diff --git a/src/common/actionApi.js b/src/common/actionApi.ts
similarity index 100%
rename from src/common/actionApi.js
rename to src/common/actionApi.ts
diff --git a/src/common/consts.js b/src/common/consts.ts
similarity index 96%
rename from src/common/consts.js
rename to src/common/consts.ts
index 7b1ef78..44e6ae8 100644
--- a/src/common/consts.js
+++ b/src/common/consts.ts
@@ -1,4 +1,12 @@
-export const isoLangs = {
+interface IsoLang {
+  name: string;
+  nativeName: string;
+};
+interface IsoLangs {
+  [key: string]: IsoLang;
+};
+
+export const isoLangs: IsoLangs = {
   'af': {'name': 'Afrikaans', 'nativeName': 'Afrikaans'},
   'ak': {'name': 'Twi', 'nativeName': 'Akan'},
   'am': {'name': 'Amharic', 'nativeName': 'አማርኛ'},
@@ -134,9 +142,13 @@
   'zu': {'name': 'Zulu', 'nativeName': 'isiZulu'},
 };
 
+interface LanguageDictionary {
+  [key: string]: string;
+};
+
 // Some languages were incorrectly set. This map serves as a conversion between
 // the previous wrong languages and the correct code.
-export const convertLanguages = {
+export const convertLanguages: LanguageDictionary = {
   'jv': 'jw',
   'zh': 'zh-CN',
 };
diff --git a/src/common/i18n.js b/src/common/i18n.ts
similarity index 63%
rename from src/common/i18n.js
rename to src/common/i18n.ts
index 053ea9b..38bb0ca 100644
--- a/src/common/i18n.js
+++ b/src/common/i18n.ts
@@ -1,5 +1,3 @@
 // Helper function which serves as a shorter alias to the chrome.i18n.getMessage
 // method, specially useful inside lit templates.
-export function msg(...args) {
-  return chrome.i18n.getMessage(...args);
-}
+export const msg = chrome.i18n.getMessage;
diff --git a/src/common/options.js b/src/common/options.ts
similarity index 70%
rename from src/common/options.js
rename to src/common/options.ts
index 8861881..6268f22 100644
--- a/src/common/options.js
+++ b/src/common/options.ts
@@ -1,6 +1,43 @@
-import {convertLanguages, isoLangs} from './consts.js';
+import {convertLanguages, isoLangs} from './consts';
 
-export const TAB_OPTIONS = [
+type TabOptionValue = ''|'yep'|'popup';
+type DeprecatedTabOptionValue = 'panel';
+type TabOptionValueIncludingDeprecated =
+    TabOptionValue|DeprecatedTabOptionValue;
+
+interface TabOption {
+  value: TabOptionValue;
+  labelMsg: string;
+  deprecatedValues: TabOptionValueIncludingDeprecated[];
+}
+
+interface TargetLangs {
+  [key: string]: string;  // Here the key is a string with a number.
+}
+interface OptionsV0 {
+  translateinto: TargetLangs;
+  uniquetab: TabOptionValue;
+}
+
+interface LegacyLanguages {
+  [key: string]: string;  // Here the key is a string with the language code.
+}
+/**
+ * Backwards-compatible interface for the information available in the sync
+ * storage area.
+ */
+interface LegacyOptions {
+  translateinto: TargetLangs;
+  languages: LegacyLanguages;
+  uniquetab: TabOptionValueIncludingDeprecated;
+}
+
+interface OptionsWrapper {
+  options: OptionsV0;
+  isFirstRun: boolean;
+}
+
+export const TAB_OPTIONS: TabOption[] = [
   // Open in new tab for each translation
   {
     value: '',
@@ -24,22 +61,25 @@
 // Class which can be used to retrieve the user options in order to act
 // accordingly.
 export default class Options {
-  constructor(options, isFirstRun) {
+  _options: OptionsV0;
+  isFirstRun: boolean;
+
+  constructor(options: OptionsV0, isFirstRun: boolean) {
     this._options = options;
     this.isFirstRun = isFirstRun;
   }
 
-  get uniqueTab() {
+  get uniqueTab(): TabOptionValue {
     return this._options.uniquetab;
   }
 
-  get targetLangs() {
+  get targetLangs(): TargetLangs {
     return this._options.translateinto;
   }
 
   // Returns a promise that resolves in an instance of the Object class with the
   // current options.
-  static getOptions(readOnly = true) {
+  static getOptions(readOnly: boolean = true): Promise<Options> {
     return Options.getOptionsRaw(readOnly).then(res => {
       return new Options(res.options, res.isFirstRun);
     });
@@ -53,20 +93,20 @@
   //
   // If the options needed to be normalized/created, they are also saved in the
   // sync storage area.
-  static getOptionsRaw(readOnly) {
+  static getOptionsRaw(readOnly: boolean): Promise<OptionsWrapper> {
     return new Promise((res, rej) => {
-      chrome.storage.sync.get(null, items => {
+      chrome.storage.sync.get(null, itemsAny => {
         if (chrome.runtime.lastError) {
           return rej(chrome.runtime.lastError);
         }
 
+        let items = <LegacyOptions>itemsAny;
         let didTranslateintoChange = false;
         let didUniquetabChange = false;
-        let returnObject = {};
 
         // If the extension sync storage area is blank, set this as being the
         // first run.
-        returnObject.isFirstRun = Object.keys(items).length === 0;
+        let isFirstRun = Object.keys(items).length === 0;
 
         // Create |translateinto| property if it doesn't exist.
         if (items.translateinto === undefined) {
@@ -75,8 +115,9 @@
           // Upgrade from a version previous to v0.7 if applicable, otherwise
           // create the property with the default values.
           if (items.languages !== undefined) {
+            let newTranslateinto: TargetLangs;
             items.translateinto =
-                Object.assign({}, Object.values(items.languages));
+                Object.assign(newTranslateinto, Object.values(items.languages));
           } else {
             let uiLocale = chrome.i18n.getMessage('@@ui_locale');
             let defaultLang1 = uiLocale.replace('_', '-');
@@ -122,20 +163,22 @@
         // value we use now.
         // - If it is set to an incorrect value or it isn't set, change it to
         // the default value.
+        let uniquetabNewValue: TabOptionValue;
         let foundValue = false;
         for (let opt of TAB_OPTIONS) {
           if (opt.value == items?.uniquetab) {
+            uniquetabNewValue = opt.value;
             foundValue = true;
             break;
           }
           if (opt.deprecatedValues.includes(items?.uniquetab)) {
             foundValue = true;
-            items.uniquetab = opt.value;
+            uniquetabNewValue = opt.value;
             break;
           }
         }
         if (!foundValue) {
-          items.uniquetab = 'popup';
+          uniquetabNewValue = 'popup';
           didUniquetabChange = true;
         }
 
@@ -145,17 +188,21 @@
           chrome.storage.sync.remove('languages');
         }
 
+        let returnObject: OptionsWrapper = {
+          isFirstRun,
+          options: {
+            translateinto: items.translateinto,
+            uniquetab: uniquetabNewValue,
+          }
+        };
+
         // Save properties that have changed if we're not in read-only mode
         if (!readOnly) {
           if (didTranslateintoChange || didUniquetabChange) {
-            chrome.storage.sync.set({
-              translateinto: items.translateinto,
-              uniquetab: items.uniquetab,
-            });
+            chrome.storage.sync.set(returnObject.options);
           }
         }
 
-        returnObject.options = items;
         res(returnObject);
       });
     });
diff --git a/src/common/sessionStorage.js b/src/common/sessionStorage.js
deleted file mode 100644
index db46c84..0000000
--- a/src/common/sessionStorage.js
+++ /dev/null
@@ -1,7 +0,0 @@
-// #!if ['chromium_mv3', 'edge_mv3'].includes(browser_target)
-import ExtSessionStorage from './sessionStorage_mv3.js'
-// #!else
-import ExtSessionStorage from './sessionStorage_mv2.js'
-// #!endif
-
-export default ExtSessionStorage;
diff --git a/src/common/sessionStorage.ts b/src/common/sessionStorage.ts
new file mode 100644
index 0000000..450e466
--- /dev/null
+++ b/src/common/sessionStorage.ts
@@ -0,0 +1,7 @@
+// #!if ['chromium_mv3', 'edge_mv3'].includes(browser_target)
+import ExtSessionStorage from './sessionStorage_mv3'
+// #!else
+import ExtSessionStorage from './sessionStorage_mv2'
+// #!endif
+
+export default ExtSessionStorage;
diff --git a/src/common/sessionStorage_mv2.js b/src/common/sessionStorage_mv2.ts
similarity index 80%
rename from src/common/sessionStorage_mv2.js
rename to src/common/sessionStorage_mv2.ts
index 4ad7daf..dda0ae1 100644
--- a/src/common/sessionStorage_mv2.js
+++ b/src/common/sessionStorage_mv2.ts
@@ -1,5 +1,11 @@
+declare global {
+  interface Window {
+    extCustomStorage: any;
+  }
+}
+
 export default class ExtSessionStorage {
-  static set(items) {
+  static set(items: any): Promise<void> {
     return new Promise((res, rej) => {
       if (window.extCustomStorage === undefined) window.extCustomStorage = {};
 
@@ -10,7 +16,7 @@
     });
   }
 
-  static get(keys) {
+  static get(keys: string|Array<string>|undefined): Promise<any> {
     return new Promise((res, rej) => {
       if (window.extCustomStorage === undefined) window.extCustomStorage = {};
 
@@ -25,7 +31,7 @@
       }
 
       if (Array.isArray(keys)) {
-        let returnObject = {};
+        let returnObject: any = {};
         for (const key of keys) {
           returnObject[key] = window.extCustomStorage[key];
         }
diff --git a/src/common/sessionStorage_mv3.js b/src/common/sessionStorage_mv3.js
deleted file mode 100644
index 9def901..0000000
--- a/src/common/sessionStorage_mv3.js
+++ /dev/null
@@ -1,9 +0,0 @@
-export default class ExtSessionStorage {
-  static set(items) {
-    return chrome.storage.session.set(items);
-  }
-
-  static get(keys) {
-    return chrome.storage.session.get(keys);
-  }
-}
diff --git a/src/common/sessionStorage_mv3.ts b/src/common/sessionStorage_mv3.ts
new file mode 100644
index 0000000..befd2a8
--- /dev/null
+++ b/src/common/sessionStorage_mv3.ts
@@ -0,0 +1,9 @@
+export default class ExtSessionStorage {
+  static set(items: any): Promise<void> {
+    return chrome.storage.session.set(items);
+  }
+
+  static get(keys?: string|string[]|undefined): Promise<any> {
+    return chrome.storage.session.get(keys);
+  }
+}