diff --git a/lib/Webhooks.js b/lib/Webhooks.js
index cfd9232527..f7570b3946 100644
--- a/lib/Webhooks.js
+++ b/lib/Webhooks.js
@@ -1,8 +1,10 @@
 'use strict';
 const utils = require('./utils');
-const {StripeError, StripeSignatureVerificationError} = require('./Error');
+const _Error = require('./Error');
+const {StripeError, StripeSignatureVerificationError} = _Error;
 const Webhook = {
   DEFAULT_TOLERANCE: 300,
+  signature: null,
   constructEvent(payload, header, secret, tolerance, cryptoProvider) {
     this.signature.verifyHeader(
       payload,
@@ -142,6 +144,7 @@ function parseEventDetails(encodedPayload, encodedHeader, expectedScheme) {
   if (!details || details.timestamp === -1) {
     throw new StripeSignatureVerificationError({
       message: 'Unable to extract timestamp and signatures from header',
+      // @ts-expect-error Type '{ decodedHeader: any; decodedPayload: any; }' is not assignable to type 'string'.
       detail: {
         decodedHeader,
         decodedPayload,
@@ -151,6 +154,7 @@ function parseEventDetails(encodedPayload, encodedHeader, expectedScheme) {
   if (!details.signatures.length) {
     throw new StripeSignatureVerificationError({
       message: 'No signatures found with expected scheme',
+      // @ts-expect-error Type '{ decodedHeader: any; decodedPayload: any; }' is not assignable to type 'string'.
       detail: {
         decodedHeader,
         decodedPayload,
@@ -179,6 +183,7 @@ function validateComputedSignature(
         'No signatures found matching the expected signature for payload.' +
         ' Are you passing the raw request body you received from Stripe?' +
         ' https://github.com/stripe/stripe-node#webhook-signing',
+      // @ts-expect-error Type '{ header: any; payload: any; }' is not assignable to type 'string'.
       detail: {
         header,
         payload,
@@ -189,6 +194,7 @@ function validateComputedSignature(
   if (tolerance > 0 && timestampAge > tolerance) {
     throw new StripeSignatureVerificationError({
       message: 'Timestamp outside the tolerance zone',
+      // @ts-expect-error Type '{ header: any; payload: any; }' is not assignable to type 'string'.
       detail: {
         header,
         payload,
@@ -205,7 +211,7 @@ function parseHeader(header, scheme) {
     (accum, item) => {
       const kv = item.split('=');
       if (kv[0] === 't') {
-        accum.timestamp = kv[1];
+        accum.timestamp = parseInt(kv[1], 10);
       }
       if (kv[0] === scheme) {
         accum.signatures.push(kv[1]);
diff --git a/lib/apiVersion.js b/lib/apiVersion.js
index 9c1e0e38d1..518f4476ab 100644
--- a/lib/apiVersion.js
+++ b/lib/apiVersion.js
@@ -1,2 +1,3 @@
+'use strict';
 // File generated from our OpenAPI spec
 module.exports = {ApiVersion: '2022-08-01'};
diff --git a/lib/crypto/SubtleCryptoProvider.js b/lib/crypto/SubtleCryptoProvider.js
index 6ff0e2b3c2..cbf5d01373 100644
--- a/lib/crypto/SubtleCryptoProvider.js
+++ b/lib/crypto/SubtleCryptoProvider.js
@@ -21,7 +21,7 @@ class SubtleCryptoProvider extends CryptoProvider {
   }
   /** @override */
   async computeHMACSignatureAsync(payload, secret) {
-    const encoder = new TextEncoder('utf-8');
+    const encoder = new TextEncoder();
     const key = await this.subtleCrypto.importKey(
       'raw',
       encoder.encode(secret),
diff --git a/lib/multipart.js b/lib/multipart.js
index bb186e00db..d88d468b1f 100644
--- a/lib/multipart.js
+++ b/lib/multipart.js
@@ -1,6 +1,7 @@
 'use strict';
 const utils = require('./utils');
-const {StripeError} = require('./Error');
+const _Error = require('./Error');
+const {StripeError} = _Error;
 class StreamProcessingError extends StripeError {}
 // Method for formatting HTTP body for the multipart/form-data specification
 // Mostly taken from Fermata.js
@@ -26,7 +27,7 @@ const multipartDataGenerator = (method, data, headers) => {
   for (const k in flattenedData) {
     const v = flattenedData[k];
     push(`--${segno}`);
-    if (v.hasOwnProperty('data')) {
+    if (Object.prototype.hasOwnProperty.call(v, 'data')) {
       push(
         `Content-Disposition: form-data; name=${q(k)}; filename=${q(
           v.name || 'blob'
@@ -79,4 +80,6 @@ const multipartRequestDataProcessor = (method, data, headers, callback) => {
   const buffer = multipartDataGenerator(method, data, headers);
   return callback(null, buffer);
 };
-module.exports.multipartRequestDataProcessor = multipartRequestDataProcessor;
+module.exports = {
+  multipartRequestDataProcessor: multipartRequestDataProcessor,
+};
diff --git a/lib/net/FetchHttpClient.js b/lib/net/FetchHttpClient.js
index e57891b8cd..195b1bb0c6 100644
--- a/lib/net/FetchHttpClient.js
+++ b/lib/net/FetchHttpClient.js
@@ -1,5 +1,6 @@
 'use strict';
-const {HttpClient, HttpClientResponse} = require('./HttpClient');
+const _HttpClient = require('./HttpClient');
+const {HttpClient, HttpClientResponse} = _HttpClient;
 /**
  * HTTP client which uses a `fetch` function to issue requests.
  *
diff --git a/lib/net/NodeHttpClient.js b/lib/net/NodeHttpClient.js
index 1f9d12c4d8..c5712dfe2f 100644
--- a/lib/net/NodeHttpClient.js
+++ b/lib/net/NodeHttpClient.js
@@ -1,7 +1,8 @@
 'use strict';
 const http = require('http');
 const https = require('https');
-const {HttpClient, HttpClientResponse} = require('./HttpClient');
+const _HttpClient = require('./HttpClient');
+const {HttpClient, HttpClientResponse} = _HttpClient;
 const defaultHttpAgent = new http.Agent({keepAlive: true});
 const defaultHttpsAgent = new https.Agent({keepAlive: true});
 /**
diff --git a/lib/utils.js b/lib/utils.js
index 9efd90fd63..a17afff790 100644
--- a/lib/utils.js
+++ b/lib/utils.js
@@ -2,7 +2,6 @@
 const EventEmitter = require('events').EventEmitter;
 const qs = require('qs');
 const crypto = require('crypto');
-const hasOwn = (obj, prop) => Object.prototype.hasOwnProperty.call(obj, prop);
 // Certain sandboxed environments (our known example right now are CloudFlare
 // Workers) may make `child_process` unavailable. Because `exec` isn't critical
 // to the operation of stripe-node, we handle this unavailability gracefully.
@@ -31,13 +30,17 @@ const DEPRECATED_OPTIONS = {
   stripeVersion: 'apiVersion',
 };
 const DEPRECATED_OPTIONS_KEYS = Object.keys(DEPRECATED_OPTIONS);
-const utils = (module.exports = {
+const utils = {
   isOptionsHash(o) {
     return (
       o &&
       typeof o === 'object' &&
-      (OPTIONS_KEYS.some((prop) => hasOwn(o, prop)) ||
-        DEPRECATED_OPTIONS_KEYS.some((prop) => hasOwn(o, prop)))
+      (OPTIONS_KEYS.some((prop) =>
+        Object.prototype.hasOwnProperty.call(o, prop)
+      ) ||
+        DEPRECATED_OPTIONS_KEYS.some((prop) =>
+          Object.prototype.hasOwnProperty.call(o, prop)
+        ))
     );
   },
   /**
@@ -189,8 +192,9 @@ const utils = (module.exports = {
    * Provide simple "Class" extension mechanism
    */
   protoExtend(sub) {
+    // eslint-disable-next-line @typescript-eslint/no-this-alias
     const Super = this;
-    const Constructor = hasOwn(sub, 'constructor')
+    const Constructor = Object.prototype.hasOwnProperty.call(sub, 'constructor')
       ? sub.constructor
       : function(...args) {
           Super.apply(this, args);
@@ -342,7 +346,10 @@ const utils = (module.exports = {
         const value = obj[key];
         const newKey = prevKey ? `${prevKey}[${key}]` : key;
         if (utils.isObject(value)) {
-          if (!Buffer.isBuffer(value) && !value.hasOwnProperty('data')) {
+          if (
+            !Buffer.isBuffer(value) &&
+            !Object.prototype.hasOwnProperty.call(value, 'data')
+          ) {
             // Non-buffer non-file Objects are recursively flattened
             return step(value, newKey);
           } else {
@@ -355,7 +362,7 @@ const utils = (module.exports = {
         }
       });
     };
-    step(data);
+    step(data, null);
     return result;
   },
   /**
@@ -386,7 +393,7 @@ const utils = (module.exports = {
           platform: process.platform,
         };
   },
-});
+};
 function emitWarning(warning) {
   if (typeof process.emitWarning !== 'function') {
     return console.warn(
@@ -395,3 +402,4 @@ function emitWarning(warning) {
   }
   return process.emitWarning(warning, 'Stripe');
 }
+module.exports = utils;
diff --git a/package.json b/package.json
index ba4f71cf36..82e2f7dc33 100644
--- a/package.json
+++ b/package.json
@@ -42,7 +42,7 @@
     "node-fetch": "^2.6.2",
     "nyc": "^15.1.0",
     "prettier": "^1.16.4",
-    "typescript": "^3.7.2"
+    "typescript": "^4.8.3"
   },
   "resolutions": {
     "ansi-regex": "5.0.1",
diff --git a/src/ResourceNamespace.js b/src/ResourceNamespace.ts
similarity index 97%
rename from src/ResourceNamespace.js
rename to src/ResourceNamespace.ts
index 0d76787d81..c4b6c5566d 100644
--- a/src/ResourceNamespace.js
+++ b/src/ResourceNamespace.ts
@@ -1,5 +1,3 @@
-'use strict';
-
 // ResourceNamespace allows you to create nested resources, i.e. `stripe.issuing.cards`.
 // It also works recursively, so you could do i.e. `stripe.billing.invoicing.pay`.
 
diff --git a/src/Webhooks.js b/src/Webhooks.ts
similarity index 91%
rename from src/Webhooks.js
rename to src/Webhooks.ts
index ffbe2004a9..14d5a4ed8f 100644
--- a/src/Webhooks.js
+++ b/src/Webhooks.ts
@@ -1,10 +1,10 @@
-'use strict';
-
-const utils = require('./utils');
-const {StripeError, StripeSignatureVerificationError} = require('./Error');
+import utils = require('./utils');
+import _Error = require('./Error');
+const {StripeError, StripeSignatureVerificationError} = _Error;
 
 const Webhook = {
   DEFAULT_TOLERANCE: 300, // 5 minutes
+  signature: null,
 
   constructEvent(payload, header, secret, tolerance, cryptoProvider) {
     this.signature.verifyHeader(
@@ -169,6 +169,7 @@ function parseEventDetails(encodedPayload, encodedHeader, expectedScheme) {
   if (!details || details.timestamp === -1) {
     throw new StripeSignatureVerificationError({
       message: 'Unable to extract timestamp and signatures from header',
+      // @ts-expect-error Type '{ decodedHeader: any; decodedPayload: any; }' is not assignable to type 'string'.
       detail: {
         decodedHeader,
         decodedPayload,
@@ -179,6 +180,7 @@ function parseEventDetails(encodedPayload, encodedHeader, expectedScheme) {
   if (!details.signatures.length) {
     throw new StripeSignatureVerificationError({
       message: 'No signatures found with expected scheme',
+      // @ts-expect-error Type '{ decodedHeader: any; decodedPayload: any; }' is not assignable to type 'string'.
       detail: {
         decodedHeader,
         decodedPayload,
@@ -210,6 +212,7 @@ function validateComputedSignature(
         'No signatures found matching the expected signature for payload.' +
         ' Are you passing the raw request body you received from Stripe?' +
         ' https://github.com/stripe/stripe-node#webhook-signing',
+      // @ts-expect-error Type '{ header: any; payload: any; }' is not assignable to type 'string'.
       detail: {
         header,
         payload,
@@ -222,6 +225,7 @@ function validateComputedSignature(
   if (tolerance > 0 && timestampAge > tolerance) {
     throw new StripeSignatureVerificationError({
       message: 'Timestamp outside the tolerance zone',
+      // @ts-expect-error Type '{ header: any; payload: any; }' is not assignable to type 'string'.
       detail: {
         header,
         payload,
@@ -242,7 +246,7 @@ function parseHeader(header, scheme) {
       const kv = item.split('=');
 
       if (kv[0] === 't') {
-        accum.timestamp = kv[1];
+        accum.timestamp = parseInt(kv[1], 10);
       }
 
       if (kv[0] === scheme) {
@@ -274,4 +278,4 @@ function getNodeCryptoProvider() {
 
 Webhook.signature = signature;
 
-module.exports = Webhook;
+export = Webhook;
diff --git a/src/crypto/CryptoProvider.js b/src/crypto/CryptoProvider.ts
similarity index 88%
rename from src/crypto/CryptoProvider.js
rename to src/crypto/CryptoProvider.ts
index 8f038d06ca..bd7387c886 100644
--- a/src/crypto/CryptoProvider.js
+++ b/src/crypto/CryptoProvider.ts
@@ -1,5 +1,3 @@
-'use strict';
-
 /**
  * Interface encapsulating the various crypto computations used by the library,
  * allowing pluggable underlying crypto implementations.
@@ -13,7 +11,7 @@ class CryptoProvider {
    * - computeHMACSignature('', 'test_secret') => 'f7f9bd47fb987337b5796fdc1fdb9ba221d0d5396814bfcaf9521f43fd8927fd'
    * - computeHMACSignature('\ud83d\ude00', 'test_secret') => '837da296d05c4fe31f61d5d7ead035099d9585a5bcde87de952012a78f0b0c43
    */
-  computeHMACSignature(payload, secret) {
+  computeHMACSignature(payload: string, secret: string): string {
     throw new Error('computeHMACSignature not implemented.');
   }
 
@@ -28,9 +26,9 @@ class CryptoProvider {
    * - computeHMACSignature('', 'test_secret') => 'f7f9bd47fb987337b5796fdc1fdb9ba221d0d5396814bfcaf9521f43fd8927fd'
    * - computeHMACSignature('\ud83d\ude00', 'test_secret') => '837da296d05c4fe31f61d5d7ead035099d9585a5bcde87de952012a78f0b0c43
    */
-  computeHMACSignatureAsync(payload, secret) {
+  computeHMACSignatureAsync(payload: string, secret: string): Promise<string> {
     throw new Error('computeHMACSignatureAsync not implemented.');
   }
 }
 
-module.exports = CryptoProvider;
+export = CryptoProvider;
diff --git a/src/crypto/NodeCryptoProvider.js b/src/crypto/NodeCryptoProvider.ts
similarity index 58%
rename from src/crypto/NodeCryptoProvider.js
rename to src/crypto/NodeCryptoProvider.ts
index 83c2037ea2..4521bc9106 100644
--- a/src/crypto/NodeCryptoProvider.js
+++ b/src/crypto/NodeCryptoProvider.ts
@@ -1,15 +1,13 @@
-'use strict';
+import crypto = require('crypto');
 
-const crypto = require('crypto');
-
-const CryptoProvider = require('./CryptoProvider');
+import CryptoProvider = require('./CryptoProvider');
 
 /**
  * `CryptoProvider which uses the Node `crypto` package for its computations.
  */
 class NodeCryptoProvider extends CryptoProvider {
   /** @override */
-  computeHMACSignature(payload, secret) {
+  computeHMACSignature(payload: string, secret: string): string {
     return crypto
       .createHmac('sha256', secret)
       .update(payload, 'utf8')
@@ -17,10 +15,13 @@ class NodeCryptoProvider extends CryptoProvider {
   }
 
   /** @override */
-  async computeHMACSignatureAsync(payload, secret) {
+  async computeHMACSignatureAsync(
+    payload: string,
+    secret: string
+  ): Promise<string> {
     const signature = await this.computeHMACSignature(payload, secret);
     return signature;
   }
 }
 
-module.exports = NodeCryptoProvider;
+export = NodeCryptoProvider;
diff --git a/src/crypto/SubtleCryptoProvider.js b/src/crypto/SubtleCryptoProvider.ts
similarity index 82%
rename from src/crypto/SubtleCryptoProvider.js
rename to src/crypto/SubtleCryptoProvider.ts
index 45aa40dd46..999181e4cc 100644
--- a/src/crypto/SubtleCryptoProvider.js
+++ b/src/crypto/SubtleCryptoProvider.ts
@@ -1,6 +1,4 @@
-'use strict';
-
-const CryptoProvider = require('./CryptoProvider');
+import CryptoProvider = require('./CryptoProvider');
 
 /**
  * `CryptoProvider which uses the SubtleCrypto interface of the Web Crypto API.
@@ -8,7 +6,9 @@ const CryptoProvider = require('./CryptoProvider');
  * This only supports asynchronous operations.
  */
 class SubtleCryptoProvider extends CryptoProvider {
-  constructor(subtleCrypto) {
+  subtleCrypto: SubtleCrypto;
+
+  constructor(subtleCrypto: SubtleCrypto) {
     super();
 
     // If no subtle crypto is interface, default to the global namespace. This
@@ -18,15 +18,18 @@ class SubtleCryptoProvider extends CryptoProvider {
   }
 
   /** @override */
-  computeHMACSignature(payload, secret) {
+  computeHMACSignature(payload: string, secret: string): string {
     throw new Error(
       'SubtleCryptoProvider cannot be used in a synchronous context.'
     );
   }
 
   /** @override */
-  async computeHMACSignatureAsync(payload, secret) {
-    const encoder = new TextEncoder('utf-8');
+  async computeHMACSignatureAsync(
+    payload: string,
+    secret: string
+  ): Promise<string> {
+    const encoder = new TextEncoder();
 
     const key = await this.subtleCrypto.importKey(
       'raw',
@@ -66,4 +69,4 @@ for (let i = 0; i < byteHexMapping.length; i++) {
   byteHexMapping[i] = i.toString(16).padStart(2, '0');
 }
 
-module.exports = SubtleCryptoProvider;
+export = SubtleCryptoProvider;
diff --git a/src/multipart.js b/src/multipart.ts
similarity index 91%
rename from src/multipart.js
rename to src/multipart.ts
index 90682aedda..dbb59a8bcf 100644
--- a/src/multipart.js
+++ b/src/multipart.ts
@@ -1,7 +1,6 @@
-'use strict';
-
-const utils = require('./utils');
-const {StripeError} = require('./Error');
+import utils = require('./utils');
+import _Error = require('./Error');
+const {StripeError} = _Error;
 
 class StreamProcessingError extends StripeError {}
 
@@ -33,7 +32,7 @@ const multipartDataGenerator = (method, data, headers) => {
   for (const k in flattenedData) {
     const v = flattenedData[k];
     push(`--${segno}`);
-    if (v.hasOwnProperty('data')) {
+    if (Object.prototype.hasOwnProperty.call(v, 'data')) {
       push(
         `Content-Disposition: form-data; name=${q(k)}; filename=${q(
           v.name || 'blob'
@@ -93,4 +92,6 @@ const multipartRequestDataProcessor = (method, data, headers, callback) => {
   return callback(null, buffer);
 };
 
-module.exports.multipartRequestDataProcessor = multipartRequestDataProcessor;
+export = {
+  multipartRequestDataProcessor: multipartRequestDataProcessor,
+};
diff --git a/src/net/FetchHttpClient.js b/src/net/FetchHttpClient.ts
similarity index 93%
rename from src/net/FetchHttpClient.js
rename to src/net/FetchHttpClient.ts
index 431282cd19..630bddc42d 100644
--- a/src/net/FetchHttpClient.js
+++ b/src/net/FetchHttpClient.ts
@@ -1,6 +1,5 @@
-'use strict';
-
-const {HttpClient, HttpClientResponse} = require('./HttpClient');
+import _HttpClient = require('./HttpClient');
+const {HttpClient, HttpClientResponse} = _HttpClient;
 
 /**
  * HTTP client which uses a `fetch` function to issue requests.
@@ -11,6 +10,12 @@ const {HttpClient, HttpClientResponse} = require('./HttpClient');
  * node-fetch package (https://github.com/node-fetch/node-fetch).
  */
 class FetchHttpClient extends HttpClient {
+  _fetchFn: (
+    input: RequestInfo | URL,
+    init?: RequestInit | undefined
+  ) => Promise<Response>;
+  _res: Response;
+
   constructor(fetchFn) {
     super();
     this._fetchFn = fetchFn;
@@ -88,6 +93,8 @@ class FetchHttpClient extends HttpClient {
 }
 
 class FetchHttpClientResponse extends HttpClientResponse {
+  _res: Response;
+
   constructor(res) {
     super(
       res.status,
@@ -135,4 +142,4 @@ class FetchHttpClientResponse extends HttpClientResponse {
   }
 }
 
-module.exports = {FetchHttpClient, FetchHttpClientResponse};
+export = {FetchHttpClient, FetchHttpClientResponse};
diff --git a/src/net/HttpClient.js b/src/net/HttpClient.ts
similarity index 77%
rename from src/net/HttpClient.js
rename to src/net/HttpClient.ts
index b309ce7d9d..2c035d7cd7 100644
--- a/src/net/HttpClient.js
+++ b/src/net/HttpClient.ts
@@ -1,4 +1,4 @@
-'use strict';
+type TimeoutError = TypeError & {code?: string};
 
 /**
  * Encapsulates the logic for issuing a request to the Stripe API.
@@ -10,6 +10,9 @@
  *    returning their own response class when making requests.
  */
 class HttpClient {
+  static CONNECTION_CLOSED_ERROR_CODES: string[];
+  static TIMEOUT_ERROR_CODE: string;
+
   /** The client name used for diagnostics. */
   getClientName() {
     throw new Error('getClientName not implemented.');
@@ -30,7 +33,9 @@ class HttpClient {
 
   /** Helper to make a consistent timeout error across implementations. */
   static makeTimeoutError() {
-    const timeoutErr = new TypeError(HttpClient.TIMEOUT_ERROR_CODE);
+    const timeoutErr: TimeoutError = new TypeError(
+      HttpClient.TIMEOUT_ERROR_CODE
+    );
     timeoutErr.code = HttpClient.TIMEOUT_ERROR_CODE;
     return timeoutErr;
   }
@@ -40,16 +45,19 @@ HttpClient.CONNECTION_CLOSED_ERROR_CODES = ['ECONNRESET', 'EPIPE'];
 HttpClient.TIMEOUT_ERROR_CODE = 'ETIMEDOUT';
 
 class HttpClientResponse {
+  _statusCode: number;
+  _headers: Record<string, string>;
+
   constructor(statusCode, headers) {
     this._statusCode = statusCode;
     this._headers = headers;
   }
 
-  getStatusCode() {
+  getStatusCode(): number {
     return this._statusCode;
   }
 
-  getHeaders() {
+  getHeaders(): Record<string, string> {
     return this._headers;
   }
 
@@ -61,9 +69,9 @@ class HttpClientResponse {
     throw new Error('toStream not implemented.');
   }
 
-  toJSON() {
+  toJSON(): any {
     throw new Error('toJSON not implemented.');
   }
 }
 
-module.exports = {HttpClient, HttpClientResponse};
+export = {HttpClient, HttpClientResponse};
diff --git a/src/net/NodeHttpClient.js b/src/net/NodeHttpClient.ts
similarity index 88%
rename from src/net/NodeHttpClient.js
rename to src/net/NodeHttpClient.ts
index 222a446ce7..1d556a0906 100644
--- a/src/net/NodeHttpClient.js
+++ b/src/net/NodeHttpClient.ts
@@ -1,9 +1,8 @@
-'use strict';
+import http = require('http');
+import https = require('https');
 
-const http = require('http');
-const https = require('https');
-
-const {HttpClient, HttpClientResponse} = require('./HttpClient');
+import _HttpClient = require('./HttpClient');
+const {HttpClient, HttpClientResponse} = _HttpClient;
 
 const defaultHttpAgent = new http.Agent({keepAlive: true});
 const defaultHttpsAgent = new https.Agent({keepAlive: true});
@@ -13,6 +12,8 @@ const defaultHttpsAgent = new https.Agent({keepAlive: true});
  * requests.`
  */
 class NodeHttpClient extends HttpClient {
+  _agent: http.Agent | https.Agent;
+
   constructor(agent) {
     super();
     this._agent = agent;
@@ -86,7 +87,9 @@ class NodeHttpClient extends HttpClient {
 }
 
 class NodeHttpClientResponse extends HttpClientResponse {
-  constructor(res) {
+  _res: http.IncomingMessage;
+
+  constructor(res: http.IncomingMessage) {
     super(res.statusCode, res.headers || {});
     this._res = res;
   }
@@ -103,7 +106,7 @@ class NodeHttpClientResponse extends HttpClientResponse {
     return this._res;
   }
 
-  toJSON() {
+  toJSON(): any {
     return new Promise((resolve, reject) => {
       let response = '';
 
@@ -122,4 +125,4 @@ class NodeHttpClientResponse extends HttpClientResponse {
   }
 }
 
-module.exports = {NodeHttpClient, NodeHttpClientResponse};
+export = {NodeHttpClient, NodeHttpClientResponse};
diff --git a/src/utils.js b/src/utils.ts
similarity index 93%
rename from src/utils.js
rename to src/utils.ts
index 7eed41defd..b3ac274cbc 100644
--- a/src/utils.js
+++ b/src/utils.ts
@@ -1,11 +1,7 @@
-'use strict';
-
 const EventEmitter = require('events').EventEmitter;
 const qs = require('qs');
 const crypto = require('crypto');
 
-const hasOwn = (obj, prop) => Object.prototype.hasOwnProperty.call(obj, prop);
-
 // Certain sandboxed environments (our known example right now are CloudFlare
 // Workers) may make `child_process` unavailable. Because `exec` isn't critical
 // to the operation of stripe-node, we handle this unavailability gracefully.
@@ -37,13 +33,30 @@ const DEPRECATED_OPTIONS = {
 };
 const DEPRECATED_OPTIONS_KEYS = Object.keys(DEPRECATED_OPTIONS);
 
-const utils = (module.exports = {
+type Settings = {
+  timeout?: number;
+  maxNetworkRetries?: number;
+};
+
+type Options = {
+  auth?: string;
+  host?: string;
+  settings?: Settings;
+  streaming?: boolean;
+  headers?: Record<string, string>;
+};
+
+const utils = {
   isOptionsHash(o) {
     return (
       o &&
       typeof o === 'object' &&
-      (OPTIONS_KEYS.some((prop) => hasOwn(o, prop)) ||
-        DEPRECATED_OPTIONS_KEYS.some((prop) => hasOwn(o, prop)))
+      (OPTIONS_KEYS.some((prop) =>
+        Object.prototype.hasOwnProperty.call(o, prop)
+      ) ||
+        DEPRECATED_OPTIONS_KEYS.some((prop) =>
+          Object.prototype.hasOwnProperty.call(o, prop)
+        ))
     );
   },
 
@@ -140,7 +153,7 @@ const utils = (module.exports = {
    * Return the options hash from a list of arguments
    */
   getOptionsFromArgs: (args) => {
-    const opts = {
+    const opts: Options = {
       auth: null,
       headers: {},
       settings: {},
@@ -210,8 +223,9 @@ const utils = (module.exports = {
    * Provide simple "Class" extension mechanism
    */
   protoExtend(sub) {
+    // eslint-disable-next-line @typescript-eslint/no-this-alias
     const Super = this;
-    const Constructor = hasOwn(sub, 'constructor')
+    const Constructor = Object.prototype.hasOwnProperty.call(sub, 'constructor')
       ? sub.constructor
       : function(...args) {
           Super.apply(this, args);
@@ -388,7 +402,10 @@ const utils = (module.exports = {
         const newKey = prevKey ? `${prevKey}[${key}]` : key;
 
         if (utils.isObject(value)) {
-          if (!Buffer.isBuffer(value) && !value.hasOwnProperty('data')) {
+          if (
+            !Buffer.isBuffer(value) &&
+            !Object.prototype.hasOwnProperty.call(value, 'data')
+          ) {
             // Non-buffer non-file Objects are recursively flattened
             return step(value, newKey);
           } else {
@@ -402,7 +419,7 @@ const utils = (module.exports = {
       });
     };
 
-    step(data);
+    step(data, null);
 
     return result;
   },
@@ -438,7 +455,7 @@ const utils = (module.exports = {
           platform: process.platform,
         };
   },
-});
+};
 
 function emitWarning(warning) {
   if (typeof process.emitWarning !== 'function') {
@@ -449,3 +466,5 @@ function emitWarning(warning) {
 
   return process.emitWarning(warning, 'Stripe');
 }
+
+export = utils;
diff --git a/tsconfig.json b/tsconfig.json
index f5cc13096a..199458365d 100644
--- a/tsconfig.json
+++ b/tsconfig.json
@@ -5,6 +5,7 @@
     "target": "es2017",
     "module": "commonjs",
     "checkJs": false,
+    "alwaysStrict": true,
   },
   "include": ["./src/**/*"]
 }
diff --git a/yarn.lock b/yarn.lock
index 7c7a6481f3..dac05c29fe 100644
--- a/yarn.lock
+++ b/yarn.lock
@@ -2395,10 +2395,10 @@ typedarray-to-buffer@^3.1.5:
   dependencies:
     is-typedarray "^1.0.0"
 
-typescript@^3.7.2:
-  version "3.9.9"
-  resolved "https://registry.yarnpkg.com/typescript/-/typescript-3.9.9.tgz#e69905c54bc0681d0518bd4d587cc6f2d0b1a674"
-  integrity sha512-kdMjTiekY+z/ubJCATUPlRDl39vXYiMV9iyeMuEuXZh2we6zz80uovNN2WlAxmmdE/Z/YQe+EbOEXB5RHEED3w==
+typescript@^4.8.3:
+  version "4.8.3"
+  resolved "https://registry.yarnpkg.com/typescript/-/typescript-4.8.3.tgz#d59344522c4bc464a65a730ac695007fdb66dd88"
+  integrity sha512-goMHfm00nWPa8UvR/CPSvykqf6dVV8x/dp0c5mFTMTIu0u0FlGWRioyy7Nn0PGAdHxpJZnuO/ut+PpQ8UiHAig==
 
 uri-js@^4.2.2:
   version "4.4.1"