diff --git a/__tests__/deepL.test.ts b/__tests__/deepL.test.ts
new file mode 100644
index 0000000..6b57ba6
--- /dev/null
+++ b/__tests__/deepL.test.ts
@@ -0,0 +1,209 @@
+// Jest tests for deepLTranslate() and deepLGetLanguages()
+
+import {
+  deepLTranslate,
+  deepLGetLanguages,
+  DeepLSupportedLanguages,
+  DeepLLanguageType,
+} from '../src/sheetsl';
+
+const ADDON_NAME = 'SheetsL';
+const DEEPL_API_BASE_URL_FREE = 'https://api-free.deepl.com/v2/';
+const translateUrl = DEEPL_API_BASE_URL_FREE + 'translate';
+const getLanguageUrl = DEEPL_API_BASE_URL_FREE + 'languages';
+const mockApiKey = 'apiKeyString:fx';
+
+PropertiesService.getUserProperties = jest.fn(() => ({
+  getProperty: jest.fn(() => mockApiKey),
+})) as any;
+
+UrlFetchApp.fetch = jest.fn((url: string, options: UrlFetchAppOptions) => ({
+  _options: options,
+  getResponseCode: jest.fn((): number => 200),
+  getContentText: jest.fn((text: string = url) => {
+    // Switch the returning value of the mock function for UrlFetchApp.fetch
+    // depending on the input URL.
+    if (text.startsWith(translateUrl)) {
+      // URL with the endpoint of DeepL API translation will return
+      // a stringified object of type DeepLTranslationResponse
+      return JSON.stringify({
+        translations: [
+          {
+            detected_source_language: 'JA',
+            text: text, // This mock translated text will return the string of the input URL for UrlFetchApp.fetch
+          },
+        ],
+      });
+    } else if (text.startsWith(getLanguageUrl)) {
+      // URL with the endpoint of DeepL API to retrieve the list of supported languages
+      // will return a stringified list of type DeepLSupportedLanguages objects.
+      return JSON.stringify([
+        {
+          language: 'EN-US',
+          name: 'English (American)',
+          supports_formality: false,
+        },
+        {
+          language: 'JA',
+          name: 'Japanese',
+          supports_formality: false,
+        },
+        {
+          language: 'MOCK',
+          name: text, // This mock language will return the string of the input URL for UrlFetchApp.fetch
+          supports_formality: true,
+        },
+      ]);
+    }
+  }),
+})) as any;
+
+type UrlFetchAppOptions = {
+  muteHttpExceptions: boolean;
+};
+
+type DeepLTranslatePattern = {
+  title: string;
+  input: DeepLTranslatePatternInput;
+  expectedOutput: string[];
+};
+
+type DeepLTranslatePatternInput = {
+  sourceText: string | string[];
+  sourceLocale: string | null | undefined;
+  targetLocale: string;
+};
+
+type DeepLGetLanguages = {
+  title: string;
+  input: DeepLLanguageType;
+  expectedOutput: DeepLSupportedLanguages[];
+};
+
+const deepLTranslatePatterns: DeepLTranslatePattern[] = [
+  {
+    title: 'sourceText as string',
+    input: {
+      sourceText: 'text to translate',
+      sourceLocale: 'JA',
+      targetLocale: 'EN-US',
+    },
+    expectedOutput: [
+      `${translateUrl}?auth_key=${mockApiKey}&target_lang=EN-US&text=${encodeURIComponent(
+        'text to translate'
+      )}&source_lang=JA`,
+    ],
+  },
+  {
+    title: 'sourceText as an array of strings',
+    input: {
+      sourceText: [
+        'text to translate 1',
+        'text to translate 2',
+        'text to translate 3',
+      ],
+      sourceLocale: 'JA',
+      targetLocale: 'EN-US',
+    },
+    expectedOutput: [
+      `${translateUrl}?auth_key=${mockApiKey}&target_lang=EN-US&text=${encodeURIComponent(
+        'text to translate 1'
+      )}&text=${encodeURIComponent(
+        'text to translate 2'
+      )}&text=${encodeURIComponent('text to translate 3')}&source_lang=JA`,
+    ],
+  },
+];
+
+const deepLTranslatePatternsWithErrors: DeepLTranslatePattern[] = [
+  {
+    title: 'Empty sourceText',
+    input: {
+      sourceText: '',
+      sourceLocale: 'JA',
+      targetLocale: 'EN-US',
+    },
+    expectedOutput: ['returns an error'],
+  },
+];
+
+const deepLGetLanguagesPatterns: DeepLGetLanguages[] = [
+  {
+    title: 'type = source',
+    input: 'source',
+    expectedOutput: [
+      {
+        language: 'EN-US',
+        name: 'English (American)',
+        supports_formality: false,
+      },
+      {
+        language: 'JA',
+        name: 'Japanese',
+        supports_formality: false,
+      },
+      {
+        language: 'MOCK',
+        name: `${getLanguageUrl}?auth_key=${mockApiKey}&type=source`,
+        supports_formality: true,
+      },
+    ],
+  },
+  {
+    title: 'type = target',
+    input: 'target',
+    expectedOutput: [
+      {
+        language: 'EN-US',
+        name: 'English (American)',
+        supports_formality: false,
+      },
+      {
+        language: 'JA',
+        name: 'Japanese',
+        supports_formality: false,
+      },
+      {
+        language: 'MOCK',
+        name: `${getLanguageUrl}?auth_key=${mockApiKey}&type=target`,
+        supports_formality: true,
+      },
+    ],
+  },
+];
+
+describe.each(deepLTranslatePatterns)(
+  'deepLTranslate',
+  ({ title, input, expectedOutput }) => {
+    test(`deepLTranslate test: ${title}`, () => {
+      expect(
+        deepLTranslate(input.sourceText, input.sourceLocale, input.targetLocale)
+      ).toEqual(expectedOutput);
+    });
+  }
+);
+
+// Error patterns in deepLTranslate
+describe.each(deepLTranslatePatternsWithErrors)(
+  'deepLTranslate Errors',
+  ({ title, input }) => {
+    test(`deepLTranslate error test: ${title}`, () => {
+      expect(() => {
+        deepLTranslate(
+          input.sourceText,
+          input.sourceLocale,
+          input.targetLocale
+        );
+      }).toThrowError(new Error(`[${ADDON_NAME}] Empty input.`));
+    });
+  }
+);
+
+describe.each(deepLGetLanguagesPatterns)(
+  'deepLGetLanguages',
+  ({ title, input, expectedOutput }) => {
+    test(`deepLGetLanguages test: ${title}`, () => {
+      expect(deepLGetLanguages(input)).toEqual(expectedOutput);
+    });
+  }
+);
diff --git a/__tests__/getDeepLApiBaseUrl.test.ts b/__tests__/getDeepLApiBaseUrl.test.ts
index 757c8d2..2a8b75f 100644
--- a/__tests__/getDeepLApiBaseUrl.test.ts
+++ b/__tests__/getDeepLApiBaseUrl.test.ts
@@ -1,8 +1,8 @@
-import {
-  DEEPL_API_BASE_URL_FREE,
-  DEEPL_API_BASE_URL_PRO,
-  getDeepLApiBaseUrl,
-} from '../src/sheetsl';
+import { getDeepLApiBaseUrl } from '../src/sheetsl';
+
+const DEEPL_API_VERSION = 'v2';
+const DEEPL_API_BASE_URL_FREE = `https://api-free.deepl.com/${DEEPL_API_VERSION}/`;
+const DEEPL_API_BASE_URL_PRO = `https://api.deepl.com/${DEEPL_API_VERSION}/`;
 
 const patterns = [
   {
diff --git a/__tests__/getDeepLApiKey.test.ts b/__tests__/getDeepLApiKey.test.ts
new file mode 100644
index 0000000..61b2e04
--- /dev/null
+++ b/__tests__/getDeepLApiKey.test.ts
@@ -0,0 +1,21 @@
+import { getDeepLApiKey } from '../src/sheetsl';
+
+const ADDON_NAME = 'SheetsL';
+
+PropertiesService.getUserProperties = jest.fn(() => ({
+  getProperty: jest.fn(() => undefined),
+})) as any;
+
+const testObj = {
+  title:
+    'Case when DeepL API Authentication Key is not saved in the user property',
+  errorMessage: `[${ADDON_NAME}] API Key Unavailable: Set the DeepL API Authentication Key from the Settings > Set Auth Key of the add-on menu.`,
+};
+
+describe('getDeepLApiKey Error', () => {
+  test(testObj.title, () => {
+    expect(() => {
+      getDeepLApiKey();
+    }).toThrowError(new Error(testObj.errorMessage));
+  });
+});
diff --git a/__tests__/handleDeepLErrors.test.ts b/__tests__/handleDeepLErrors.test.ts
new file mode 100644
index 0000000..dd78f2e
--- /dev/null
+++ b/__tests__/handleDeepLErrors.test.ts
@@ -0,0 +1,70 @@
+import { handleDeepLErrors } from '../src/sheetsl';
+
+const ADDON_NAME = 'SheetsL';
+
+const successPattern = {
+  title: 'HTTP Response Code 200',
+  inputResponse: {
+    getResponseCode: () => 200,
+    getContentText: () => 'Testing HTTP Response Code 200',
+  },
+} as any;
+const errorPatterns = [
+  {
+    title: 'HTTP Response Code 429',
+    inputResponse: {
+      getResponseCode: () => 429,
+      getContentText: () => 'Testing HTTP Response Code 429',
+    },
+    expectedErrorMessage: `[${ADDON_NAME}] Too Many Requests: Try again after some time.`,
+  },
+  {
+    title: 'HTTP Response Code 456',
+    inputResponse: {
+      getResponseCode: () => 456,
+      getContentText: () => 'Testing HTTP Response Code 456',
+    },
+    expectedErrorMessage: `[${ADDON_NAME}] Quota Exceeded: The translation limit of your account has been reached.`,
+  },
+  {
+    title: 'HTTP Response Code 500',
+    inputResponse: {
+      getResponseCode: () => 500,
+      getContentText: () => 'Testing HTTP Response Code 500',
+    },
+    expectedErrorMessage: `[${ADDON_NAME}] Temporary errors in the DeepL service. Please retry after waiting for a while.`,
+  },
+  {
+    title: 'HTTP Response Code 501',
+    inputResponse: {
+      getResponseCode: () => 501,
+      getContentText: () => 'Testing HTTP Response Code 501',
+    },
+    expectedErrorMessage: `[${ADDON_NAME}] Temporary errors in the DeepL service. Please retry after waiting for a while.`,
+  },
+  {
+    title: 'HTTP Response Code 300',
+    inputResponse: {
+      getResponseCode: () => 300,
+      getContentText: () => 'Testing HTTP Response Code 300',
+    },
+    expectedErrorMessage: `[${ADDON_NAME}] Error on Calling DeepL API: Testing HTTP Response Code 300`,
+  },
+] as any[];
+
+describe('handleDeepLErrors Success', () => {
+  test(successPattern.title, () => {
+    expect(handleDeepLErrors(successPattern.inputResponse)).toBeUndefined();
+  });
+});
+
+describe.each(errorPatterns)(
+  'handleDeepLErrors Errors',
+  ({ title, inputResponse, expectedErrorMessage }) => {
+    test(title, () => {
+      expect(() => {
+        handleDeepLErrors(inputResponse);
+      }).toThrowError(new Error(expectedErrorMessage));
+    });
+  }
+);
diff --git a/__tests__/onInstall.test.ts b/__tests__/onInstall.test.ts
new file mode 100644
index 0000000..c6af9f4
--- /dev/null
+++ b/__tests__/onInstall.test.ts
@@ -0,0 +1,57 @@
+import { onInstall } from '../src/sheetsl';
+
+const ADDON_NAME = 'SheetsL';
+
+SpreadsheetApp.getUi = jest.fn(() => ({
+  createAddonMenu: jest.fn(() => new MockMenu(true)),
+  createMenu: jest.fn((title: string) => new MockMenu(false, title)),
+})) as any;
+
+class MockMenu {
+  mockMenu: MockMenuObj;
+  isAddonMenu: boolean;
+  constructor(isAddonMenu: boolean, title: string = '') {
+    this.mockMenu = {
+      title: isAddonMenu ? ADDON_NAME : title,
+      menu: [],
+      isAddonMenu: isAddonMenu,
+    };
+  }
+  addItem(itemName: string, functionName: string): this {
+    this.mockMenu.menu.push({ itemName: itemName, functionName: functionName });
+    return this;
+  }
+  addSeparator(): this {
+    this.mockMenu.menu.push('---');
+    return this;
+  }
+  addSubMenu(menu: MockMenuObj): this {
+    this.mockMenu.menu.push(menu);
+    return this;
+  }
+  addToUi(): MockMenuObj {
+    return this.mockMenu;
+  }
+}
+
+type MockMenuObj = {
+  title: string;
+  menu: any[];
+  isAddonMenu: boolean;
+};
+
+describe('onOpen/onInstall', () => {
+  test('onOpen/onInstall test', () => {
+    expect(onInstall).toEqual({
+      title: ADDON_NAME,
+      menu: [
+        {
+          title: 'Settings',
+          menu: [{ itemName: 'Set Auth Key', functionName: 'setDeeplAuthKey' }],
+          isAddonMenu: false,
+        },
+      ],
+      isAddonMenu: true,
+    });
+  });
+});
diff --git a/src/sheetsl.ts b/src/sheetsl.ts
index 0c04ff8..a65e9be 100644
--- a/src/sheetsl.ts
+++ b/src/sheetsl.ts
@@ -13,13 +13,13 @@
    limitations under the License.
  */
 
-export const ADDON_NAME = 'SheetsL';
+const ADDON_NAME = 'SheetsL';
 const UP_KEY_DEEPL_API_KEY = 'deeplApiKey'; // User property key for saving the DeepL API key
 const UP_KEY_SOURCE_LOCALE = 'sourceLocale'; // User property key for saving the source language for DeepL
 const UP_KEY_TARGET_LOCALE = 'targetLocale'; // User property key for saving the target language for DeepL
 const DEEPL_API_VERSION = 'v2'; // DeepL API version
-export const DEEPL_API_BASE_URL_FREE = `https://api-free.deepl.com/${DEEPL_API_VERSION}/`;
-export const DEEPL_API_BASE_URL_PRO = `https://api.deepl.com/${DEEPL_API_VERSION}/`;
+const DEEPL_API_BASE_URL_FREE = `https://api-free.deepl.com/${DEEPL_API_VERSION}/`;
+const DEEPL_API_BASE_URL_PRO = `https://api.deepl.com/${DEEPL_API_VERSION}/`;
 const ROW_SEPARATOR = '|||';
 
 // Threshold value of the length of the text to translate, in bytes. See https://developers.google.com/apps-script/guides/services/quotas#current_limitations
@@ -30,7 +30,7 @@ const THRESHOLD_BYTES = 1900;
  * GET request on /v2/languages returns an array of this object.
  * @see https://www.deepl.com/docs-api/general/get-languages/
  */
-type DeepLSupportedLanguages = {
+export type DeepLSupportedLanguages = {
   language: string;
   name: string;
   supports_formality: boolean;
@@ -59,7 +59,7 @@ type DeepLTranslationObj = {
  * to the DeepL API to retrieve its supported languages.
  * @see https://www.deepl.com/docs-api/general/get-languages/
  */
-type DeepLLanguageType = 'source' | 'target';
+export type DeepLLanguageType = 'source' | 'target';
 
 /**
  * Create add-on menu on opening spreadsheet file.
@@ -341,13 +341,7 @@ export function deepLTranslate(
   // console.log(`sourceTextCasted: ${sourceTextCasted}`);
 
   // API key
-  const apiKey =
-    PropertiesService.getUserProperties().getProperty(UP_KEY_DEEPL_API_KEY);
-  if (!apiKey) {
-    throw new Error(
-      `[${ADDON_NAME}] API Key Unavailable: Set the DeepL API Authentication Key from the Settings > Set Auth Key of the add-on menu.`
-    );
-  }
+  const apiKey = getDeepLApiKey();
   const baseUrl = getDeepLApiBaseUrl(apiKey);
   // Call the DeepL API
   let url =
@@ -389,13 +383,7 @@ export function deepLGetLanguages(
 ): DeepLSupportedLanguages[] {
   const endpoint = 'languages';
   // API key
-  const apiKey =
-    PropertiesService.getUserProperties().getProperty(UP_KEY_DEEPL_API_KEY);
-  if (!apiKey) {
-    throw new Error(
-      `[${ADDON_NAME}] API Key Unavailable: Set the DeepL API Authentication Key from the Settings > Set Auth Key of the add-on menu.`
-    );
-  }
+  const apiKey = getDeepLApiKey();
   const baseUrl = getDeepLApiBaseUrl(apiKey);
   // Call the DeepL API
   let url = baseUrl + endpoint + `?auth_key=${apiKey}&type=${type}`;
@@ -407,19 +395,6 @@ export function deepLGetLanguages(
   return JSON.parse(response.getContentText());
 }
 
-/**
- * Returns the DeepL API base URL. The URL depends on whether the user's account
- * is DeepL API Free or Pro. Auth keys of DeepL API Free end with `:fx`
- * @param apiKey The DeepL API Free/Pro Authentication Key
- * @returns The relevant base URL for DeepL API
- * @see https://support.deepl.com/hc/en-us/articles/360021183620-DeepL-API-Free-vs-DeepL-API-Pro
- */
-export function getDeepLApiBaseUrl(apiKey: string): string {
-  return apiKey.endsWith(':fx')
-    ? DEEPL_API_BASE_URL_FREE
-    : DEEPL_API_BASE_URL_PRO;
-}
-
 /**
  * Get the length of a given string in bytes.
  * @param text The string of which to get the bytes.
@@ -456,3 +431,33 @@ export function handleDeepLErrors(
     );
   }
 }
+
+/**
+ * Get the string of DeepL API Authentication Key saved as a user property of the add-on.
+ * Throws an error if the key is not save in the user property.
+ * @returns The string of DeepL API Authentication Key saved as a user property of the add-on.
+ */
+export function getDeepLApiKey(): string {
+  const apiKey =
+    PropertiesService.getUserProperties().getProperty(UP_KEY_DEEPL_API_KEY);
+  if (!apiKey) {
+    throw new Error(
+      `[${ADDON_NAME}] API Key Unavailable: Set the DeepL API Authentication Key from the Settings > Set Auth Key of the add-on menu.`
+    );
+  } else {
+    return apiKey;
+  }
+}
+
+/**
+ * Returns the DeepL API base URL. The URL depends on whether the user's account
+ * is DeepL API Free or Pro. Auth keys of DeepL API Free end with `:fx`
+ * @param apiKey The DeepL API Free/Pro Authentication Key
+ * @returns The relevant base URL for DeepL API
+ * @see https://support.deepl.com/hc/en-us/articles/360021183620-DeepL-API-Free-vs-DeepL-API-Pro
+ */
+export function getDeepLApiBaseUrl(apiKey: string): string {
+  return apiKey.endsWith(':fx')
+    ? DEEPL_API_BASE_URL_FREE
+    : DEEPL_API_BASE_URL_PRO;
+}