diff --git a/react/features/base/react/Platform.native.js b/react/features/base/react/Platform.native.js
index 1710b20d3..2c5130848 100644
--- a/react/features/base/react/Platform.native.js
+++ b/react/features/base/react/Platform.native.js
@@ -1,3 +1,5 @@
+/* @flow */
+
 // Re-export react-native's Platform because we want to provide a minimal
 // equivalent on Web.
 import { Platform } from 'react-native';
diff --git a/react/features/base/react/Platform.web.js b/react/features/base/react/Platform.web.js
index e09dda133..6d03b9a4e 100644
--- a/react/features/base/react/Platform.web.js
+++ b/react/features/base/react/Platform.web.js
@@ -1,3 +1,5 @@
+/* @flow */
+
 const userAgent = navigator.userAgent;
 let OS;
 
diff --git a/react/features/base/react/RouteRegistry.js b/react/features/base/react/RouteRegistry.js
index 86403af97..b2734801a 100644
--- a/react/features/base/react/RouteRegistry.js
+++ b/react/features/base/react/RouteRegistry.js
@@ -1,3 +1,7 @@
+/* @flow */
+
+import { Component } from 'react';
+
 /**
  * Object describing application route.
  *
@@ -5,12 +9,18 @@
  * @property {Component} component - React Component constructor.
  * @property {string} path - URL route, required for web routing.
  */
+type Route = {
+    component: Class<Component<*>>, // eslint-disable-line no-undef
+    path: string
+};
 
 /**
  * A registry for Navigator routes, allowing features to register themselves
  * without needing to create additional inter-feature dependencies.
  */
 class RouteRegistry {
+    _elements: Route[];
+
     /**
      * Initializes a new RouteRegistry instance.
      */
@@ -19,8 +29,9 @@ class RouteRegistry {
          * The set of registered routes.
          *
          * @private
+         * @type {Route[]}
          */
-        this._elements = new Set();
+        this._elements = [];
     }
 
     /**
@@ -32,7 +43,7 @@ class RouteRegistry {
      * @returns {boolean} True if the specified a and b describe one and the
      * same abstract route; otherwise, false.
      */
-    areRoutesEqual(a, b) {
+    areRoutesEqual(a: Route, b: Route) {
         if (a === b) { // reflexive
             return true;
         }
@@ -60,21 +71,25 @@ class RouteRegistry {
         // We use the destructuring operator to 'clone' the route object to
         // prevent modifications from outside (e.g. React Native's Navigator
         // extends it with additional properties).
-        return [ ...this._elements ].map(r => {
+        return this._elements.map(r => {
             return { ...r };
         });
     }
 
+/* eslint-disable no-undef */
+
     /**
      * Returns registered route by name if any.
      *
-     * @param {Object} component - The React Component (class) of the route to
-     * retrieve.
+     * @param {Component} component - The React Component (class) of the route
+     * to retrieve.
      * @returns {Route|null}
      */
-    getRouteByComponent(component) {
-        const route
-            = [ ...this._elements ].find(r => r.component === component);
+    getRouteByComponent(component: Class<Component<*>>) {
+
+/* eslint-enable no-undef */
+
+        const route = this._elements.find(r => r.component === component);
 
         // We use destructuring operator to 'clone' route object to prevent
         // modifications from outside (e.g. React Native's Navigator extends
@@ -88,12 +103,13 @@ class RouteRegistry {
      * @param {Route} route - Route definition object.
      * @returns {void}
      */
-    register(route) {
-        if (this._elements.has(route)) {
-            throw new Error(`Route ${route.component} is registered already!`);
+    register(route: Route) {
+        if (this._elements.includes(route)) {
+            throw new Error(
+                    `Route ${String(route.component)} is registered already!`);
         }
 
-        this._elements.add(route);
+        this._elements.push(route);
     }
 }
 
diff --git a/react/features/base/react/functions.js b/react/features/base/react/functions.js
index 2c27bce85..457418d6e 100644
--- a/react/features/base/react/functions.js
+++ b/react/features/base/react/functions.js
@@ -1,3 +1,5 @@
+/* @flow */
+
 /**
  * Prevents further propagation of the events to be handler by a specific event
  * handler/listener in the capturing and bubbling phases.
@@ -7,8 +9,9 @@
  * @returns {Function} An event handler/listener to be used in place of the
  * specified eventHandler in order to stop the events from propagating.
  */
-export function stopEventPropagation(eventHandler) {
-    return ev => {
+export function stopEventPropagation<T>(eventHandler: (ev: Event) => T)
+        : (ev: Event) => T {
+    return (ev: Event) => {
         const r = eventHandler(ev);
 
         // React Native does not propagate the press event so, for the sake of
diff --git a/react/features/base/redux/MiddlewareRegistry.js b/react/features/base/redux/MiddlewareRegistry.js
index 5cfdd44e5..6ca51e09d 100644
--- a/react/features/base/redux/MiddlewareRegistry.js
+++ b/react/features/base/redux/MiddlewareRegistry.js
@@ -1,29 +1,38 @@
+/* @flow */
+
 import { applyMiddleware } from 'redux';
 
+type Middleware = Function;
+
 /**
  * A registry for Redux middleware, allowing features to register their
  * middleware without needing to create additional inter-feature dependencies.
  */
 class MiddlewareRegistry {
+    _elements: Middleware[];
+
     /**
      * Creates a MiddlewareRegistry instance.
      */
     constructor() {
         /**
          * The set of registered middleware.
+         *
+         * @private
+         * @type {Route[]}
          */
-        this._elements = new Set();
+        this._elements = [];
     }
 
     /**
      * Applies all registered middleware into a store enhancer.
      * (@link http://redux.js.org/docs/api/applyMiddleware.html).
      *
-     * @param {Function[]} additional - Any additional middleware that need to
+     * @param {Middleware[]} additional - Any additional middleware that need to
      * be included (such as middleware from third-party modules).
-     * @returns {Function}
+     * @returns {Middleware}
      */
-    applyMiddleware(...additional) {
+    applyMiddleware(...additional: Middleware[]) {
         return applyMiddleware(
             ...this._elements,
             ...additional
@@ -35,11 +44,11 @@ class MiddlewareRegistry {
      *
      * The method is to be invoked only before {@link #applyMiddleware()}.
      *
-     * @param {Function} middleware - A Redux middleware.
+     * @param {Middleware} middleware - A Redux middleware.
      * @returns {void}
      */
-    register(middleware) {
-        this._elements.add(middleware);
+    register(middleware: Middleware) {
+        this._elements.push(middleware);
     }
 }
 
diff --git a/react/features/base/util/randomUtil.js b/react/features/base/util/randomUtil.js
index dc9106e24..e9f177136 100644
--- a/react/features/base/util/randomUtil.js
+++ b/react/features/base/util/randomUtil.js
@@ -30,7 +30,7 @@ export function randomAlphanumString(length: number) {
  * @param {Array|string} arr - Source.
  * @returns {Array|string} Array element or string character.
  */
-export function randomElement(arr: Array<any> | string) {
+export function randomElement(arr: [any] | string) {
     return arr[randomInt(0, arr.length - 1)];
 }