @param object The object to test.
+ * @param obj The object to test.
  * @returns Whether the provided object can be used as an IFocusableNode.
  */
-export function isFocusableNode(object: any | null): object is IFocusableNode {
+export function isFocusableNode(obj: any): obj is IFocusableNode {
   return (
-    object &&
-    'getFocusableElement' in object &&
-    'getFocusableTree' in object &&
-    'onNodeFocus' in object &&
-    'onNodeBlur' in object &&
-    'canBeFocused' in object
+    obj &&
+    typeof obj.getFocusableElement === 'function' &&
+    typeof obj.getFocusableTree === 'function' &&
+    typeof obj.onNodeFocus === 'function' &&
+    typeof obj.onNodeBlur === 'function' &&
+    typeof obj.canBeFocused === 'function'
   );
 }
diff --git a/core/interfaces/i_focusable_tree.ts b/core/interfaces/i_focusable_tree.ts
index f4f25f7f5..c33189fcd 100644
--- a/core/interfaces/i_focusable_tree.ts
+++ b/core/interfaces/i_focusable_tree.ts
@@ -128,17 +128,17 @@ export interface IFocusableTree {
  * Determines whether the provided object fulfills the contract of
  * IFocusableTree.
  *
- * @param object The object to test.
+ * @param obj The object to test.
  * @returns Whether the provided object can be used as an IFocusableTree.
  */
-export function isFocusableTree(object: any | null): object is IFocusableTree {
+export function isFocusableTree(obj: any): obj is IFocusableTree {
   return (
-    object &&
-    'getRootFocusableNode' in object &&
-    'getRestoredFocusableNode' in object &&
-    'getNestedTrees' in object &&
-    'lookUpFocusableNode' in object &&
-    'onTreeFocus' in object &&
-    'onTreeBlur' in object
+    obj &&
+    typeof obj.getRootFocusableNode === 'function' &&
+    typeof obj.getRestoredFocusableNode === 'function' &&
+    typeof obj.getNestedTrees === 'function' &&
+    typeof obj.lookUpFocusableNode === 'function' &&
+    typeof obj.onTreeFocus === 'function' &&
+    typeof obj.onTreeBlur === 'function'
   );
 }
diff --git a/core/interfaces/i_has_bubble.ts b/core/interfaces/i_has_bubble.ts
index 85c6f0990..0c2e257a4 100644
--- a/core/interfaces/i_has_bubble.ts
+++ b/core/interfaces/i_has_bubble.ts
@@ -30,6 +30,8 @@ export interface IHasBubble {
 /** Type guard that checks whether the given object is a IHasBubble. */
 export function hasBubble(obj: any): obj is IHasBubble {
   return (
-    obj.bubbleIsVisible !== undefined && obj.setBubbleVisible !== undefined
+    typeof obj.bubbleIsVisible === 'function' &&
+    typeof obj.setBubbleVisible === 'function' &&
+    typeof obj.getBubble === 'function'
   );
 }
diff --git a/core/interfaces/i_icon.ts b/core/interfaces/i_icon.ts
index 74489dc5e..06f416424 100644
--- a/core/interfaces/i_icon.ts
+++ b/core/interfaces/i_icon.ts
@@ -98,19 +98,19 @@ export interface IIcon extends IFocusableNode {
 /** Type guard that checks whether the given object is an IIcon. */
 export function isIcon(obj: any): obj is IIcon {
   return (
-    obj.getType !== undefined &&
-    obj.initView !== undefined &&
-    obj.dispose !== undefined &&
-    obj.getWeight !== undefined &&
-    obj.getSize !== undefined &&
-    obj.applyColour !== undefined &&
-    obj.hideForInsertionMarker !== undefined &&
-    obj.updateEditable !== undefined &&
-    obj.updateCollapsed !== undefined &&
-    obj.isShownWhenCollapsed !== undefined &&
-    obj.setOffsetInBlock !== undefined &&
-    obj.onLocationChange !== undefined &&
-    obj.onClick !== undefined &&
-    isFocusableNode(obj)
+    isFocusableNode(obj) &&
+    typeof (obj as IIcon).getType === 'function' &&
+    typeof (obj as IIcon).initView === 'function' &&
+    typeof (obj as IIcon).dispose === 'function' &&
+    typeof (obj as IIcon).getWeight === 'function' &&
+    typeof (obj as IIcon).getSize === 'function' &&
+    typeof (obj as IIcon).applyColour === 'function' &&
+    typeof (obj as IIcon).hideForInsertionMarker === 'function' &&
+    typeof (obj as IIcon).updateEditable === 'function' &&
+    typeof (obj as IIcon).updateCollapsed === 'function' &&
+    typeof (obj as IIcon).isShownWhenCollapsed === 'function' &&
+    typeof (obj as IIcon).setOffsetInBlock === 'function' &&
+    typeof (obj as IIcon).onLocationChange === 'function' &&
+    typeof (obj as IIcon).onClick === 'function'
   );
 }
diff --git a/core/interfaces/i_legacy_procedure_blocks.ts b/core/interfaces/i_legacy_procedure_blocks.ts
index d74eaec22..c723a5ed7 100644
--- a/core/interfaces/i_legacy_procedure_blocks.ts
+++ b/core/interfaces/i_legacy_procedure_blocks.ts
@@ -28,9 +28,9 @@ export interface LegacyProcedureDefBlock {
 
 /** @internal */
 export function isLegacyProcedureDefBlock(
-  block: object,
-): block is LegacyProcedureDefBlock {
-  return (block as any).getProcedureDef !== undefined;
+  obj: any,
+): obj is LegacyProcedureDefBlock {
+  return obj && typeof obj.getProcedureDef === 'function';
 }
 
 /** @internal */
@@ -41,10 +41,11 @@ export interface LegacyProcedureCallBlock {
 
 /** @internal */
 export function isLegacyProcedureCallBlock(
-  block: object,
-): block is LegacyProcedureCallBlock {
+  obj: any,
+): obj is LegacyProcedureCallBlock {
   return (
-    (block as any).getProcedureCall !== undefined &&
-    (block as any).renameProcedure !== undefined
+    obj &&
+    typeof obj.getProcedureCall === 'function' &&
+    typeof obj.renameProcedure === 'function'
   );
 }
diff --git a/core/interfaces/i_observable.ts b/core/interfaces/i_observable.ts
index 96a2a0bc4..8db0c2378 100644
--- a/core/interfaces/i_observable.ts
+++ b/core/interfaces/i_observable.ts
@@ -20,5 +20,9 @@ export interface IObservable {
  * @internal
  */
 export function isObservable(obj: any): obj is IObservable {
-  return obj.startPublishing !== undefined && obj.stopPublishing !== undefined;
+  return (
+    obj &&
+    typeof obj.startPublishing === 'function' &&
+    typeof obj.stopPublishing === 'function'
+  );
 }
diff --git a/core/interfaces/i_paster.ts b/core/interfaces/i_paster.ts
index 321ff118f..128913a26 100644
--- a/core/interfaces/i_paster.ts
+++ b/core/interfaces/i_paster.ts
@@ -21,5 +21,5 @@ export interface IPaster<U extends ICopyData, T extends ICopyable<U>> {
 export function isPaster(
   obj: any,
 ): obj is IPaster<ICopyData, ICopyable<ICopyData>> {
-  return obj.paste !== undefined;
+  return obj && typeof obj.paste === 'function';
 }
diff --git a/core/interfaces/i_procedure_block.ts b/core/interfaces/i_procedure_block.ts
index f85380527..3a6dc4847 100644
--- a/core/interfaces/i_procedure_block.ts
+++ b/core/interfaces/i_procedure_block.ts
@@ -20,9 +20,10 @@ export interface IProcedureBlock {
 export function isProcedureBlock(
   block: Block | IProcedureBlock,
 ): block is IProcedureBlock {
+  block = block as IProcedureBlock;
   return (
-    (block as IProcedureBlock).getProcedureModel !== undefined &&
-    (block as IProcedureBlock).doProcedureUpdate !== undefined &&
-    (block as IProcedureBlock).isProcedureDef !== undefined
+    typeof block.getProcedureModel === 'function' &&
+    typeof block.doProcedureUpdate === 'function' &&
+    typeof block.isProcedureDef === 'function'
   );
 }
diff --git a/core/interfaces/i_rendered_element.ts b/core/interfaces/i_rendered_element.ts
index fe9460c7f..2f82487e9 100644
--- a/core/interfaces/i_rendered_element.ts
+++ b/core/interfaces/i_rendered_element.ts
@@ -15,5 +15,5 @@ export interface IRenderedElement {
  * @returns True if the given object is an IRenderedElement.
  */
 export function isRenderedElement(obj: any): obj is IRenderedElement {
-  return obj['getSvgRoot'] !== undefined;
+  return obj && typeof obj.getSvgRoot === 'function';
 }
diff --git a/core/interfaces/i_selectable.ts b/core/interfaces/i_selectable.ts
index 639972e45..5374f50cd 100644
--- a/core/interfaces/i_selectable.ts
+++ b/core/interfaces/i_selectable.ts
@@ -30,12 +30,12 @@ export interface ISelectable extends IFocusableNode {
 }
 
 /** Checks whether the given object is an ISelectable. */
-export function isSelectable(obj: object): obj is ISelectable {
+export function isSelectable(obj: any): obj is ISelectable {
   return (
-    typeof (obj as any).id === 'string' &&
-    (obj as any).workspace !== undefined &&
-    (obj as any).select !== undefined &&
-    (obj as any).unselect !== undefined &&
-    isFocusableNode(obj)
+    isFocusableNode(obj) &&
+    typeof (obj as ISelectable).id === 'string' &&
+    typeof (obj as ISelectable).workspace === 'object' &&
+    typeof (obj as ISelectable).select === 'function' &&
+    typeof (obj as ISelectable).unselect === 'function'
   );
 }
diff --git a/core/interfaces/i_serializable.ts b/core/interfaces/i_serializable.ts
index 380a27709..99e597da3 100644
--- a/core/interfaces/i_serializable.ts
+++ b/core/interfaces/i_serializable.ts
@@ -24,5 +24,9 @@ export interface ISerializable {
 
 /** Type guard that checks whether the given object is a ISerializable. */
 export function isSerializable(obj: any): obj is ISerializable {
-  return obj.saveState !== undefined && obj.loadState !== undefined;
+  return (
+    obj &&
+    typeof obj.saveState === 'function' &&
+    typeof obj.loadState === 'function'
+  );
 }
diff --git a/tests/mocha/block_test.js b/tests/mocha/block_test.js
index eda2d82a5..62c61ce00 100644
--- a/tests/mocha/block_test.js
+++ b/tests/mocha/block_test.js
@@ -7,7 +7,10 @@
 import {ConnectionType} from '../../build/src/core/connection_type.js';
 import {EventType} from '../../build/src/core/events/type.js';
 import * as eventUtils from '../../build/src/core/events/utils.js';
+import {IconType} from '../../build/src/core/icons/icon_types.js';
 import {EndRowInput} from '../../build/src/core/inputs/end_row_input.js';
+import {isCommentIcon} from '../../build/src/core/interfaces/i_comment_icon.js';
+import {Size} from '../../build/src/core/utils/size.js';
 import {assert} from '../../node_modules/chai/chai.js';
 import {createRenderedBlock} from './test_helpers/block_definitions.js';
 import {
@@ -1426,9 +1429,9 @@ suite('Blocks', function () {
     });
 
     suite('Constructing registered comment classes', function () {
-      class MockComment extends MockIcon {
+      class MockComment extends MockBubbleIcon {
         getType() {
-          return Blockly.icons.IconType.COMMENT;
+          return IconType.COMMENT;
         }
 
         setText() {}
@@ -1440,19 +1443,13 @@ suite('Blocks', function () {
         setBubbleSize() {}
 
         getBubbleSize() {
-          return Blockly.utils.Size(0, 0);
+          return Size(0, 0);
         }
 
         setBubbleLocation() {}
 
         getBubbleLocation() {}
 
-        bubbleIsVisible() {
-          return true;
-        }
-
-        setBubbleVisible() {}
-
         saveState() {
           return {};
         }
@@ -1460,6 +1457,10 @@ suite('Blocks', function () {
         loadState() {}
       }
 
+      if (!isCommentIcon(new MockComment())) {
+        throw new TypeError('MockComment not an ICommentIcon');
+      }
+
       setup(function () {
         this.workspace = Blockly.inject('blocklyDiv', {});
 
diff --git a/tests/mocha/test_helpers/icon_mocks.js b/tests/mocha/test_helpers/icon_mocks.js
index 5d117c712..0e549b976 100644
--- a/tests/mocha/test_helpers/icon_mocks.js
+++ b/tests/mocha/test_helpers/icon_mocks.js
@@ -4,7 +4,24 @@
  * SPDX-License-Identifier: Apache-2.0
  */
 
-export class MockIcon {
+import {isFocusableNode} from '../../../build/src/core/interfaces/i_focusable_node.js';
+import {hasBubble} from '../../../build/src/core/interfaces/i_has_bubble.js';
+import {isIcon} from '../../../build/src/core/interfaces/i_icon.js';
+import {isSerializable} from '../../../build/src/core/interfaces/i_serializable.js';
+
+export class MockFocusable {
+  getFocusableElement() {}
+  getFocusableTree() {}
+  onNodeFocus() {}
+  onNodeBlur() {}
+  canBeFocused() {}
+}
+
+if (!isFocusableNode(new MockFocusable())) {
+  throw new TypeError('MockFocusable not an IFocuableNode');
+}
+
+export class MockIcon extends MockFocusable {
   getType() {
     return new Blockly.icons.IconType('mock icon');
   }
@@ -52,6 +69,10 @@ export class MockIcon {
   }
 }
 
+if (!isIcon(new MockIcon())) {
+  throw new TypeError('MockIcon not an IIcon');
+}
+
 export class MockSerializableIcon extends MockIcon {
   constructor() {
     super();
@@ -75,6 +96,10 @@ export class MockSerializableIcon extends MockIcon {
   }
 }
 
+if (!isSerializable(new MockSerializableIcon())) {
+  throw new TypeError('MockSerializableIcon not an ISerializable');
+}
+
 export class MockBubbleIcon extends MockIcon {
   constructor() {
     super();
@@ -94,4 +119,12 @@ export class MockBubbleIcon extends MockIcon {
   setBubbleVisible(visible) {
     this.visible = visible;
   }
+
+  getBubble() {
+    return null;
+  }
+}
+
+if (!hasBubble(new MockBubbleIcon())) {
+  throw new TypeError('MockBubbleIcon not an IHasBubble');
 }