diff --git a/tools/test-js-with-node b/tools/test-js-with-node
index b967924cb2..e0b34191cc 100755
--- a/tools/test-js-with-node
+++ b/tools/test-js-with-node
@@ -44,7 +44,7 @@ def make_set(files: List[str]) -> Set[str]:
 # We do not yet require 100% line coverage for these files:
 EXEMPT_FILES = make_set(
     [
-        "web/shared/src/poll_data.js",
+        "web/shared/src/poll_data.ts",
         "web/src/about_zulip.js",
         "web/src/add_subscribers_pill.js",
         "web/src/admin.js",
diff --git a/web/shared/src/poll_data.js b/web/shared/src/poll_data.ts
similarity index 61%
rename from web/shared/src/poll_data.js
rename to web/shared/src/poll_data.ts
index 25df9d2578..a788b14d7a 100644
--- a/web/shared/src/poll_data.js
+++ b/web/shared/src/poll_data.ts
@@ -1,3 +1,74 @@
+import assert from "minimalistic-assert";
+import z from "zod";
+
+export type PollDataConfig = {
+    message_sender_id: number;
+    current_user_id: number;
+    is_my_poll: boolean;
+    question: string;
+    options: string[];
+    comma_separated_names: (user_ids: number[]) => string;
+    report_error_function: (msg: string, more_info?: unknown) => void;
+};
+
+export type PollOptionData = {
+    option: string;
+    names: string;
+    count: number;
+    key: string;
+    current_user_vote: boolean;
+};
+
+type PollOption = {
+    option: string;
+    user_id: number;
+    votes: Map<number, number>;
+};
+
+export type WidgetData = {
+    options: PollOptionData[];
+    question: string;
+};
+
+export type InboundData = Record<string, unknown> & {type: string};
+
+export type PollHandle = {
+    // Add generic key property to allow string indexing on PollHandle type in `handle_event` method.
+    [key: string]: {
+        outbound: (arg: string) => InboundData | undefined;
+        inbound: (sender_id: number, data: InboundData) => void;
+    };
+    new_option: {
+        outbound: (option: string) => {type: string; idx: number; option: string};
+        inbound: (sender_id: number, data: InboundData) => void;
+    };
+    question: {
+        outbound: (question: string) => {type: string; question: string} | undefined;
+        inbound: (sender_id: number, data: InboundData) => void;
+    };
+    vote: {
+        outbound: (key: string) => {type: string; key: string; vote: number};
+        inbound: (sender_id: number, data: InboundData) => void;
+    };
+};
+
+const inbound_option_schema = z.object({
+    idx: z.number(),
+    option: z.string(),
+    type: z.literal("new_option"),
+});
+
+const inbound_question_schema = z.object({
+    question: z.string(),
+    type: z.literal("question"),
+});
+
+const inbound_vote_schema = z.object({
+    key: z.string(),
+    type: z.literal("vote"),
+    vote: z.number(),
+});
+
 // Any single user should send add a finite number of options
 // to a poll. We arbitrarily pick this value.
 const MAX_IDX = 1000;
@@ -8,8 +79,16 @@ export class PollData {
     // should be represented for rendering, plus how the
     // server sends us data.
 
-    key_to_option = new Map();
+    key_to_option = new Map<string, PollOption>();
     my_idx = 1;
+    message_sender_id: number;
+    me: number;
+    is_my_poll: boolean;
+    poll_question: string;
+    input_mode: boolean;
+    comma_separated_names: (user_ids: number[]) => string;
+    report_error_function: (error_message: string) => void;
+    handle: PollHandle;
 
     constructor({
         message_sender_id,
@@ -19,7 +98,7 @@ export class PollData {
         options,
         comma_separated_names,
         report_error_function,
-    }) {
+    }: PollDataConfig) {
         this.message_sender_id = message_sender_id;
         this.me = current_user_id;
         this.is_my_poll = is_my_poll;
@@ -28,6 +107,10 @@ export class PollData {
         this.comma_separated_names = comma_separated_names;
         this.report_error_function = report_error_function;
 
+        if (question) {
+            this.set_question(question);
+        }
+
         this.handle = {
             new_option: {
                 outbound: (option) => {
@@ -43,9 +126,11 @@ export class PollData {
                 },
 
                 inbound: (sender_id, data) => {
+                    const safe_data = inbound_option_schema.parse(data);
+
                     // All message readers may add a new option to the poll.
-                    const idx = data.idx;
-                    const option = data.option;
+                    const idx = safe_data.idx;
+                    const option = safe_data.option;
                     const options = this.get_widget_data().options;
 
                     // While the UI doesn't allow adding duplicate options
@@ -55,17 +140,12 @@ export class PollData {
                         return;
                     }
 
-                    if (!Number.isInteger(idx) || idx < 0 || idx > MAX_IDX) {
-                        this.report_error_function("poll widget: bad type for inbound option idx");
+                    if (idx < 0 || idx > MAX_IDX) {
+                        this.report_error_function("poll widget: idx out of bound");
                         return;
                     }
 
-                    if (typeof option !== "string") {
-                        this.report_error_function("poll widget: bad type for inbound option");
-                        return;
-                    }
-
-                    const key = sender_id + "," + idx;
+                    const key = `${sender_id},${idx}`;
                     const votes = new Map();
 
                     this.key_to_option.set(key, {
@@ -94,6 +174,8 @@ export class PollData {
                 },
 
                 inbound: (sender_id, data) => {
+                    const safe_data = inbound_question_schema.parse(data);
+
                     // Only the message author can edit questions.
                     if (sender_id !== this.message_sender_id) {
                         this.report_error_function(
@@ -102,12 +184,7 @@ export class PollData {
                         return;
                     }
 
-                    if (typeof data.question !== "string") {
-                        this.report_error_function("poll widget: bad type for inbound question");
-                        return;
-                    }
-
-                    this.set_question(data.question);
+                    this.set_question(safe_data.question);
                 },
             },
 
@@ -116,7 +193,8 @@ export class PollData {
                     let vote = 1;
 
                     // toggle
-                    if (this.key_to_option.get(key).votes.get(this.me)) {
+                    assert(this.key_to_option.has(key), `option key not found: ${key}`);
+                    if (this.key_to_option.get(key)!.votes.get(this.me)) {
                         vote = -1;
                     }
 
@@ -130,16 +208,13 @@ export class PollData {
                 },
 
                 inbound: (sender_id, data) => {
+                    const safe_data = inbound_vote_schema.parse(data);
+
                     // All message readers may vote on poll options.
-                    const key = data.key;
-                    const vote = data.vote;
+                    const key = safe_data.key;
+                    const vote = safe_data.vote;
 
-                    if (typeof key !== "string") {
-                        this.report_error_function("poll widget: bad type for inbound vote key");
-                        return;
-                    }
-
-                    if (!Number.isInteger(vote) || !(vote === 1 || vote === -1)) {
+                    if (!(vote === 1 || vote === -1)) {
                         this.report_error_function("poll widget: bad value for inbound vote count");
                         return;
                     }
@@ -162,41 +237,38 @@ export class PollData {
             },
         };
 
-        if (question) {
-            this.set_question(question);
-        }
-
         for (const [i, option] of options.entries()) {
-            this.handle.new_option.inbound("canned", {
+            this.handle.new_option.inbound(-1, {
                 idx: i,
                 option,
+                type: "new_option",
             });
         }
     }
 
-    set_question(new_question) {
+    set_question(new_question: string): void {
         this.input_mode = false;
         this.poll_question = new_question;
     }
 
-    get_question() {
+    get_question(): string {
         return this.poll_question;
     }
 
-    set_input_mode() {
+    set_input_mode(): void {
         this.input_mode = true;
     }
 
-    clear_input_mode() {
+    clear_input_mode(): void {
         this.input_mode = false;
     }
 
-    get_input_mode() {
+    get_input_mode(): boolean {
         return this.input_mode;
     }
 
-    get_widget_data() {
-        const options = [];
+    get_widget_data(): WidgetData {
+        const options: PollOptionData[] = [];
 
         for (const [key, obj] of this.key_to_option) {
             const voters = [...obj.votes.keys()];
@@ -219,9 +291,9 @@ export class PollData {
         return widget_data;
     }
 
-    handle_event(sender_id, data) {
+    handle_event(sender_id: number, data: InboundData): void {
         const type = data.type;
-        if (this.handle[type] && this.handle[type].inbound) {
+        if (this.handle[type]) {
             this.handle[type].inbound(sender_id, data);
         } else {
             this.report_error_function(`poll widget: unknown inbound type: ${type}`);
@@ -229,7 +301,7 @@ export class PollData {
     }
 
     // function to check whether option already exists
-    is_option_present(data, latest_option) {
+    is_option_present(data: PollOptionData[], latest_option: string): boolean {
         return data.some((el) => el.option === latest_option);
     }
 }