From 93e68fbfed5021bb368d7451f43b8b2743be1e2c Mon Sep 17 00:00:00 2001
From: Justintime50 <39606064+Justintime50@users.noreply.github.com>
Date: Fri, 7 Aug 2026 14:48:19 -0600
Subject: [PATCH] TSM-10: convert Group E services to TypeScript

---
 ...{api_key_service.js => api_key_service.ts} | 26 +++---
 ..._service.js => customer_portal_service.ts} |  2 +-
 ...dable_service.js => embeddable_service.ts} |  2 +-
 ...pper_service.js => end_shipper_service.ts} | 10 ++-
 ...rvice.js => fedex_registration_service.ts} | 37 +++++---
 .../{luma_service.js => luma_service.ts}      |  4 +-
 ...ervice.js => referral_customer_service.ts} | 88 +++++++++++++------
 .../{user_service.js => user_service.ts}      | 48 +++++++---
 8 files changed, 150 insertions(+), 67 deletions(-)
 rename src/services/{api_key_service.js => api_key_service.ts} (85%)
 rename src/services/{customer_portal_service.js => customer_portal_service.ts} (90%)
 rename src/services/{embeddable_service.js => embeddable_service.ts} (90%)
 rename src/services/{end_shipper_service.js => end_shipper_service.ts} (88%)
 rename src/services/{fedex_registration_service.js => fedex_registration_service.ts} (81%)
 rename src/services/{luma_service.js => luma_service.ts} (89%)
 rename src/services/{referral_customer_service.js => referral_customer_service.ts} (76%)
 rename src/services/{user_service.js => user_service.ts} (78%)

diff --git a/src/services/api_key_service.js b/src/services/api_key_service.ts
similarity index 85%
rename from src/services/api_key_service.js
rename to src/services/api_key_service.ts
index 20f1d1b63..c661426e8 100644
--- a/src/services/api_key_service.js
+++ b/src/services/api_key_service.ts
@@ -4,6 +4,12 @@ import Constants from '../constants';
 import FilteringError from '../errors/general/filtering_error';
 import baseService from './base_service';
 
+type ApiKeyUser = Record<string, unknown> & {
+  id?: string;
+  keys?: unknown[];
+  children?: Array<Record<string, unknown> & { id?: string; keys?: unknown[] }>;
+};
+
 export default (easypostClient) =>
   /**
    * The ApiKeyService class provides methods for interacting with EasyPost {@link ApiKey} objects.
@@ -17,20 +23,20 @@ export default (easypostClient) =>
      * @returns {Array} - List of associated API Keys.
      * @throws {FilteringError} If user or API Keys are not found.
      */
-    static async retrieveApiKeysForUser(id) {
+    static async retrieveApiKeysForUser(id: string): Promise<unknown[]> {
       const url = `api_keys`;
 
       try {
         const response = await easypostClient._get(url);
-        const user = this._convertToEasyPostObject(response.body);
+        const user = this._convertToEasyPostObject(response.body) as ApiKeyUser;
 
         if (user.id == id) {
-          return user.keys;
+          return user.keys ?? [];
         }
 
-        user.children.forEach((child) => {
+        user.children?.forEach((child) => {
           if (child.id == id) {
-            return child.keys;
+            return child.keys ?? [];
           }
         });
       } catch (e) {
@@ -45,7 +51,7 @@ export default (easypostClient) =>
      * See {@link https://docs.easypost.com/docs/api-keys#retrieve-an-api-key EasyPost API Documentation} for more information.
      * @returns {Object} - An object containing the API keys associated with the current authenticated user and its child users.
      */
-    static async all(params = {}) {
+    static async all(params: Record<string, unknown> = {}): Promise<unknown> {
       const url = 'api_keys';
 
       return this._all(url, params);
@@ -57,7 +63,7 @@ export default (easypostClient) =>
      * @param {string} mode - The mode for the API key (either "production" or "test").
      * @returns {ApiKey} - The created API key.
      */
-    static async create(mode) {
+    static async create(mode: string): Promise<unknown> {
       const url = 'api_keys';
       const params = { mode };
 
@@ -70,7 +76,7 @@ export default (easypostClient) =>
      * @param {string} id - The ID of the API key to delete.
      * @returns {Promise|Promise<never>} - A promise that resolves if the API key was successfully deleted.
      */
-    static async delete(id) {
+    static async delete(id: string): Promise<void> {
       const url = `api_keys/${id}`;
 
       try {
@@ -88,7 +94,7 @@ export default (easypostClient) =>
      * @param {string} id - The ID of the API key to enable.
      * @returns {ApiKey} - The enabled API key.
      */
-    static async enable(id) {
+    static async enable(id: string): Promise<unknown> {
       const url = `api_keys/${id}/enable`;
 
       try {
@@ -106,7 +112,7 @@ export default (easypostClient) =>
      * @param {string} id - The ID of the API key to disable.
      * @returns {ApiKey} - The disabled API key.
      */
-    static async disable(id) {
+    static async disable(id: string): Promise<unknown> {
       const url = `api_keys/${id}/disable`;
 
       try {
diff --git a/src/services/customer_portal_service.js b/src/services/customer_portal_service.ts
similarity index 90%
rename from src/services/customer_portal_service.js
rename to src/services/customer_portal_service.ts
index c58a3f8c8..9642d6a5e 100644
--- a/src/services/customer_portal_service.js
+++ b/src/services/customer_portal_service.ts
@@ -11,7 +11,7 @@ export default (easypostClient) =>
      * @param {Object} [params] - The parameters to create a session from.
      * @returns {Object} - An object containing the created session.
      */
-    static async createAccountLink(params = {}) {
+    static async createAccountLink(params: Record<string, unknown> = {}): Promise<unknown> {
       const url = 'customer_portal/account_link';
 
       try {
diff --git a/src/services/embeddable_service.js b/src/services/embeddable_service.ts
similarity index 90%
rename from src/services/embeddable_service.js
rename to src/services/embeddable_service.ts
index d27f056e1..df90a3347 100644
--- a/src/services/embeddable_service.js
+++ b/src/services/embeddable_service.ts
@@ -11,7 +11,7 @@ export default (easypostClient) =>
      * @param {Object} [params] - The parameters to create a session from.
      * @returns {Object} - An object containing the created session.
      */
-    static async createSession(params = {}) {
+    static async createSession(params: Record<string, unknown> = {}): Promise<unknown> {
       const url = 'embeddables/session';
 
       try {
diff --git a/src/services/end_shipper_service.js b/src/services/end_shipper_service.ts
similarity index 88%
rename from src/services/end_shipper_service.js
rename to src/services/end_shipper_service.ts
index 7c2515422..ad5b59b6f 100644
--- a/src/services/end_shipper_service.js
+++ b/src/services/end_shipper_service.ts
@@ -1,5 +1,7 @@
 import baseService from './base_service';
 
+type EndShipperParams = Record<string, unknown>;
+
 export default (easypostClient) =>
   /**
    * The EndShipperService class provides methods for interacting with EasyPost {@link EndShipper} objects.
@@ -12,7 +14,7 @@ export default (easypostClient) =>
      * @param {Object} params - Parameters for the end shipper to be created.
      * @returns {EndShipper} - The created end shipper.
      */
-    static async create(params) {
+    static async create(params: EndShipperParams): Promise<unknown> {
       const url = 'end_shippers';
       const wrappedParams = { address: params };
 
@@ -26,7 +28,7 @@ export default (easypostClient) =>
      * @param {Object} params - Parameters for the end shipper to be updated.
      * @returns {EndShipper} - The updated end shipper.
      */
-    static async update(id, params) {
+    static async update(id: string, params: EndShipperParams): Promise<unknown> {
       const url = `end_shippers/${id}`;
       const wrappedParams = { address: params };
 
@@ -45,7 +47,7 @@ export default (easypostClient) =>
      * @param {string} id - The ID of the end shipper to retrieve.
      * @returns {EndShipper} - The retrieved end shipper.
      */
-    static async retrieve(id) {
+    static async retrieve(id: string): Promise<unknown> {
       const url = `end_shippers/${id}`;
 
       return this._retrieve(url);
@@ -57,7 +59,7 @@ export default (easypostClient) =>
      * @param {Object} [params] - Parameters to filter the list of end shippers.
      * @returns {Object} - An object containing a list of {@link EndShipper end shippers} and pagination information.
      */
-    static async all(params = {}) {
+    static async all(params: Record<string, unknown> = {}): Promise<unknown> {
       const url = 'end_shippers';
 
       return this._all(url, params);
diff --git a/src/services/fedex_registration_service.js b/src/services/fedex_registration_service.ts
similarity index 81%
rename from src/services/fedex_registration_service.js
rename to src/services/fedex_registration_service.ts
index 2f11f901d..659377815 100644
--- a/src/services/fedex_registration_service.js
+++ b/src/services/fedex_registration_service.ts
@@ -2,6 +2,14 @@ import { v4 as uuid } from 'uuid';
 
 import baseService from './base_service';
 
+type FedExValidationMap = Record<string, unknown> & { name?: string | null };
+type FedExParams = Record<string, unknown> & {
+  address_validation?: FedExValidationMap;
+  pin_validation?: FedExValidationMap;
+  invoice_validation?: FedExValidationMap;
+  easypost_details?: Record<string, unknown>;
+};
+
 export default (easypostClient) =>
   /**
    * The FedExRegistrationService class provides methods for registering FedEx carrier accounts with MFA.
@@ -14,7 +22,10 @@ export default (easypostClient) =>
      * @param {Object} params - Map of parameters.
      * @returns {Object}
      */
-    static async registerAddress(fedexAccountNumber, params) {
+    static async registerAddress(
+      fedexAccountNumber: string,
+      params: FedExParams,
+    ): Promise<unknown> {
       const wrappedParams = this._wrapAddressValidation(params);
       const endpoint = `fedex_registrations/${fedexAccountNumber}/address`;
 
@@ -33,7 +44,11 @@ export default (easypostClient) =>
      * @param {Object} params - Map of parameters.
      * @returns {Object}
      */
-    static async requestPin(fedexAccountNumber, pinMethodOption, params) {
+    static async requestPin(
+      fedexAccountNumber: string,
+      pinMethodOption: string,
+      params: FedExParams,
+    ): Promise<unknown> {
       const wrappedParams = this._wrapPinValidation(params);
       wrappedParams.pin_method = {
         option: pinMethodOption,
@@ -54,7 +69,7 @@ export default (easypostClient) =>
      * @param {Object} params - Map of parameters.
      * @returns {Object}
      */
-    static async validatePin(fedexAccountNumber, params) {
+    static async validatePin(fedexAccountNumber: string, params: FedExParams): Promise<unknown> {
       const wrappedParams = this._wrapPinValidation(params);
       const endpoint = `fedex_registrations/${fedexAccountNumber}/pin/validate`;
 
@@ -72,7 +87,7 @@ export default (easypostClient) =>
      * @param {Object} params - Map of parameters.
      * @returns {Object}
      */
-    static async submitInvoice(fedexAccountNumber, params) {
+    static async submitInvoice(fedexAccountNumber: string, params: FedExParams): Promise<unknown> {
       const wrappedParams = this._wrapInvoiceValidation(params);
       const endpoint = `fedex_registrations/${fedexAccountNumber}/invoice`;
 
@@ -91,8 +106,8 @@ export default (easypostClient) =>
      * @param {Object} params - The original parameters map.
      * @returns {Object} - A new map with properly wrapped address_validation and easypost_details.
      */
-    static _wrapAddressValidation(params) {
-      const wrappedParams = {};
+    static _wrapAddressValidation(params: FedExParams): Record<string, unknown> {
+      const wrappedParams: Record<string, unknown> = {};
 
       if (params.address_validation) {
         const addressValidation = { ...params.address_validation };
@@ -114,8 +129,8 @@ export default (easypostClient) =>
      * @param {Object} params - The original parameters map.
      * @returns {Object} - A new map with properly wrapped pin_validation and easypost_details.
      */
-    static _wrapPinValidation(params) {
-      const wrappedParams = {};
+    static _wrapPinValidation(params: FedExParams): Record<string, unknown> {
+      const wrappedParams: Record<string, unknown> = {};
 
       if (params.pin_validation) {
         const pinValidation = { ...params.pin_validation };
@@ -137,8 +152,8 @@ export default (easypostClient) =>
      * @param {Object} params - The original parameters map.
      * @returns {Object} - A new map with properly wrapped invoice_validation and easypost_details.
      */
-    static _wrapInvoiceValidation(params) {
-      const wrappedParams = {};
+    static _wrapInvoiceValidation(params: FedExParams): Record<string, unknown> {
+      const wrappedParams: Record<string, unknown> = {};
 
       if (params.invoice_validation) {
         const invoiceValidation = { ...params.invoice_validation };
@@ -160,7 +175,7 @@ export default (easypostClient) =>
      * @private
      * @param {Object} map - The map to ensure the "name" field in.
      */
-    static _ensureNameField(map) {
+    static _ensureNameField(map: FedExValidationMap): void {
       if (!map.name || map.name === null) {
         const uuidValue = uuid().replace(/-/g, '');
         map.name = uuidValue;
diff --git a/src/services/luma_service.js b/src/services/luma_service.ts
similarity index 89%
rename from src/services/luma_service.js
rename to src/services/luma_service.ts
index 3947a5421..79db1c314 100644
--- a/src/services/luma_service.js
+++ b/src/services/luma_service.ts
@@ -1,5 +1,7 @@
 import baseService from './base_service';
 
+type LumaParams = Record<string, unknown>;
+
 export default (easypostClient) =>
   /**
    * The LumaService class provides methods for interacting with EasyPost Luma objects.
@@ -11,7 +13,7 @@ export default (easypostClient) =>
      * @param {Object} params - The parameters to get a Luma promise with.
      * @returns {Object} - An object containing the Luma promise.
      */
-    static async getPromise(params) {
+    static async getPromise(params: LumaParams): Promise<unknown> {
       const url = `luma/promise`;
 
       const wrappedParams = {
diff --git a/src/services/referral_customer_service.js b/src/services/referral_customer_service.ts
similarity index 76%
rename from src/services/referral_customer_service.js
rename to src/services/referral_customer_service.ts
index 70cbe1f8b..de716b06d 100644
--- a/src/services/referral_customer_service.js
+++ b/src/services/referral_customer_service.ts
@@ -5,6 +5,20 @@ import EasyPostClient from '../easypost';
 import ExternalApiError from '../errors/api/external_api_error';
 import baseService from './base_service';
 
+type ReferralCustomerParams = Record<string, unknown>;
+type MandateData = Record<string, unknown>;
+/* eslint-disable no-unused-vars */
+type ReferralScopedClient = {
+  _post: (
+    ...args: [string, Record<string, unknown>?]
+  ) => Promise<{ body: Record<string, unknown> }>;
+};
+type EasyPostHttpClient = {
+  _get: (...args: [string, Record<string, unknown>?]) => Promise<{ body: Record<string, unknown> }>;
+  _put: (...args: [string, Record<string, unknown>?]) => Promise<unknown>;
+};
+/* eslint-enable no-unused-vars */
+
 /**
  * Get an instance of the EasyPostClient using the referral user's API key.
  * @private
@@ -12,7 +26,7 @@ import baseService from './base_service';
  * @param {string} referralApiKey - The referral user's API key.
  * @returns {EasyPostClient} - An instance of the EasyPostClient.
  */
-function _getReferralClient(client, referralApiKey) {
+function _getReferralClient(client: EasyPostClient, referralApiKey: string): EasyPostClient {
   return EasyPostClient.copyClient(client, {
     apiKey: referralApiKey,
   });
@@ -24,12 +38,13 @@ function _getReferralClient(client, referralApiKey) {
  * @param {EasyPostClient} easypostClient - The EasyPostClient to use.
  * @returns {string} - The Stripe API key.
  */
-async function _getEasyPostStripeKey(easypostClient) {
+async function _getEasyPostStripeKey(easypostClient: EasyPostHttpClient): Promise<string> {
   const url = 'partners/stripe_public_key';
 
   const response = await easypostClient._get(url);
 
-  return response.body.public_key;
+  const body = response.body as Record<string, unknown>;
+  return body.public_key as string;
 }
 
 /**
@@ -42,7 +57,13 @@ async function _getEasyPostStripeKey(easypostClient) {
  * @param {string} cvc - Credit card CVC.
  * @returns {Promise<string>} - Stripe credit card token.
  */
-async function _sendCardDetailsToStripe(stripeKey, number, expirationMonth, expirationYear, cvc) {
+async function _sendCardDetailsToStripe(
+  stripeKey: string,
+  number: string,
+  expirationMonth: string,
+  expirationYear: string,
+  cvc: string,
+): Promise<string> {
   const searchParams = new URLSearchParams({
     'card[number]': number,
     'card[exp_month]': expirationMonth,
@@ -66,10 +87,13 @@ async function _sendCardDetailsToStripe(stripeKey, number, expirationMonth, expi
 
     const body = await response.json();
 
-    return body.id;
+    return body.id as string;
   } catch (error) {
     throw new ExternalApiError({
       message: util.format(Constants.EXTERNAL_API_CALL_FAILED, 'Stripe'),
+      code: undefined,
+      statusCode: undefined,
+      errors: undefined,
     });
   }
 }
@@ -83,12 +107,17 @@ async function _sendCardDetailsToStripe(stripeKey, number, expirationMonth, expi
  * @param {string} priority - Whether to add the card as the 'primary' or 'secondary' card.
  * @returns {Object} - Response body (EasyPost payment method object).
  */
-async function _sendCardDetailsToEasyPost(client, referralApiKey, stripeCreditCardToken, priority) {
+async function _sendCardDetailsToEasyPost(
+  client: EasyPostClient,
+  referralApiKey: string,
+  stripeCreditCardToken: string,
+  priority: string,
+): Promise<Record<string, unknown>> {
   const _client = _getReferralClient(client, referralApiKey);
   const url = 'credit_cards';
   const params = { credit_card: { stripe_object_id: stripeCreditCardToken, priority } };
 
-  const response = await _client._post(url, params);
+  const response = await (_client as ReferralScopedClient)._post(url, params);
 
   return response.body;
 }
@@ -105,7 +134,7 @@ export default (easypostClient) =>
      * @param {Object} params - The referral customer's information.
      * @returns {User} - The newly created referral customer.
      */
-    static async create(params) {
+    static async create(params: ReferralCustomerParams): Promise<unknown> {
       const url = 'referral_customers';
 
       const wrappedParams = {
@@ -122,7 +151,7 @@ export default (easypostClient) =>
      * @param {string} email - The new email address.
      * @returns {boolean} - Returns true if the referral was updated successfully, false otherwise.
      */
-    static async updateEmail(referralUserId, email) {
+    static async updateEmail(referralUserId: string, email: string): Promise<boolean> {
       const url = `referral_customers/${referralUserId}`;
       const wrappedParams = { user: { email } };
 
@@ -143,13 +172,13 @@ export default (easypostClient) =>
      * @returns {Object} - An object representing the newly-added credit card.
      */
     static async addCreditCard(
-      referralApiKey,
-      number,
-      expirationMonth,
-      expirationYear,
-      cvc,
-      priority = 'primary',
-    ) {
+      referralApiKey: string,
+      number: string,
+      expirationMonth: string,
+      expirationYear: string,
+      cvc: string,
+      priority: string = 'primary',
+    ): Promise<Record<string, unknown>> {
       const stripeKey = await _getEasyPostStripeKey(easypostClient); // will throw if there's an error
 
       const stripeCreditCardId = await _sendCardDetailsToStripe(
@@ -175,7 +204,11 @@ export default (easypostClient) =>
      * This function requires the ReferralCustomer User's API key.
      * @returns {object} - A JSON object representing the credit card.
      */
-    static async addCreditCardFromStripe(referralApiKey, paymentMethodId, priority = 'primary') {
+    static async addCreditCardFromStripe(
+      referralApiKey: string,
+      paymentMethodId: string,
+      priority: string = 'primary',
+    ): Promise<unknown> {
       const _client = _getReferralClient(easypostClient, referralApiKey);
       const params = {
         credit_card: {
@@ -185,7 +218,7 @@ export default (easypostClient) =>
       };
       const url = 'credit_cards';
 
-      const response = await _client._post(url, params);
+      const response = await (_client as ReferralScopedClient)._post(url, params);
 
       return this._convertToEasyPostObject(response.body, params);
     }
@@ -196,11 +229,11 @@ export default (easypostClient) =>
      * @returns {object} - A JSON object representing the bank account.
      */
     static async addBankAccountFromStripe(
-      referralApiKey,
-      financialConnectionsId,
-      mandateData,
-      priority = 'primary',
-    ) {
+      referralApiKey: string,
+      financialConnectionsId: string,
+      mandateData: MandateData,
+      priority: string = 'primary',
+    ): Promise<unknown> {
       const _client = _getReferralClient(easypostClient, referralApiKey);
       const params = {
         financial_connections_id: financialConnectionsId,
@@ -210,7 +243,7 @@ export default (easypostClient) =>
 
       const url = 'bank_accounts';
 
-      const response = await _client._post(url, params);
+      const response = await (_client as ReferralScopedClient)._post(url, params);
 
       return this._convertToEasyPostObject(response.body, params);
     }
@@ -221,7 +254,7 @@ export default (easypostClient) =>
      * @param {Object} [params] - Parameters to filter the referral customers by.
      * @returns {Object} - An object containing a list of {@link User referral customers} and pagination information.
      */
-    static async all(params = {}) {
+    static async all(params: Record<string, unknown> = {}): Promise<unknown> {
       const url = 'referral_customers';
 
       return this._all(url, params);
@@ -233,7 +266,10 @@ export default (easypostClient) =>
      * @param {Number} pageSize The number of records to return on each page
      * @returns {EasyPostObject|Promise<never>} The retrieved {@link EasyPostObject}-based class instance, or a `Promise` that rejects with an error.
      */
-    static async getNextPage(referralCustomers, pageSize = null) {
+    static async getNextPage(
+      referralCustomers: Record<string, unknown>,
+      pageSize: number | null = null,
+    ): Promise<unknown> {
       const url = 'referral_customers';
       return this._getNextPage(url, 'referral_customers', referralCustomers, pageSize);
     }
diff --git a/src/services/user_service.js b/src/services/user_service.ts
similarity index 78%
rename from src/services/user_service.js
rename to src/services/user_service.ts
index 7bfc6e382..d91abada1 100644
--- a/src/services/user_service.js
+++ b/src/services/user_service.ts
@@ -1,6 +1,13 @@
 import EndOfPaginationError from '../errors/general/end_of_pagination_error';
 import baseService from './base_service';
 
+type UserParams = Record<string, unknown>;
+type BrandParams = Record<string, unknown>;
+type UserCollection = Record<string, unknown> & {
+  has_more?: boolean;
+  _params?: Record<string, unknown>;
+};
+
 export default (easypostClient) =>
   /**
    * The UserService class provides methods for interacting with EasyPost {@link User} objects.
@@ -13,7 +20,7 @@ export default (easypostClient) =>
      * @param {Object} params - The parameters to create a child user with.
      * @returns {User} - The created child user.
      */
-    static async create(params) {
+    static async create(params: UserParams): Promise<unknown> {
       const url = 'users';
 
       const wrappedParams = {
@@ -30,7 +37,7 @@ export default (easypostClient) =>
      * @param {Object} params - The parameters to update the user with.
      * @returns {User} - The updated user.
      */
-    static async update(id, params) {
+    static async update(id: string, params: UserParams): Promise<unknown> {
       const url = `users/${id}`;
       const wrappedParams = {
         user: params,
@@ -51,7 +58,7 @@ export default (easypostClient) =>
      * @param {string} id - The ID of the child user to retrieve.
      * @returns {User} - The retrieved child user.
      */
-    static async retrieve(id) {
+    static async retrieve(id: string): Promise<unknown> {
       const url = `users/${id}`;
 
       try {
@@ -68,7 +75,7 @@ export default (easypostClient) =>
      * See {@link https://docs.easypost.com/docs/users#retrieve-a-user EasyPost API Documentation} for more information.
      * @returns {User} - The retrieved user.
      */
-    static async retrieveMe() {
+    static async retrieveMe(): Promise<unknown> {
       const url = 'users';
 
       try {
@@ -86,7 +93,7 @@ export default (easypostClient) =>
      * @param {string} id - The ID of the child user to delete.
      * @returns {Promise|Promise<never>} - A promise that resolves when the child user is deleted successfully.
      */
-    static async delete(id) {
+    static async delete(id: string): Promise<void> {
       const url = `users/${id}`;
 
       try {
@@ -105,7 +112,7 @@ export default (easypostClient) =>
      * @param {Object} params - The parameters to update the brand with.
      * @returns {Brand} - The updated brand.
      */
-    static async updateBrand(id, params) {
+    static async updateBrand(id: string, params: BrandParams): Promise<unknown> {
       const url = `users/${id}/brand`;
       const wrappedParams = { brand: params };
 
@@ -124,7 +131,7 @@ export default (easypostClient) =>
      * @param {Object} params - Parameters to filter the list of children users.
      * @returns {Object} - An object containing a list of {@link Children User} and pagination information.
      */
-    static async allChildren(params) {
+    static async allChildren(params: Record<string, unknown>): Promise<unknown> {
       const url = 'users/children';
 
       try {
@@ -142,18 +149,28 @@ export default (easypostClient) =>
      * @param {Number} pageSize The number of records to return on each page
      * @returns {EasyPostObject|Promise<never>} The retrieved {@link EasyPostObject}-based class instance, or a `Promise` that rejects with an error.
      */
-    static async getNextPage(children, pageSize = null) {
+    static async getNextPage(
+      children: UserCollection,
+      pageSize: number | null = null,
+    ): Promise<unknown> {
       const url = 'users/children';
       return this._getNextPage(url, 'children', children, pageSize);
     }
 
-    static async _getNextPage(url, key, collection, pageSize = null) {
-      const collectionArray = collection[key];
+    static async _getNextPage(
+      url: string,
+      key: string,
+      collection: UserCollection,
+      pageSize: number | null = null,
+    ): Promise<unknown> {
+      const collectionArray = collection[key] as Array<Record<string, unknown>> | undefined;
       if (collectionArray == undefined || collectionArray.length == 0 || !collection.has_more) {
         throw new EndOfPaginationError();
       }
 
-      const defaultParams = collection._params ?? collectionArray[0]._params ?? {};
+      const firstObjectParams =
+        (collectionArray[0]?._params as Record<string, unknown> | undefined) ?? {};
+      const defaultParams = collection._params ?? firstObjectParams;
 
       const params = {
         ...defaultParams,
@@ -161,8 +178,13 @@ export default (easypostClient) =>
         after_id: collectionArray[collectionArray.length - 1].id,
       };
 
-      const response = await this._all(url, params);
-      if (response == undefined || response[key].length == 0) {
+      const response = (await this._all(url, params)) as Record<string, unknown>;
+      const responseCollection = response[key] as Array<Record<string, unknown>> | undefined;
+      if (
+        response == undefined ||
+        responseCollection == undefined ||
+        responseCollection.length == 0
+      ) {
         throw new EndOfPaginationError();
       }