diff --git a/README.md b/README.md
index 6393b30b..e8ba74ef 100644
--- a/README.md
+++ b/README.md
@@ -96,7 +96,7 @@ let payload = {
 };
 
 const response = await PAYPAY.QRCodeCreate(payload);
-const body = JSON.parse(response.BODY);
+const body = response.BODY;
 console.log(response.STATUS, body.resultInfo.code);
 ```
 
@@ -117,7 +117,7 @@ Now that you have created a Code, the next  step is to implement polling to get
 let merchantPaymentId = 'merchantPaymentId';
 
 const response = await PAYPAY.GetCodePaymentDetails([merchantPaymentId]);
-const body = JSON.parse(response.BODY);
+const body = response.BODY;
 console.log(body.resultInfo.code);
 console.log(body.data.status);
 ```
@@ -139,7 +139,7 @@ Following are the important parameters that you can provide for this method:
 
 ```javascript
 const response = await PAYPAY.QRCodeDelete([codeId]);
-const body = JSON.parse(response.BODY);
+const body = response.BODY;
 console.log(body.resultInfo.code);
 ```
 
@@ -162,7 +162,7 @@ Following are the important parameters that you can provide for this method:
 
 ```javascript
 const response = await PAYPAY.PaymentCancel([merchantPaymentId]);
-const body = JSON.parse(response.BODY);
+const body = response.BODY;
 console.log(body.resultInfo.code);
 ```
 
@@ -190,7 +190,7 @@ let payload = {
 };
 
 const response = await PAYPAY.PaymentRefund(payload);
-const body = JSON.parse(response.BODY);
+const body = response.BODY;
 console.log(body.resultInfo.code);
 ```
 
@@ -220,7 +220,7 @@ let payload = {
 };
 
 const response = await PAYPAY.PaymentAuthCapture(payload);
-const body = JSON.parse(response.BODY);
+const body = response.BODY;
 console.log(body.resultInfo.code);
 ```
 For details of all the request and response parameters , check our [API Documentation guide](https://www.paypay.ne.jp/opa/doc/v1.0/dynamicqrcode#operation/capturePaymentAuth).
@@ -243,7 +243,7 @@ let payload = {
 };
 
 const response = await PAYPAY.PaymentAuthRevert(payload);
-const body = JSON.parse(response.BODY);
+const body = response.BODY:
 console.log(body.resultInfo.code);
 ```
 For List of params refer to the API guide :
@@ -260,7 +260,7 @@ So you want to confirm the status of the refund, maybe because the request for t
 ```javascript
 let merchantPaymentId = 'merchantRefundId';
 const response = await PAYPAY.GetRefundDetails([merchantRefundId]);
-const body = JSON.parse(response.BODY);
+const body = response.BODY;
 console.log(body.resultInfo.code);
 ```
 For details of all the request and response parameters , check our [API Documentation guide](https://www.paypay.ne.jp/opa/doc/v1.0/dynamicqrcode#operation/getRefundDetails).
@@ -305,7 +305,7 @@ let payload = {
 };
 // Calling the method to create the account linking QR Code
 const response = await PAYPAY.AccountLinkQRCodeCreate(payload);
-const body = JSON.parse(response.BODY);
+const body = response.BODY;
 
 // Printing if the method call was SUCCESS
 console.log(body.resultInfo.code);
@@ -332,7 +332,7 @@ const userAuthorizationId = jwtResponse["userAuthorizationId"];
 ```javascript
 // Calling the method to unlink a Payment
 const result = await PAYPAY.unlinkUser([userAuthorizationId]);
-const body = JSON.parse(result.BODY);
+const body = result.BODY;
 
 // Printing if the method call was SUCCESS
 console.log(body.resultInfo.code);
@@ -365,7 +365,7 @@ let payload = {
 
 // Calling the method to create a payment
 const response = await PAYPAY.CreatePayment(payload);
-const body = JSON.parse(response.BODY);
+const body = response.BODY;
 
 // Printing if the method call was SUCCESS
 console.log(body.resultInfo.code);
@@ -424,7 +424,7 @@ Now that you have created a payment, in case the payment request timeout, you ca
 ```javascript
 let merchantPaymentId = 'merchantPaymentId';
 const response = await PAYPAY.GetPaymentDetails([merchantPaymentId]);
-const body = JSON.parse(response.BODY);
+const body = response.BODY;
 
 // Printing if the method call was SUCCESS, this does not mean the payment was a success
 console.log(body.resultInfo.code);
@@ -454,7 +454,7 @@ Following are the important parameters that you can provide for this method:
 
 ```javascript
 const response = await PAYPAY.PaymentCancel([merchantPaymentId]);
-const body = JSON.parse(response.BODY);
+const body = response.BODY;
 // Printing if the method call was SUCCESS
 console.log(body.resultInfo.code);
 ```
@@ -475,7 +475,7 @@ So the user has decided to return the goods they have purchased and needs to be
 |reason   |  No |integer <= 11 characters  |The reason for refund |
 
 ```javascript
- let payload = {
+let payload = {
     merchantRefundId: 'merchant_refund_id',
     paymentId: 'paypay_payment_id',
     amount: {
@@ -486,7 +486,7 @@ So the user has decided to return the goods they have purchased and needs to be
 };
 // Calling the method to refund a Payment
 const response = await PAYPAY.PaymentRefund(payload);
-const body = JSON.parse(response.BODY);
+const body = response.BODY;
 // Printing if the method call was SUCCESS
 console.log(body.resultInfo.code);
 });
@@ -509,7 +509,7 @@ So you want to confirm the status of the refund, maybe because the request for t
 ```javascript
 let merchantPaymentId = 'merchantRefundId'
 const response = await PAYPAY.GetRefundDetails([merchantRefundId]);
-const body = JSON.parse(response.BODY);
+const body = response.BODY;
 // Printing if the method call was SUCCESS
 console.log(body.resultInfo.code);
 ```
diff --git a/src/lib/httpsClient.ts b/src/lib/httpsClient.ts
index af80483e..0574c742 100644
--- a/src/lib/httpsClient.ts
+++ b/src/lib/httpsClient.ts
@@ -2,7 +2,7 @@ import * as https from "https";
 
 export interface HttpsClientSuccess {
   STATUS: number;
-  BODY: string;
+  BODY: object | null;
 }
 
 export interface HttpsClientError {
@@ -52,7 +52,20 @@ export class HttpsClient {
         if (status < 200 || status > 299) {
           this.printDebugMessage(status, body, apiName);
         }
-        callback({ STATUS: status, BODY: body });
+        let parsed;
+        try {
+          parsed = body.match(/\S/) ? JSON.parse(body) : null;
+        } catch (e: any) {
+          callback({ STATUS: 500, ERROR: e.message });
+          return;
+        }
+
+        // Make the `BODY.toString()` return the raw JSON.
+        // This makes the library compatible with calls like `JSON.parse(response.BODY)`,
+        // which were required prior to version 2.
+        const responseObject = parsed && Object.assign(Object.create({ toString() { return body; } }), parsed);
+
+        callback({ STATUS: status, BODY: responseObject });
       });
     });
 
@@ -66,5 +79,3 @@ export class HttpsClient {
     req.end();
   }
 }
-
-export const httpsClient = new HttpsClient();
diff --git a/src/lib/paypay-rest-sdk.ts b/src/lib/paypay-rest-sdk.ts
index 87a15656..b4a44fa7 100644
--- a/src/lib/paypay-rest-sdk.ts
+++ b/src/lib/paypay-rest-sdk.ts
@@ -3,7 +3,7 @@
  */
 import { Auth } from "./auth";
 import { Conf } from "./conf";
-import { httpsClient, HttpsClientError, HttpsClientMessage, HttpsClientSuccess } from "./httpsClient";
+import { HttpsClient, HttpsClientError, HttpsClientMessage, HttpsClientSuccess } from "./httpsClient";
 import { HmacSHA256, enc, algo } from "crypto-js";
 import { v4 as uuidv4 } from "uuid";
 import * as jwt from "jsonwebtoken";
@@ -19,12 +19,20 @@ class PayPayRestSDK {
   private perfMode: boolean = false;
   private readonly auth: Auth;
   private config: Conf;
+  private httpsClient: HttpsClient = new HttpsClient();
 
   constructor() {
     this.config = new Conf(this.productionMode, this.perfMode);
     this.auth = new Auth();
   }
 
+  /**
+   * Replace the HttpsClient that this SDK client instance uses for API calls.
+   */
+  setHttpsClient(httpsClient: HttpsClient) {
+    this.httpsClient = httpsClient;
+  }
+
   /**
    * Set authentication passed by end-user
    * @param clientConfig
@@ -130,7 +138,7 @@ class PayPayRestSDK {
     callback?: HttpsClientMessage): Promise<HttpsClientSuccess | HttpsClientError> => {
     const options = this.paypaySetupOptions(endpoint, payload);
     return new Promise((resolve) => {
-      httpsClient.httpsCall(options, payload, (result) => {
+      this.httpsClient.httpsCall(options, payload, (result) => {
         resolve(result);
         if (callback !== undefined) {
           callback(result);
diff --git a/test/accountLinkQRCodeCreate.test.ts b/test/accountLinkQRCodeCreate.test.ts
index 42463b27..c42837df 100644
--- a/test/accountLinkQRCodeCreate.test.ts
+++ b/test/accountLinkQRCodeCreate.test.ts
@@ -1,5 +1,5 @@
 import { payPayRestSDK } from "../src/lib/paypay-rest-sdk";
-import { httpsClient } from '../src/lib/httpsClient';
+import { HttpsClient } from '../src/lib/httpsClient';
 
 const conf = {
     clientId: '5345435fsdfsr54353454',
@@ -10,6 +10,9 @@ const conf = {
 
 payPayRestSDK.configure(conf);
 
+const httpsClient = new HttpsClient();
+payPayRestSDK.setHttpsClient(httpsClient);
+
 test('Unit Test - Account Link Create QR code', async () => {
 
     const response = {
diff --git a/test/cancelPayment.test.ts b/test/cancelPayment.test.ts
index e7d6edf8..9e965514 100644
--- a/test/cancelPayment.test.ts
+++ b/test/cancelPayment.test.ts
@@ -1,5 +1,5 @@
 import { payPayRestSDK } from "../src/lib/paypay-rest-sdk";
-import { httpsClient } from '../src/lib/httpsClient';
+import { HttpsClient } from '../src/lib/httpsClient';
 
 const conf = {
     clientId: '5345435fsdfsr54353454',
@@ -10,6 +10,9 @@ const conf = {
 
 payPayRestSDK.configure(conf);
 
+const httpsClient = new HttpsClient();
+payPayRestSDK.setHttpsClient(httpsClient);
+
 test('Unit Test - Cancel Payment', async () => {
 
     const merchantPaymentId = [12393849];
diff --git a/test/cancelPendingOrder.test.ts b/test/cancelPendingOrder.test.ts
index 85aeb388..937ffd84 100644
--- a/test/cancelPendingOrder.test.ts
+++ b/test/cancelPendingOrder.test.ts
@@ -1,5 +1,5 @@
 import { payPayRestSDK } from "../src/lib/paypay-rest-sdk";
-import { httpsClient } from '../src/lib/httpsClient';
+import { HttpsClient } from '../src/lib/httpsClient';
 
 const conf = {
     clientId: '5345435fsdfsr54353454',
@@ -10,6 +10,9 @@ const conf = {
 
 payPayRestSDK.configure(conf);
 
+const httpsClient = new HttpsClient();
+payPayRestSDK.setHttpsClient(httpsClient);
+
 test('Unit Test - Cancel Pending Order', async () => {
 
     const merchantPaymentId = [12393849];
diff --git a/test/cashback.test.ts b/test/cashback.test.ts
index 7c8ce7c5..49c8b581 100644
--- a/test/cashback.test.ts
+++ b/test/cashback.test.ts
@@ -1,5 +1,5 @@
 import { payPayRestSDK } from "../src/lib/paypay-rest-sdk";
-import { httpsClient } from '../src/lib/httpsClient';
+import { HttpsClient } from '../src/lib/httpsClient';
 const { v4: uuidv4 } = require('uuid');
 
 const conf = {
@@ -11,6 +11,9 @@ const conf = {
 
 payPayRestSDK.configure(conf);
 
+const httpsClient = new HttpsClient();
+payPayRestSDK.setHttpsClient(httpsClient);
+
 test('Unit Test - Cash Back', async () => {
 
     const payload = {
diff --git a/test/checkCashBackDetails.test.ts b/test/checkCashBackDetails.test.ts
index 1c26fe84..584441ef 100644
--- a/test/checkCashBackDetails.test.ts
+++ b/test/checkCashBackDetails.test.ts
@@ -1,5 +1,5 @@
 import { payPayRestSDK } from "../src/lib/paypay-rest-sdk";
-import { httpsClient } from '../src/lib/httpsClient';
+import { HttpsClient } from '../src/lib/httpsClient';
 
 const conf = {
     clientId: 'testId',
@@ -10,13 +10,35 @@ const conf = {
 
 payPayRestSDK.configure(conf);
 
+const httpsClient = new HttpsClient();
+payPayRestSDK.setHttpsClient(httpsClient);
+
 test('Unit Test - Check cashback details', async () => {
 
     const merchantPaymentId = [12393849];
     const response = {
         STATUS: 200,
-        BODY: '{"resultInfo":{"code":"SUCCESS","message":"Success","codeId":"08100001"},"data":{"status":"SUCCESS","acceptedAt":1611747653,"merchantAlias":"test","amount":{"amount":1,"currency":"JPY"},"requestedAt":1611747650,"metadata":"","cashbackId":"test","merchantCashbackId":"test","userAuthorizationId":"test","orderDescription":"order description","walletType":"PREPAID"}}'
-    }
+        BODY: {
+            "resultInfo": {
+                "code": "SUCCESS",
+                "message": "Success",
+                "codeId": "08100001",
+            },
+            "data": {
+                "status": "SUCCESS",
+                "acceptedAt": 1611747653,
+                "merchantAlias": "test",
+                "amount": { "amount": 1, "currency": "JPY" },
+                "requestedAt": 1611747650,
+                "metadata": "",
+                "cashbackId": "test",
+                "merchantCashbackId": "test",
+                "userAuthorizationId": "test",
+                "orderDescription": "order description",
+                "walletType": "PREPAID",
+            },
+        },
+    };
 
     const mockHttpsCall = jest.spyOn(httpsClient, 'httpsCall');
     mockHttpsCall.mockImplementation(jest.fn((_options: any, _payload = '', _callback: any) => {
diff --git a/test/checkCashBackReversalDetails.test.ts b/test/checkCashBackReversalDetails.test.ts
index cf1a23cd..f94dd790 100644
--- a/test/checkCashBackReversalDetails.test.ts
+++ b/test/checkCashBackReversalDetails.test.ts
@@ -1,5 +1,5 @@
 import { payPayRestSDK } from "../src/lib/paypay-rest-sdk";
-import { httpsClient } from '../src/lib/httpsClient';
+import { HttpsClient } from '../src/lib/httpsClient';
 
 const conf = {
     clientId: 'test',
@@ -10,13 +10,33 @@ const conf = {
 
 payPayRestSDK.configure(conf);
 
+const httpsClient = new HttpsClient();
+payPayRestSDK.setHttpsClient(httpsClient);
+
 test('Unit Test - Check cashback reversal details', async () => {
 
     const merchantPaymentId = [12393849];
     const response = {
         STATUS: 200,
-        BODY: '{"resultInfo":{"code":"SUCCESS","message":"Success","codeId":"08100001"},"data":{"status":"SUCCESS","acceptedAt":1611747702,"merchantAlias":"test","amount":{"amount":1,"currency":"JPY"},"requestedAt":1611747699,"metadata":"","cashbackReversalId":"test","merchantCashbackReversalId":"test","merchantCashbackId":"test"}}'
-    }
+        BODY: {
+            "resultInfo": {
+                "code": "SUCCESS",
+                "message": "Success",
+                "codeId": "08100001",
+            },
+            "data": {
+                "status": "SUCCESS",
+                "acceptedAt": 1611747702,
+                "merchantAlias": "test",
+                "amount": { "amount": 1, "currency": "JPY" },
+                "requestedAt": 1611747699,
+                "metadata": "",
+                "cashbackReversalId": "test",
+                "merchantCashbackReversalId": "test",
+                "merchantCashbackId": "test",
+            },
+        },
+    };
 
     const mockHttpsCall = jest.spyOn(httpsClient, 'httpsCall');
     mockHttpsCall.mockImplementation(jest.fn((_options: any, _payload = '', _callback: any) => {
diff --git a/test/createPayment.test.ts b/test/createPayment.test.ts
index 9f2f8476..63528088 100644
--- a/test/createPayment.test.ts
+++ b/test/createPayment.test.ts
@@ -1,5 +1,5 @@
 import { payPayRestSDK } from "../src/lib/paypay-rest-sdk";
-import { httpsClient } from '../src/lib/httpsClient';
+import { HttpsClient } from '../src/lib/httpsClient';
 const { v4: uuidv4 } = require('uuid');
 
 const conf = {
@@ -11,6 +11,9 @@ const conf = {
 
 payPayRestSDK.configure(conf);
 
+const httpsClient = new HttpsClient();
+payPayRestSDK.setHttpsClient(httpsClient);
+
 test('Unit Test - Create Payment', async () => {
 
     const payload = {
@@ -39,7 +42,7 @@ test('Unit Test - Create Payment', async () => {
             "assumeMerchant": "1609155081"
         }
     }
-    const expected = { STATUS: 200, BODY: JSON.stringify(response) };
+    const expected = { STATUS: 200, BODY: response };
 
     const mockHttpsCall = jest.spyOn(httpsClient, 'httpsCall');
     mockHttpsCall.mockImplementation(jest.fn((_options: any, _payload = '', _callback: any) => {
@@ -85,7 +88,7 @@ test('Unit Test - Create Payment with agreeSimilarTransaction=false', async () =
             "assumeMerchant": "1609155081"
         }
     }
-    const expected = { STATUS: 200, BODY: JSON.stringify(response) };
+    const expected = { STATUS: 200, BODY: response };
 
     const mockHttpsCall = jest.spyOn(httpsClient, 'httpsCall');
     mockHttpsCall.mockImplementation(jest.fn((_options: any, _payload = '', _callback: any) => {
@@ -131,7 +134,7 @@ test('Unit Test - Create Payment with agreeSimilarTransaction=true', async () =>
             "assumeMerchant": "1609155081"
         }
     }
-    const expected = { STATUS: 200, BODY: JSON.stringify(response) };
+    const expected = { STATUS: 200, BODY: response };
 
     const mockHttpsCall = jest.spyOn(httpsClient, 'httpsCall');
     mockHttpsCall.mockImplementation(jest.fn((_options: any, _payload = '', _callback: any) => {
diff --git a/test/createPendingPayment.test.ts b/test/createPendingPayment.test.ts
index 16772b6f..6beea37f 100644
--- a/test/createPendingPayment.test.ts
+++ b/test/createPendingPayment.test.ts
@@ -1,5 +1,5 @@
 import { payPayRestSDK } from "../src/lib/paypay-rest-sdk";
-import { httpsClient } from '../src/lib/httpsClient';
+import { HttpsClient } from '../src/lib/httpsClient';
 
 const conf = {
     clientId: '5345435fsdfsr54353454',
@@ -10,6 +10,9 @@ const conf = {
 
 payPayRestSDK.configure(conf);
 
+const httpsClient = new HttpsClient();
+payPayRestSDK.setHttpsClient(httpsClient);
+
 test('Unit Test - Create Pending Payment', async () => {
 
     const payload = {
diff --git a/test/getCodePaymentDetails.test.ts b/test/getCodePaymentDetails.test.ts
index 4a841cd4..d87880ba 100644
--- a/test/getCodePaymentDetails.test.ts
+++ b/test/getCodePaymentDetails.test.ts
@@ -1,5 +1,5 @@
 import { payPayRestSDK } from "../src/lib/paypay-rest-sdk";
-import { httpsClient } from '../src/lib/httpsClient';
+import { HttpsClient } from '../src/lib/httpsClient';
 
 const conf = {
     clientId: '5345435fsdfsr54353454',
@@ -10,6 +10,9 @@ const conf = {
 
 payPayRestSDK.configure(conf);
 
+const httpsClient = new HttpsClient();
+payPayRestSDK.setHttpsClient(httpsClient);
+
 test('Unit Test - Get Code Payment Details', async () => {
 
     const merchantPaymentId = [12393849];
diff --git a/test/getPaymentDetails.test.ts b/test/getPaymentDetails.test.ts
index 52d3057d..68404153 100644
--- a/test/getPaymentDetails.test.ts
+++ b/test/getPaymentDetails.test.ts
@@ -1,5 +1,5 @@
 import { payPayRestSDK } from "../src/lib/paypay-rest-sdk";
-import { httpsClient } from '../src/lib/httpsClient';
+import { HttpsClient } from '../src/lib/httpsClient';
 
 const conf = {
     clientId: '5345435fsdfsr54353454',
@@ -10,6 +10,9 @@ const conf = {
 
 payPayRestSDK.configure(conf);
 
+const httpsClient = new HttpsClient();
+payPayRestSDK.setHttpsClient(httpsClient);
+
 test('Unit Test - Get Payment Details', async () => {
 
     const merchantPaymentId = [12393849];
diff --git a/test/getPendingPaymentDetails.test.ts b/test/getPendingPaymentDetails.test.ts
index 61b2d9a7..b34ac7f7 100644
--- a/test/getPendingPaymentDetails.test.ts
+++ b/test/getPendingPaymentDetails.test.ts
@@ -1,5 +1,5 @@
 import { payPayRestSDK } from "../src/lib/paypay-rest-sdk";
-import { httpsClient } from '../src/lib/httpsClient';
+import { HttpsClient } from '../src/lib/httpsClient';
 
 const conf = {
     clientId: '5345435fsdfsr54353454',
@@ -10,6 +10,9 @@ const conf = {
 
 payPayRestSDK.configure(conf);
 
+const httpsClient = new HttpsClient();
+payPayRestSDK.setHttpsClient(httpsClient);
+
 test('Unit Test - Get Pending Order Details', async () => {
 
     const merchantPaymentId = [12393849];
diff --git a/test/getRefundDetails.test.ts b/test/getRefundDetails.test.ts
index ea72def3..f6282725 100644
--- a/test/getRefundDetails.test.ts
+++ b/test/getRefundDetails.test.ts
@@ -1,5 +1,5 @@
 import { payPayRestSDK } from "../src/lib/paypay-rest-sdk";
-import { httpsClient } from '../src/lib/httpsClient';
+import { HttpsClient } from '../src/lib/httpsClient';
 
 const conf = {
     clientId: '5345435fsdfsr54353454',
@@ -10,6 +10,9 @@ const conf = {
 
 payPayRestSDK.configure(conf);
 
+const httpsClient = new HttpsClient();
+payPayRestSDK.setHttpsClient(httpsClient);
+
 test('Unit Test - Get Refund Details', async () => {
 
     const merchantRefundId = [34243242];
diff --git a/test/getUserAuthorizationStatus.test.ts b/test/getUserAuthorizationStatus.test.ts
index 2a2a4d4b..49b78251 100644
--- a/test/getUserAuthorizationStatus.test.ts
+++ b/test/getUserAuthorizationStatus.test.ts
@@ -1,5 +1,5 @@
 import { payPayRestSDK } from "../src/lib/paypay-rest-sdk";
-import { httpsClient } from '../src/lib/httpsClient';
+import { HttpsClient } from '../src/lib/httpsClient';
 
 const conf = {
     clientId: '5345435fsdfsr54353454',
@@ -10,6 +10,9 @@ const conf = {
 
 payPayRestSDK.configure(conf);
 
+const httpsClient = new HttpsClient();
+payPayRestSDK.setHttpsClient(httpsClient);
+
 test('Unit Test - Get User Authorization Status', async () => {
     const userAuthorizationId = [6787435345];
     const response = {
@@ -20,14 +23,14 @@ test('Unit Test - Get User Authorization Status', async () => {
         },
         "data": {
             "userAuthorizationId": "6787435345",
-            "referenceIds": [ ],
+            "referenceIds": [],
             "status": "ACTIVE",
             "scopes": [
-            "continuous_payments"
+                "continuous_payments"
             ],
             "expireAt": 34534543534,
             "issuedAt": 45353535343
-            }
+        }
     };
 
     const mockHttpsCall = jest.spyOn(httpsClient, 'httpsCall');
diff --git a/test/httpsClient.test.ts b/test/httpsClient.test.ts
index 987a3ebd..3116e795 100644
--- a/test/httpsClient.test.ts
+++ b/test/httpsClient.test.ts
@@ -1,4 +1,6 @@
-import { httpsClient } from "../src/lib/httpsClient";
+import { HttpsClient } from "../src/lib/httpsClient";
+
+const httpsClient = new HttpsClient();
 
 jest.mock("https", () => ({
     __esModule: true,
@@ -13,12 +15,67 @@ jest.mock("https", () => ({
 
 var httpsRequestMock: any;
 
+test("httpsClient.httpsCall 200 handle empty response", () => {
+    jest.clearAllMocks();
+
+    const options = { apiKey: "options", path: "/some/path", method: "POST" };
+    const payload = { payload: true };
+
+    const expectedBodyChunks: string[] = [];
+    const expected = { STATUS: 200, BODY: null };
+
+    const callback = jest.fn();
+
+    httpsClient.httpsCall(options, payload, callback);
+
+    expect(httpsRequestMock).toHaveBeenCalledWith({ path: "/some/path", method: "POST" }, expect.anything());
+    expect("apiKey" in options).toEqual(false);
+
+    const req = httpsRequestMock.mock.results[0].value;
+
+    // Error listener must be set up.
+    expect(req.on).toHaveBeenCalledWith("error", expect.anything());
+    expect(req.write).toHaveBeenCalledWith(JSON.stringify(payload));
+    expect(req.end).toHaveBeenCalled();
+
+    const res = {
+        statusCode: 200,
+        setEncoding: jest.fn(),
+        on: jest.fn(),
+    };
+    const resCallback = httpsRequestMock.mock.calls[0][1];
+    resCallback(res);
+
+    expect(res.setEncoding).toHaveBeenCalledWith("utf8");
+    expect(res.on).toHaveBeenCalledWith("data", expect.anything());
+    expect(res.on).toHaveBeenCalledWith("end", expect.anything());
+
+    // The data is received chunk-by-chunk.
+    const onDataCallback = res.on.mock.calls[0];
+    expect(onDataCallback[0]).toEqual("data");
+    for (const chunk of expectedBodyChunks) {
+        onDataCallback[1](chunk);
+
+        // The callback is not invoked until the response is completed.
+        expect(callback).toHaveBeenCalledTimes(0);
+    }
+
+    const onEndCallback = res.on.mock.calls[1];
+    expect(onEndCallback[0]).toEqual("end");
+    onEndCallback[1]();
+
+    // The passed callback is invoked after the on-end event.
+    expect(callback).toHaveBeenCalledWith(expected);
+});
+
 test("httpsClient.httpsCall POST 200", () => {
+    jest.clearAllMocks();
+
     const options = { apiKey: "options", path: "/some/path", method: "POST" };
     const payload = { payload: true };
 
-    const expectedBodyChunks = ["ab", "cdef", "g", "hij"];
-    const expected = { STATUS: 200, BODY: expectedBodyChunks.join("") };
+    const expectedBodyChunks = ["{", "\"", "k", "\": ", "1}"];
+    const expected = { STATUS: 200, BODY: { k: 1 } };
 
     const callback = jest.fn();
 
@@ -62,4 +119,72 @@ test("httpsClient.httpsCall POST 200", () => {
 
     // The passed callback is invoked after the on-end event.
     expect(callback).toHaveBeenCalledWith(expected);
+
+    // Prior to version 2, the SDK returned a string instead of an object.
+    // For compatibility, ensure that `JSON.parse(response.BODY)` still works.
+    const call = callback.mock.calls[0];
+    expect(JSON.parse(call[0].BODY)).toEqual(expected.BODY);
+});
+
+test("httpsClient.httpsCall POST 400", () => {
+    jest.clearAllMocks();
+
+    const consoleLogSpy = jest.spyOn(console, "log").mockImplementation(() => { });
+
+    const options = { apiKey: "options", path: "/some/path", method: "POST" };
+    const payload = { payload: true };
+
+    const expectedBodyChunks = [`{"resultInfo": {"code": "CODE", "codeId": "CODEID"}}`];
+    const expected = {
+        STATUS: 400,
+        BODY: { "resultInfo": { "code": "CODE", "codeId": "CODEID" } },
+    };
+
+    const callback = jest.fn();
+
+    httpsClient.httpsCall(options, payload, callback);
+
+    expect(httpsRequestMock).toHaveBeenCalledWith({ path: "/some/path", method: "POST" }, expect.anything());
+    expect("apiKey" in options).toEqual(false);
+
+    const req = httpsRequestMock.mock.results[0].value;
+
+    // Error listener must be set up.
+    expect(req.on).toHaveBeenCalledWith("error", expect.anything());
+    expect(req.write).toHaveBeenCalledWith(JSON.stringify(payload));
+    expect(req.end).toHaveBeenCalled();
+
+    const res = {
+        statusCode: 400,
+        setEncoding: jest.fn(),
+        on: jest.fn(),
+    };
+    const resCallback = httpsRequestMock.mock.calls[0][1];
+    resCallback(res);
+
+    expect(res.setEncoding).toHaveBeenCalledWith("utf8");
+    expect(res.on).toHaveBeenCalledWith("data", expect.anything());
+    expect(res.on).toHaveBeenCalledWith("end", expect.anything());
+
+    // The data is received chunk-by-chunk.
+    const onDataCallback = res.on.mock.calls[0];
+    expect(onDataCallback[0]).toEqual("data");
+    for (const chunk of expectedBodyChunks) {
+        onDataCallback[1](chunk);
+
+        // The callback is not invoked until the response is completed.
+        expect(callback).toHaveBeenCalledTimes(0);
+    }
+
+    const onEndCallback = res.on.mock.calls[1];
+    expect(onEndCallback[0]).toEqual("end");
+    onEndCallback[1]();
+
+    // The passed callback is invoked after the on-end event.
+    expect(callback).toHaveBeenCalledWith(expected);
+
+    // Expect a console log to have been produced.
+    expect(consoleLogSpy).toHaveBeenCalled();
+    expect(consoleLogSpy.mock.calls[0][0]).toContain("https://developer.paypay.ne.jp/develop/resolve?api_name=options&code=CODE&code_id=CODEID");
+    consoleLogSpy.mockRestore();
 });
diff --git a/test/paymentAuthCapture.test.ts b/test/paymentAuthCapture.test.ts
index 693e84ad..d08fe2ce 100644
--- a/test/paymentAuthCapture.test.ts
+++ b/test/paymentAuthCapture.test.ts
@@ -1,5 +1,5 @@
 import { payPayRestSDK } from "../src/lib/paypay-rest-sdk";
-import { httpsClient } from '../src/lib/httpsClient';
+import { HttpsClient } from '../src/lib/httpsClient';
 
 const conf = {
     clientId: '5345435fsdfsr54353454',
@@ -10,6 +10,9 @@ const conf = {
 
 payPayRestSDK.configure(conf);
 
+const httpsClient = new HttpsClient();
+payPayRestSDK.setHttpsClient(httpsClient);
+
 test('Unit Test - Payment Auth Capture', async () => {
 
     const payload = {
@@ -20,7 +23,7 @@ test('Unit Test - Payment Auth Capture', async () => {
         },
         merchantCaptureId: "3432424",
         requestedAt: 324234234,
-        orderDescription: "Test Description"    
+        orderDescription: "Test Description",
     };
 
     const response = {
diff --git a/test/paymentAuthRevert.test.ts b/test/paymentAuthRevert.test.ts
index b6d79cd5..01621af9 100644
--- a/test/paymentAuthRevert.test.ts
+++ b/test/paymentAuthRevert.test.ts
@@ -1,5 +1,5 @@
 import { payPayRestSDK } from "../src/lib/paypay-rest-sdk";
-import { httpsClient } from '../src/lib/httpsClient';
+import { HttpsClient } from '../src/lib/httpsClient';
 
 const conf = {
     clientId: '5345435fsdfsr54353454',
@@ -10,6 +10,9 @@ const conf = {
 
 payPayRestSDK.configure(conf);
 
+const httpsClient = new HttpsClient();
+payPayRestSDK.setHttpsClient(httpsClient);
+
 test('Unit Test - Payment Auth Revert', async () => {
 
     const payload = {
diff --git a/test/paymentPreauthorize.test.ts b/test/paymentPreauthorize.test.ts
index 3aeab59e..c7405bba 100644
--- a/test/paymentPreauthorize.test.ts
+++ b/test/paymentPreauthorize.test.ts
@@ -1,5 +1,5 @@
 import { payPayRestSDK } from "../src/lib/paypay-rest-sdk";
-import { httpsClient } from '../src/lib/httpsClient';
+import { HttpsClient } from '../src/lib/httpsClient';
 const { v4: uuidv4 } = require('uuid');
 
 const conf = {
@@ -11,6 +11,9 @@ const conf = {
 
 payPayRestSDK.configure(conf);
 
+const httpsClient = new HttpsClient();
+payPayRestSDK.setHttpsClient(httpsClient);
+
 test('Unit Test - Preauthorize Payment', async () => {
 
     const payload = {
@@ -42,10 +45,10 @@ test('Unit Test - Preauthorize Payment', async () => {
 
     const mockHttpsCall = jest.spyOn(httpsClient, 'httpsCall');
     mockHttpsCall.mockImplementation(jest.fn((_options: any, _payload = '', _callback: any) => {
-        _callback({ STATUS: 200, BODY: JSON.stringify(response) });
+        _callback({ STATUS: 200, BODY: response });
     }));
 
-    const expected = { STATUS: 200, BODY: JSON.stringify(response) };
+    const expected = { STATUS: 200, BODY: response };
     const promiseResult = await payPayRestSDK.paymentPreauthorize(payload, (result: any) => {
         expect(result).toEqual(expected);
     });
@@ -88,10 +91,10 @@ test('Unit Test - Preauthorize Payment with agreeSimilarTransaction=false', asyn
 
     const mockHttpsCall = jest.spyOn(httpsClient, 'httpsCall');
     mockHttpsCall.mockImplementation(jest.fn((_options: any, _payload = '', _callback: any) => {
-        _callback({ STATUS: 200, BODY: JSON.stringify(response) });
+        _callback({ STATUS: 200, BODY: response });
     }));
 
-    const expected = { STATUS: 200, BODY: JSON.stringify(response) };
+    const expected = { STATUS: 200, BODY: response };
     const promiseResult = await payPayRestSDK.paymentPreauthorize(payload, false, (result: any) => {
         expect(result).toEqual(expected);
     });
@@ -134,10 +137,10 @@ test('Unit Test - Preauthorize Payment with agreeSimilarTransaction=true', async
 
     const mockHttpsCall = jest.spyOn(httpsClient, 'httpsCall');
     mockHttpsCall.mockImplementation(jest.fn((_options: any, _payload = '', _callback: any) => {
-        _callback({ STATUS: 200, BODY: JSON.stringify(response) });
+        _callback({ STATUS: 200, BODY: response });
     }));
 
-    const expected = { STATUS: 200, BODY: JSON.stringify(response) };
+    const expected = { STATUS: 200, BODY: response };
     const promiseResult = await payPayRestSDK.paymentPreauthorize(payload, true, (result: any) => {
         expect(result).toEqual(expected);
     });
diff --git a/test/paymentSubscription.test.ts b/test/paymentSubscription.test.ts
index 98693e2e..bf2eb96c 100644
--- a/test/paymentSubscription.test.ts
+++ b/test/paymentSubscription.test.ts
@@ -1,5 +1,5 @@
 import { payPayRestSDK } from "../src/lib/paypay-rest-sdk";
-import { httpsClient } from '../src/lib/httpsClient';
+import { HttpsClient } from '../src/lib/httpsClient';
 
 const conf = {
     clientId: '5345435fsdfsr54353454',
@@ -10,6 +10,9 @@ const conf = {
 
 payPayRestSDK.configure(conf);
 
+const httpsClient = new HttpsClient();
+payPayRestSDK.setHttpsClient(httpsClient);
+
 test('Unit Test - Payment Subscription', async () => {
 
     const payload = {
diff --git a/test/qrCodeCreate.test.ts b/test/qrCodeCreate.test.ts
index ec13bc75..12b4ab1c 100644
--- a/test/qrCodeCreate.test.ts
+++ b/test/qrCodeCreate.test.ts
@@ -1,5 +1,5 @@
 import { payPayRestSDK } from "../src/lib/paypay-rest-sdk";
-import { httpsClient } from '../src/lib/httpsClient';
+import { HttpsClient } from '../src/lib/httpsClient';
 
 const conf = {
     clientId: '5345435fsdfsr54353454',
@@ -10,6 +10,9 @@ const conf = {
 
 payPayRestSDK.configure(conf);
 
+const httpsClient = new HttpsClient();
+payPayRestSDK.setHttpsClient(httpsClient);
+
 test('Unit Test - Create QR code', async () => {
 
     const payload = {
diff --git a/test/qrCodeCreateWithMerchantNotSpecified.test.ts b/test/qrCodeCreateWithMerchantNotSpecified.test.ts
index 023f5e1f..e726cd21 100644
--- a/test/qrCodeCreateWithMerchantNotSpecified.test.ts
+++ b/test/qrCodeCreateWithMerchantNotSpecified.test.ts
@@ -1,5 +1,5 @@
 import { payPayRestSDK } from "../src/lib/paypay-rest-sdk";
-import { httpsClient } from '../src/lib/httpsClient';
+import { HttpsClient } from '../src/lib/httpsClient';
 
 const conf = {
     clientId: '5345435fsdfsr54353454',
@@ -9,6 +9,9 @@ const conf = {
 
 payPayRestSDK.configure(conf);
 
+const httpsClient = new HttpsClient();
+payPayRestSDK.setHttpsClient(httpsClient);
+
 test('Unit Test - Create QR code without specifying the merchant id', async () => {
 
     const payload = {
diff --git a/test/qrCodeDelete.test.ts b/test/qrCodeDelete.test.ts
index 65663d46..3bb89537 100644
--- a/test/qrCodeDelete.test.ts
+++ b/test/qrCodeDelete.test.ts
@@ -1,5 +1,5 @@
 import { payPayRestSDK } from "../src/lib/paypay-rest-sdk";
-import { httpsClient } from '../src/lib/httpsClient';
+import { HttpsClient } from '../src/lib/httpsClient';
 
 const conf = {
     clientId: '5345435fsdfsr54353454',
@@ -10,6 +10,9 @@ const conf = {
 
 payPayRestSDK.configure(conf);
 
+const httpsClient = new HttpsClient();
+payPayRestSDK.setHttpsClient(httpsClient);
+
 test('Unit Test - Delete QR code', async () => {
 
     const qrCodeId = [874878];
diff --git a/test/refundPayment.test.ts b/test/refundPayment.test.ts
index b4507824..17c28895 100644
--- a/test/refundPayment.test.ts
+++ b/test/refundPayment.test.ts
@@ -1,5 +1,5 @@
 import { payPayRestSDK } from "../src/lib/paypay-rest-sdk";
-import { httpsClient } from '../src/lib/httpsClient';
+import { HttpsClient } from '../src/lib/httpsClient';
 
 const conf = {
     clientId: '5345435fsdfsr54353454',
@@ -10,6 +10,9 @@ const conf = {
 
 payPayRestSDK.configure(conf);
 
+const httpsClient = new HttpsClient();
+payPayRestSDK.setHttpsClient(httpsClient);
+
 test('Unit Test - Refund Payment', async () => {
 
     const payload = {
diff --git a/test/refundPendingPayment.test.ts b/test/refundPendingPayment.test.ts
index 24329569..11abfd0c 100644
--- a/test/refundPendingPayment.test.ts
+++ b/test/refundPendingPayment.test.ts
@@ -1,5 +1,5 @@
 import { payPayRestSDK } from "../src/lib/paypay-rest-sdk";
-import { httpsClient } from '../src/lib/httpsClient';
+import { HttpsClient } from '../src/lib/httpsClient';
 
 const conf = {
     clientId: '5345435fsdfsr54353454',
@@ -10,6 +10,9 @@ const conf = {
 
 payPayRestSDK.configure(conf);
 
+const httpsClient = new HttpsClient();
+payPayRestSDK.setHttpsClient(httpsClient);
+
 test('Unit Test - Refund Pending Payment', async () => {
 
     const payload = {
diff --git a/test/reversalCashBack.test.ts b/test/reversalCashBack.test.ts
index 103f4a0e..94b566ff 100644
--- a/test/reversalCashBack.test.ts
+++ b/test/reversalCashBack.test.ts
@@ -1,5 +1,5 @@
 import { payPayRestSDK } from "../src/lib/paypay-rest-sdk";
-import { httpsClient } from '../src/lib/httpsClient';
+import { HttpsClient } from '../src/lib/httpsClient';
 const { v4: uuidv4 } = require('uuid');
 
 const conf = {
@@ -11,6 +11,9 @@ const conf = {
 
 payPayRestSDK.configure(conf);
 
+const httpsClient = new HttpsClient();
+payPayRestSDK.setHttpsClient(httpsClient);
+
 test('Unit Test - Reversal Cash Back', async () => {
 
     const payload = {
diff --git a/test/unlinkUser.test.ts b/test/unlinkUser.test.ts
index b851e833..918888f5 100644
--- a/test/unlinkUser.test.ts
+++ b/test/unlinkUser.test.ts
@@ -1,5 +1,5 @@
 import { payPayRestSDK } from "../src/lib/paypay-rest-sdk";
-import { httpsClient } from '../src/lib/httpsClient';
+import { HttpsClient } from '../src/lib/httpsClient';
 
 const conf = {
     clientId: '5345435fsdfsr54353454',
@@ -10,6 +10,9 @@ const conf = {
 
 payPayRestSDK.configure(conf);
 
+const httpsClient = new HttpsClient();
+payPayRestSDK.setHttpsClient(httpsClient);
+
 test('Unit Test - Unlink user', async () => {
     const userAuthorizationId = [6787435345];
     const response = {