diff --git a/types/i18next/i18next-tests.ts b/types/i18next/i18next-tests.ts
index a96901cbb7..cdf30bb9c1 100644
--- a/types/i18next/i18next-tests.ts
+++ b/types/i18next/i18next-tests.ts
@@ -6,13 +6,14 @@ i18n.init({
         en: {
             translation: {
                 helloWorld: 'Hello, world!',
-                helloWorldInterpolated: 'Hello, {{name}}!'
+                helloWorldInterpolated: 'Hello, {{name}}!',
+                uppercaseFormatted: '{{text^uppercase}} just uppercased',
             }
         },
         ru: {
             translation: {
                 helloWorld: 'Привет, мир!',
-                helloWorldInterpolated: 'Привет, {{name}}!'
+                helloWorldInterpolated: 'Привет, {{name}}!',
             }
         }
     },
@@ -31,23 +32,29 @@ i18n.init({
     contextSeparator: '_',
     saveMissing: true,
     saveMissingTo: 'all',
-    missingKeyHandler: (lng:string, ns:string, key:string, fallbackValue:string) => {
+    missingKeyHandler: (lng: string, ns: string, key: string, fallbackValue: string) => {
         console.log('Lng: ' + lng + ', ns: ' + ns + ', key' + key + ', fallbackValue: ' + fallbackValue);
     },
-    parseMissingKeyHandler: (key:string) => {
+    parseMissingKeyHandler: (key: string) => {
         console.log(key);
     },
     appendNamespaceToMissingKey: true,
     returnNull: false,
     returnEmptyString: false,
     returnObjects: false,
-    returnedObjectHandler: (key:string, value:string, options:any) => {
+    returnedObjectHandler: (key: string, value: string, options: any) => {
     },
     joinArrays: '\n',
-    overloadTranslationOptionHandler: (args:any[]) => {
+    overloadTranslationOptionHandler: (args: any[]) => {
         return <i18n.TranslationOptions>{}
     },
-    interpolation: <i18n.InterpolationOptions>{},
+    interpolation: {
+        format: function(value, format, lng) {
+            if (format === 'uppercase') return value.toUpperCase();
+            return value;
+        },
+        formatSeparator: '^',
+    },
     detection: null,
     backend: null,
     cache: null,
@@ -77,6 +84,10 @@ i18n.t('helloWorldInterpolated', <i18n.TranslationOptions> {
     name: "world"
 });
 
+i18n.t('uppercaseFormatted', <i18n.TranslationOptions> {
+    text: 'can you hear me',
+});
+
 i18n.t('helloSingleFallbackLng', <i18n.TranslationOptions> {
     fallbackLng: 'en'
 });
diff --git a/types/i18next/index.d.ts b/types/i18next/index.d.ts
index 1cacdeb7c3..8ea39a6352 100644
--- a/types/i18next/index.d.ts
+++ b/types/i18next/index.d.ts
@@ -1,11 +1,10 @@
-// Type definitions for i18next v2.3.5
+// Type definitions for i18next v8.4.2
 // Project: http://i18next.com
 // Definitions by: Michael Ledin <https://github.com/mxl>, Budi Irawan <https://github.com/deerawan>
 // Definitions: https://github.com/DefinitelyTyped/DefinitelyTyped
 
 // Sources: https://github.com/i18next/i18next/
 
-
 declare namespace i18n {
     interface ResourceStore {
         [language: string]: ResourceStoreLanguage;
@@ -25,7 +24,12 @@ declare namespace i18n {
 
     type FallbackLng = string | string[] | FallbackLngObjList;
 
+    type FormatFunction = (value: any, format: string, lng: string) => string;
+
     interface InterpolationOptions {
+        format?: FormatFunction;
+        formatSeparator?: string;
+        escape?: (str: string) => string;
         escapeValue?: boolean;
         prefix?: string;
         suffix?: string;
@@ -99,40 +103,28 @@ declare namespace i18n {
     }
 
     type TranslationFunction = (key: string, options?: TranslationOptions) => string;
+    type LoadCallback = (error: any, t: TranslationFunction) => void;
 
     interface I18n {
-        //constructor(options?: Options, callback?: (err: any, t: TranslationFunction) => void);
-
-        init(options?: Options&ReactOptions, callback?: (err: any, t: TranslationFunction) => void): I18n;
-
-        loadResources(callback?: (err: any) => void): void;
-
-        language: string;
-
-        languages: string[];
-
+        // api
+        init(options?: Options & ReactOptions, callback?: LoadCallback): I18n;
         use(module: any): I18n;
-
-        changeLanguage(lng: string, callback?: (err: any, t: TranslationFunction) => void): void;
-
-        getFixedT(lng?: string, ns?: string | string[]): TranslationFunction;
-
-        t(key: string, options?: TranslationOptions): string | any | Array<any>;
-
+        t(key: string, options?: TranslationOptions): string | any | any[];
         exists(key: string, options?: TranslationOptions): boolean;
-
+        getFixedT(lng?: string, ns?: string | string[]): TranslationFunction;
+        changeLanguage(lng: string, callback?: LoadCallback): void;
+        language: string;
+        languages: string[];
+        loadNamespaces(ns: string[], callback?: LoadCallback): void;
+        loadLanguages(lngs: string[], callback?: LoadCallback): void;
+        reloadResources(lng?: string[], ns?: string[]): void;
         setDefaultNamespace(ns: string): void;
-
-        loadNamespaces(ns: string[], callback?: () => void): void;
-
-        loadLanguages(lngs: string[], callback?: () => void): void;
-
         dir(lng?: string): string;
-
-        createInstance(options?: Options, callback?: (err: any, t: TranslationFunction) => void): I18n;
-
-        cloneInstance(options?: Options, callback?: (err: any, t: TranslationFunction) => void): I18n;
-
+        format: FormatFunction;  // introduced in v8.4.0;
+        // instance creation
+        createInstance(options?: Options, callback?: LoadCallback): I18n;
+        cloneInstance(options?: Options, callback?: LoadCallback): I18n;
+        // events
         on(event: string, listener: () => void): void;
         on(initialized: 'initialized', listener: (options: i18n.Options) => void): void;
         on(loaded: 'loaded', listener: (loaded: any) => void): void;
@@ -141,8 +133,20 @@ declare namespace i18n {
         on(added: 'added', listener: (lng: string, ns: string) => void): void;
         on(removed: 'removed', listener: (lng: string, ns: string) => void): void;
         on(languageChanged: 'languageChanged', listener: (lng: string) => void): void;
-
         off(event: string, listener: () => void): void;
+        // resource handling
+        getResource(lng: string, ns: string, key: string, options?: {
+            keySeparator?: string,
+        }): ResourceStore;
+        addResource(lng: string, ns: string, key: string, value: string, options?: {
+            keySeparator?: string,
+            silent?: boolean,
+        }): void;
+        addResources(lng: string, ns: string, resources: ResourceStore): void;
+        addResourceBundle(lng: string, ns: string, resources: ResourceStore, deep: boolean, overwrite: boolean): void;
+        hasResourceBundle(lng: string, ns: string): boolean;
+        getResourceBundle(lng: string, ns: string): ResourceStore;
+        removeResourceBundle(lng: string, ns: string): void;
 
         options: Options;
     }