diff --git a/src/cli/extract.ts b/src/cli/extract.ts
index 7a1f730..a5f537a 100644
--- a/src/cli/extract.ts
+++ b/src/cli/extract.ts
@@ -23,7 +23,7 @@ const FORMAT_METHOD_NAMES = new Set(['format', '_', 't']);
 function isFormatCall(node: Node, imports: Set<string>) {
   if (node.type !== 'CallExpression') return false;
 
-  let identifier: Identifier;
+  let identifier: Identifier | undefined;
 
   if (node.callee.type === 'Identifier') {
     identifier = node.callee;
@@ -125,7 +125,9 @@ export function collectMessageDefinitions(ast: Ast) {
     definitionDict.properties.map((propNode) => {
       if (propNode.type !== 'Property') {
         throw new Error(
-          `Found invalid '${propNode.type}' at L${propNode.loc.start.line}:${propNode.loc.start.column}`,
+          `Found invalid '${propNode.type}' at L${propNode.loc!.start.line}:${
+            propNode.loc!.start.column
+          }`,
         );
       }
 
@@ -143,7 +145,7 @@ export function collectMessages(markup: string): Message[] {
     ...definitions.map((definition) => getObjFromExpression(definition)),
     ...calls.map((call) => {
       const [pathNode, options] = call.arguments;
-      let messageObj;
+      let messageObj: Partial<Message>;
 
       if (pathNode.type === 'ObjectExpression') {
         // _({ ...opts })
@@ -166,7 +168,7 @@ export function collectMessages(markup: string): Message[] {
 
       return messageObj;
     }),
-  ].filter(Boolean);
+  ].filter((Boolean as unknown) as (x: Message | null) => x is Message);
 }
 
 export function extractMessages(
diff --git a/src/cli/includes/getObjFromExpression.ts b/src/cli/includes/getObjFromExpression.ts
index 81938c4..cd65b75 100644
--- a/src/cli/includes/getObjFromExpression.ts
+++ b/src/cli/includes/getObjFromExpression.ts
@@ -2,8 +2,10 @@ import type { ObjectExpression, Property, Identifier } from 'estree';
 
 import type { Message } from '../types';
 
-export function getObjFromExpression(exprNode: ObjectExpression) {
-  return exprNode.properties.reduce<Message>((acc, prop: Property) => {
+export function getObjFromExpression(
+  exprNode: ObjectExpression,
+): Partial<Message> {
+  return exprNode.properties.reduce<Partial<Message>>((acc, prop: Property) => {
     // we only want primitives
     if (
       prop.value.type === 'Literal' &&
diff --git a/src/cli/types/index.ts b/src/cli/types/index.ts
index 4f1c93b..cdd0c04 100644
--- a/src/cli/types/index.ts
+++ b/src/cli/types/index.ts
@@ -1,5 +1,5 @@
 export interface Message {
-  id?: string;
-  default?: string;
+  id: string;
+  default: string;
   [key: string]: any;
 }
diff --git a/src/runtime/configs.ts b/src/runtime/configs.ts
index 53fdaa4..204ca17 100644
--- a/src/runtime/configs.ts
+++ b/src/runtime/configs.ts
@@ -1,4 +1,4 @@
-import type { ConfigureOptions } from './types';
+import type { ConfigureOptions, ConfigureOptionsInit } from './types';
 import { $locale } from './stores/locale';
 
 interface Formats {
@@ -47,13 +47,13 @@ export const defaultOptions: ConfigureOptions = {
   ignoreTag: true,
 };
 
-const options: ConfigureOptions = defaultOptions;
+const options: ConfigureOptions = defaultOptions as any;
 
 export function getOptions() {
   return options;
 }
 
-export function init(opts: ConfigureOptions) {
+export function init(opts: ConfigureOptionsInit) {
   const { formats, ...rest } = opts;
   const initialLocale = opts.initialLocale || opts.fallbackLocale;
 
diff --git a/src/runtime/includes/formatters.ts b/src/runtime/includes/formatters.ts
index caedceb..46dd2ad 100644
--- a/src/runtime/includes/formatters.ts
+++ b/src/runtime/includes/formatters.ts
@@ -1,6 +1,9 @@
 import IntlMessageFormat from 'intl-messageformat';
 
-import type { MemoizedIntlFormatter } from '../types';
+import type {
+  MemoizedIntlFormatter,
+  MemoizedIntlFormatterOptional,
+} from '../types';
 import { getCurrentLocale } from '../stores/locale';
 import { getOptions } from '../configs';
 import { monadicMemoize } from './memoize';
@@ -15,6 +18,16 @@ type MemoizedDateTimeFormatterFactory = MemoizedIntlFormatter<
   Intl.DateTimeFormatOptions
 >;
 
+type MemoizedNumberFormatterFactoryOptional = MemoizedIntlFormatterOptional<
+  Intl.NumberFormat,
+  Intl.NumberFormatOptions
+>;
+
+type MemoizedDateTimeFormatterFactoryOptional = MemoizedIntlFormatterOptional<
+  Intl.DateTimeFormat,
+  Intl.DateTimeFormatOptions
+>;
+
 const getIntlFormatterOptions = (
   type: 'time' | 'number' | 'date',
   name: string,
@@ -76,17 +89,17 @@ const createTimeFormatter: MemoizedDateTimeFormatterFactory = monadicMemoize(
   },
 );
 
-export const getNumberFormatter: MemoizedNumberFormatterFactory = ({
+export const getNumberFormatter: MemoizedNumberFormatterFactoryOptional = ({
   locale = getCurrentLocale(),
   ...args
 } = {}) => createNumberFormatter({ locale, ...args });
 
-export const getDateFormatter: MemoizedDateTimeFormatterFactory = ({
+export const getDateFormatter: MemoizedDateTimeFormatterFactoryOptional = ({
   locale = getCurrentLocale(),
   ...args
 } = {}) => createDateFormatter({ locale, ...args });
 
-export const getTimeFormatter: MemoizedDateTimeFormatterFactory = ({
+export const getTimeFormatter: MemoizedDateTimeFormatterFactoryOptional = ({
   locale = getCurrentLocale(),
   ...args
 } = {}) => createTimeFormatter({ locale, ...args });
diff --git a/src/runtime/includes/loaderQueue.ts b/src/runtime/includes/loaderQueue.ts
index 6b01161..007d892 100644
--- a/src/runtime/includes/loaderQueue.ts
+++ b/src/runtime/includes/loaderQueue.ts
@@ -62,7 +62,7 @@ function loadLocaleQueue(locale: string, localeQueue: MessagesLoader[]) {
 
 const activeFlushes: { [key: string]: Promise<void> } = {};
 
-export function flush(locale: string): Promise<void> {
+export async function flush(locale: string): Promise<void> {
   if (!hasLocaleQueue(locale)) {
     if (locale in activeFlushes) {
       return activeFlushes[locale];
diff --git a/src/runtime/includes/lookup.ts b/src/runtime/includes/lookup.ts
index 921fe7c..dcfad8c 100644
--- a/src/runtime/includes/lookup.ts
+++ b/src/runtime/includes/lookup.ts
@@ -15,7 +15,7 @@ const addToCache = (path: string, locale: string, message: string) => {
   return message;
 };
 
-export const lookup = (path: string, refLocale: string) => {
+export const lookup = (path: string, refLocale: string | null | undefined) => {
   if (refLocale == null) return undefined;
 
   if (refLocale in lookupCache && path in lookupCache[refLocale]) {
diff --git a/src/runtime/index.ts b/src/runtime/index.ts
index ad58480..842022e 100644
--- a/src/runtime/index.ts
+++ b/src/runtime/index.ts
@@ -31,7 +31,7 @@ export function defineMessages(i: Record<string, MessageObject>) {
 }
 
 export function waitLocale(locale?: string) {
-  return flush(locale || getCurrentLocale() || getOptions().initialLocale);
+  return flush(locale || getCurrentLocale() || getOptions().initialLocale!);
 }
 
 export {
diff --git a/src/runtime/stores/dictionary.ts b/src/runtime/stores/dictionary.ts
index bce55ea..701c385 100644
--- a/src/runtime/stores/dictionary.ts
+++ b/src/runtime/stores/dictionary.ts
@@ -34,7 +34,9 @@ export function getMessageFromDictionary(locale: string, id: string) {
   return match;
 }
 
-export function getClosestAvailableLocale(refLocale: string): string | null {
+export function getClosestAvailableLocale(
+  refLocale: string,
+): string | null | undefined {
   if (refLocale == null) return undefined;
 
   const relatedLocales = getPossibleLocales(refLocale);
diff --git a/src/runtime/stores/formatters.ts b/src/runtime/stores/formatters.ts
index 3a0eb1b..12af044 100644
--- a/src/runtime/stores/formatters.ts
+++ b/src/runtime/stores/formatters.ts
@@ -23,7 +23,7 @@ import { getCurrentLocale, getPossibleLocales, $locale } from './locale';
 const formatMessage: MessageFormatter = (id, options = {}) => {
   if (typeof id === 'object') {
     options = id as MessageObject;
-    id = options.id;
+    id = options.id!;
   }
 
   const {
@@ -90,11 +90,8 @@ const formatNumber: NumberFormatter = (n, options) => {
   return getNumberFormatter(options).format(n);
 };
 
-const getJSON: JSONGetter = <T = any>(
-  id: string,
-  locale = getCurrentLocale(),
-) => {
-  return lookup(id, locale) as T;
+const getJSON: JSONGetter = (id: string, locale = getCurrentLocale()): any => {
+  return lookup(id, locale);
 };
 
 export const $format = derived([$locale, $dictionary], () => formatMessage);
diff --git a/src/runtime/stores/locale.ts b/src/runtime/stores/locale.ts
index 4ada4dd..04cfc96 100644
--- a/src/runtime/stores/locale.ts
+++ b/src/runtime/stores/locale.ts
@@ -6,7 +6,7 @@ import { getClosestAvailableLocale } from './dictionary';
 import { $isLoading } from './loading';
 
 let current: string;
-const $locale = writable(null);
+const $locale = writable<string | null | undefined>(null);
 
 function getSubLocales(refLocale: string) {
   return refLocale
@@ -77,7 +77,8 @@ $locale.set = (newLocale: string): void | Promise<void> => {
 };
 
 // istanbul ignore next
-$locale.update = (fn: (locale: string) => void | Promise<void>) =>
-  localeSet(fn(current));
+$locale.update = (
+  fn: (value: string | null | undefined) => string | null | undefined,
+) => localeSet(fn(current));
 
 export { $locale };
diff --git a/src/runtime/types/index.ts b/src/runtime/types/index.ts
index 76a6b05..a2b5e1b 100644
--- a/src/runtime/types/index.ts
+++ b/src/runtime/types/index.ts
@@ -1,7 +1,11 @@
 import type { FormatXMLElementFn, Formats } from 'intl-messageformat';
 
 export interface LocaleDictionary {
-  [key: string]: LocaleDictionary | string | Array<string | LocaleDictionary>;
+  [key: string]:
+    | LocaleDictionary
+    | string
+    | Array<string | LocaleDictionary>
+    | null;
 }
 
 export type LocalesDictionary = {
@@ -49,7 +53,7 @@ export type NumberFormatter = (
   options?: IntlFormatterOptions<Intl.NumberFormatOptions>,
 ) => string;
 
-export type JSONGetter = <T>(id: string, locale?: string) => T;
+export type JSONGetter = (id: string, locale?: string) => any;
 
 type IntlFormatterOptions<T> = T & {
   format?: string;
@@ -57,6 +61,10 @@ type IntlFormatterOptions<T> = T & {
 };
 
 export interface MemoizedIntlFormatter<T, U> {
+  (options: IntlFormatterOptions<U>): T;
+}
+
+export interface MemoizedIntlFormatterOptional<T, U> {
   (options?: IntlFormatterOptions<U>): T;
 }
 
@@ -65,10 +73,14 @@ export interface MessagesLoader {
 }
 
 export interface ConfigureOptions {
-  fallbackLocale: string;
-  formats?: Partial<Formats>;
-  initialLocale?: string;
-  loadingDelay?: number;
-  warnOnMissingMessages?: boolean;
-  ignoreTag?: boolean;
+  fallbackLocale: string | null | undefined;
+  formats: Formats;
+  initialLocale: string | null;
+  loadingDelay: number;
+  warnOnMissingMessages: boolean;
+  ignoreTag: boolean;
 }
+
+export type ConfigureOptionsInit = Pick<ConfigureOptions, 'fallbackLocale'> &
+  Partial<Record<'formats', Partial<ConfigureOptions['formats']>>> &
+  Partial<Omit<ConfigureOptions, 'fallbackLocale' | 'formats'>>;
diff --git a/test/runtime/includes/loaderQueue.test.ts b/test/runtime/includes/loaderQueue.test.ts
index ebc9bcc..0b0f03f 100644
--- a/test/runtime/includes/loaderQueue.test.ts
+++ b/test/runtime/includes/loaderQueue.test.ts
@@ -23,8 +23,8 @@ test('checks if exist queues of locale and its fallbacks', () => {
   expect(hasLocaleQueue('en-US')).toBe(true);
 });
 
-test("does nothing if there's no queue for a locale", () => {
-  expect(flush('foo')).toBeUndefined();
+test("does nothing if there's no queue for a locale", async () => {
+  expect(await flush('foo')).toBeUndefined();
 });
 
 test('flushes the queue of a locale and its fallbacks and merge the result with the dictionary', async () => {
diff --git a/test/runtime/includes/utils.test.ts b/test/runtime/includes/utils.test.ts
index c2fac9a..46d4647 100644
--- a/test/runtime/includes/utils.test.ts
+++ b/test/runtime/includes/utils.test.ts
@@ -8,7 +8,7 @@ import {
 
 describe('getting client locale', () => {
   beforeEach(() => {
-    delete window.location;
+    delete (window as any).location;
     window.location = {
       pathname: '/',
       hostname: 'example.com',
diff --git a/tsconfig.json b/tsconfig.json
index 2c88793..d85e826 100644
--- a/tsconfig.json
+++ b/tsconfig.json
@@ -1,10 +1,12 @@
 {
   "compilerOptions": {
     "allowJs": true,
+    "strictNullChecks": true,
     "noImplicitAny": true,
     "sourceMap": false,
     "module": "esnext",
     "moduleResolution": "node",
+    "skipLibCheck": true,
     "esModuleInterop": true,
     "resolveJsonModule": true,
     "target": "es2017",
diff --git a/yarn.lock b/yarn.lock
index d4917ff..fbdbffc 100644
--- a/yarn.lock
+++ b/yarn.lock
@@ -1263,9 +1263,9 @@
   integrity sha512-lg55ArB+ZiHHbBBttLpzD07akz0QPrZgUODNakeC09i62dnrywr9mFErHuaPlB6I7z+sEbK+IYmplahvplCj2g==
 
 "@types/node@^14.14.35":
-  version "14.14.35"
-  resolved "https://registry.yarnpkg.com/@types/node/-/node-14.14.35.tgz#42c953a4e2b18ab931f72477e7012172f4ffa313"
-  integrity sha512-Lt+wj8NVPx0zUmUwumiVXapmaLUcAk3yPuHCFVXras9k5VT9TdhJqKqGVUQCD60OTMCl0qxJ57OiTL0Mic3Iag==
+  version "14.17.11"
+  resolved "https://registry.yarnpkg.com/@types/node/-/node-14.17.11.tgz#82d266d657aec5ff01ca59f2ffaff1bb43f7bf0f"
+  integrity sha512-n2OQ+0Bz6WEsUjrvcHD1xZ8K+Kgo4cn9/w94s1bJS690QMUWfJPW/m7CCb7gPkA1fcYwL2UpjXP/rq/Eo41m6w==
 
 "@types/normalize-package-data@^2.4.0":
   version "2.4.0"