diff --git a/web/src/input_pill.ts b/web/src/input_pill.ts
index 5184fc2821..754d763ef9 100644
--- a/web/src/input_pill.ts
+++ b/web/src/input_pill.ts
@@ -1,6 +1,7 @@
 // todo: Refactor pills subsystem to use modern javascript classes?
 
 import $ from "jquery";
+import assert from "minimalistic-assert";
 
 import render_input_pill from "../templates/input_pill.hbs";
 
@@ -408,9 +409,8 @@ export function create<T>(opts: InputPillCreateOptions<T>): InputPillContainer<T
             e.preventDefault();
 
             // get text representation of clipboard
-            const text = ((e.originalEvent ?? e) as ClipboardEvent).clipboardData?.getData(
-                "text/plain",
-            );
+            assert(e.originalEvent instanceof ClipboardEvent);
+            const text = e.originalEvent.clipboardData?.getData("text/plain");
 
             // insert text manually
             document.execCommand("insertText", false, text);
@@ -440,10 +440,8 @@ export function create<T>(opts: InputPillCreateOptions<T>): InputPillContainer<T
         store.$parent.on("copy", ".pill", (e) => {
             const element: HTMLElement = e.currentTarget;
             const {item} = funcs.getByElement(element)!;
-            (e.originalEvent as ClipboardEvent).clipboardData?.setData(
-                "text/plain",
-                store.get_text_from_item(item),
-            );
+            assert(e.originalEvent instanceof ClipboardEvent);
+            e.originalEvent.clipboardData?.setData("text/plain", store.get_text_from_item(item));
             e.preventDefault();
         });
     }
diff --git a/web/src/modals.ts b/web/src/modals.ts
index fb0033b75f..ac05e3ada9 100644
--- a/web/src/modals.ts
+++ b/web/src/modals.ts
@@ -1,5 +1,6 @@
 import $ from "jquery";
 import Micromodal from "micromodal";
+import assert from "minimalistic-assert";
 
 import * as blueslip from "./blueslip";
 import * as overlay_util from "./overlay_util";
@@ -105,7 +106,8 @@ export function open(
     const $micromodal = $(id_selector);
 
     $micromodal.find(".modal__container").on("animationend", (event) => {
-        const animation_name = (event.originalEvent as AnimationEvent).animationName;
+        assert(event.originalEvent instanceof AnimationEvent);
+        const animation_name = event.originalEvent.animationName;
         if (animation_name === "mmfadeIn") {
             // Micromodal adds the is-open class before the modal animation
             // is complete, which isn't really helpful since a modal is open after the
@@ -198,7 +200,8 @@ export function close(modal_id: string, conf: Pick<ModalConfig, "on_hidden"> = {
     // mechanism as a convenience for hooks only known when
     // closing the modal.
     $micromodal.find(".modal__container").on("animationend", (event) => {
-        const animation_name = (event.originalEvent as AnimationEvent).animationName;
+        assert(event.originalEvent instanceof AnimationEvent);
+        const animation_name = event.originalEvent.animationName;
         if (animation_name === "mmfadeOut" && conf.on_hidden) {
             conf.on_hidden();
         }
diff --git a/web/src/portico/portico_modals.ts b/web/src/portico/portico_modals.ts
index 46dec13d85..243247318a 100644
--- a/web/src/portico/portico_modals.ts
+++ b/web/src/portico/portico_modals.ts
@@ -1,5 +1,6 @@
 import $ from "jquery";
 import Micromodal from "micromodal";
+import assert from "minimalistic-assert";
 
 import * as blueslip from "../blueslip";
 
@@ -77,7 +78,8 @@ export function open(modal_id: string, recursive_call_count: number = 0): void {
     const $micromodal = $(id_selector);
 
     $micromodal.find(".modal__container").on("animationend", (event) => {
-        const animation_name = (event.originalEvent as AnimationEvent).animationName;
+        assert(event.originalEvent instanceof AnimationEvent);
+        const animation_name = event.originalEvent.animationName;
         if (animation_name === "mmfadeIn") {
             // Micromodal adds the is-open class before the modal animation
             // is complete, which isn't really helpful since a modal is open after the
diff --git a/web/tests/input_pill.test.js b/web/tests/input_pill.test.js
index c34e96f794..a6bfd206b4 100644
--- a/web/tests/input_pill.test.js
+++ b/web/tests/input_pill.test.js
@@ -8,6 +8,8 @@ const blueslip = require("./lib/zblueslip");
 const $ = require("./lib/zjquery");
 
 set_global("document", {});
+class ClipboardEvent {}
+set_global("ClipboardEvent", ClipboardEvent);
 
 const noop = () => {};
 const example_img_link = "http://example.com/example.png";
@@ -163,16 +165,16 @@ run_test("copy from pill", ({mock_template}) => {
 
     const $pill_stub = "<pill-stub RED>";
 
+    const originalEvent = new ClipboardEvent();
+    originalEvent.clipboardData = {
+        setData(format, text) {
+            assert.equal(format, "text/plain");
+            copied_text = text;
+        },
+    };
     const e = {
         currentTarget: $pill_stub,
-        originalEvent: {
-            clipboardData: {
-                setData(format, text) {
-                    assert.equal(format, "text/plain");
-                    copied_text = text;
-                },
-            },
-        },
+        originalEvent,
         preventDefault: noop,
     };
 
@@ -198,15 +200,15 @@ run_test("paste to input", ({mock_template}) => {
 
     const paste_text = "blue,yellow";
 
-    const e = {
-        originalEvent: {
-            clipboardData: {
-                getData(format) {
-                    assert.equal(format, "text/plain");
-                    return paste_text;
-                },
-            },
+    const originalEvent = new ClipboardEvent();
+    originalEvent.clipboardData = {
+        getData(format) {
+            assert.equal(format, "text/plain");
+            return paste_text;
         },
+    };
+    const e = {
+        originalEvent,
         preventDefault: noop,
     };