diff --git a/modules/API/API.js b/modules/API/API.js
index 4f801672c4..7c105132e0 100644
--- a/modules/API/API.js
+++ b/modules/API/API.js
@@ -1,3 +1,5 @@
+// @flow
+
 import * as JitsiMeetConferenceEvents from '../../ConferenceEvents';
 import { parseJWTFromURLParams } from '../../react/features/jwt';
 import { getJitsiMeetTransport } from '../transport';
@@ -207,7 +209,10 @@ class API {
      * @param {Object} options - Object with the message properties.
      * @returns {void}
      */
-    notifyReceivedChatMessage({ body, id, nick, ts } = {}) {
+    notifyReceivedChatMessage(
+            { body, id, nick, ts }: {
+                body: *, id: string, nick: string, ts: *
+            } = {}) {
         if (APP.conference.isLocalId(id)) {
             return;
         }
diff --git a/react/features/base/conference/actions.js b/react/features/base/conference/actions.js
index a1b94911df..77c7b04a00 100644
--- a/react/features/base/conference/actions.js
+++ b/react/features/base/conference/actions.js
@@ -1,3 +1,5 @@
+// @flow
+
 import { JitsiConferenceEvents } from '../lib-jitsi-meet';
 import { setAudioMuted, setVideoMuted } from '../media';
 import {
@@ -161,7 +163,7 @@ function _setLocalParticipantData(conference, state) {
  * }}
  * @public
  */
-export function conferenceFailed(conference, error) {
+export function conferenceFailed(conference: Object, error: string) {
     return {
         type: CONFERENCE_FAILED,
         conference,
@@ -179,7 +181,7 @@ export function conferenceFailed(conference, error) {
  *     conference: JitsiConference
  * }}
  */
-export function conferenceJoined(conference) {
+export function conferenceJoined(conference: Object) {
     return {
         type: CONFERENCE_JOINED,
         conference
@@ -196,7 +198,7 @@ export function conferenceJoined(conference) {
  *     conference: JitsiConference
  * }}
  */
-export function conferenceLeft(conference) {
+export function conferenceLeft(conference: Object) {
     return {
         type: CONFERENCE_LEFT,
         conference
@@ -212,8 +214,8 @@ export function conferenceLeft(conference) {
  * local participant will (try to) join.
  * @returns {Function}
  */
-function _conferenceWillJoin(conference) {
-    return (dispatch, getState) => {
+function _conferenceWillJoin(conference: Object) {
+    return (dispatch: Dispatch<*>, getState: Function) => {
         const localTracks
             = getState()['features/base/tracks']
                 .filter(t => t.local)
@@ -243,7 +245,7 @@ function _conferenceWillJoin(conference) {
  *     conference: JitsiConference
  * }}
  */
-export function conferenceWillLeave(conference) {
+export function conferenceWillLeave(conference: Object) {
     return {
         type: CONFERENCE_WILL_LEAVE,
         conference
@@ -256,7 +258,7 @@ export function conferenceWillLeave(conference) {
  * @returns {Function}
  */
 export function createConference() {
-    return (dispatch: Dispatch<*>, getState: Function) => {
+    return (dispatch: Function, getState: Function) => {
         const state = getState();
         const { connection, locationURL } = state['features/base/connection'];
 
@@ -331,7 +333,7 @@ export function dataChannelOpened() {
  *     locked: boolean
  * }}
  */
-export function lockStateChanged(conference, locked) {
+export function lockStateChanged(conference: Object, locked: boolean) {
     return {
         type: LOCK_STATE_CHANGED,
         conference,
@@ -348,7 +350,7 @@ export function lockStateChanged(conference, locked) {
  *     p2p: boolean
  * }}
  */
-export function p2pStatusChanged(p2p) {
+export function p2pStatusChanged(p2p: boolean) {
     return {
         type: P2P_STATUS_CHANGED,
         p2p
@@ -365,7 +367,7 @@ export function p2pStatusChanged(p2p) {
  *     audioOnly: boolean
  * }}
  */
-export function setAudioOnly(audioOnly) {
+export function setAudioOnly(audioOnly: boolean) {
     return {
         type: SET_AUDIO_ONLY,
         audioOnly
@@ -412,7 +414,10 @@ export function setLastN(lastN: ?number) {
  * is to be joined or locked.
  * @returns {Function}
  */
-export function setPassword(conference, method: Function, password: string) {
+export function setPassword(
+        conference: Object,
+        method: Function,
+        password: string) {
     return (dispatch: Dispatch<*>, getState: Function) => {
         switch (method) {
         case conference.join: {
diff --git a/react/features/base/util/uri.js b/react/features/base/util/uri.js
index 9a10a100d1..2022a4170f 100644
--- a/react/features/base/util/uri.js
+++ b/react/features/base/util/uri.js
@@ -1,3 +1,5 @@
+// @flow
+
 /**
  * The {@link RegExp} pattern of the authority of a URI.
  *
@@ -41,7 +43,7 @@ function _fixURIStringHierPart(uri) {
         = new RegExp(
             `^${_URI_PROTOCOL_PATTERN}//hipchat\\.com/video/call/`,
             'gi');
-    let match = regex.exec(uri);
+    let match: Array<string> | null = regex.exec(uri);
 
     if (!match) {
         // enso.me
@@ -80,7 +82,7 @@ function _fixURIStringHierPart(uri) {
  */
 function _fixURIStringScheme(uri: string) {
     const regex = new RegExp(`^${_URI_PROTOCOL_PATTERN}+`, 'gi');
-    const match = regex.exec(uri);
+    const match: Array<string> | null = regex.exec(uri);
 
     if (match) {
         // As an implementation convenience, pick up the last scheme and make
@@ -115,8 +117,7 @@ function _fixURIStringScheme(uri: string) {
  * @returns {string} - The (Web application) context root defined by the
  * specified {@code location} (URI).
  */
-export function getLocationContextRoot(location: Object) {
-    const pathname = location.pathname;
+export function getLocationContextRoot({ pathname }: { pathname: string }) {
     const contextRootEndIndex = pathname.lastIndexOf('/');
 
     return (
@@ -169,12 +170,12 @@ function _objectToURLParamsArray(obj = {}) {
 export function parseStandardURIString(str: string) {
     /* eslint-disable no-param-reassign */
 
-    const obj = {
+    const obj: Object = {
         toString: _standardURIToString
     };
 
     let regex;
-    let match;
+    let match: Array<string> | null;
 
     // protocol
     regex = new RegExp(`^${_URI_PROTOCOL_PATTERN}`, 'gi');
@@ -188,7 +189,7 @@ export function parseStandardURIString(str: string) {
     regex = new RegExp(`^${_URI_AUTHORITY_PATTERN}`, 'gi');
     match = regex.exec(str);
     if (match) {
-        let authority = match[1].substring(/* // */ 2);
+        let authority: string = match[1].substring(/* // */ 2);
 
         str = str.substring(regex.lastIndex);
 
@@ -217,7 +218,7 @@ export function parseStandardURIString(str: string) {
     regex = new RegExp(`^${_URI_PATH_PATTERN}`, 'gi');
     match = regex.exec(str);
 
-    let pathname;
+    let pathname: ?string;
 
     if (match) {
         pathname = match[1];
@@ -360,7 +361,7 @@ export function urlObjectToString(o: Object): ?string {
 
     // protocol
     if (!url.protocol) {
-        let protocol = o.protocol || o.scheme;
+        let protocol: ?string = o.protocol || o.scheme;
 
         if (protocol) {
             // Protocol is supposed to be the scheme and the final ':'. Anyway,
@@ -378,7 +379,7 @@ export function urlObjectToString(o: Object): ?string {
         //
         // It may be host/hostname and pathname with the latter denoting the
         // tenant.
-        const domain = o.domain || o.host || o.hostname;
+        const domain: ?string = o.domain || o.host || o.hostname;
 
         if (domain) {
             const { host, hostname, pathname: contextRoot, port }
diff --git a/react/features/connection-indicator/statsEmitter.js b/react/features/connection-indicator/statsEmitter.js
index 939de4ab22..35626a7d94 100644
--- a/react/features/connection-indicator/statsEmitter.js
+++ b/react/features/connection-indicator/statsEmitter.js
@@ -1,3 +1,5 @@
+// @flow
+
 import _ from 'lodash';
 
 import JitsiMeetJS from '../base/lib-jitsi-meet';
@@ -25,7 +27,7 @@ const statsEmitter = {
      * {@code statsEmitter} should subscribe for stat updates.
      * @returns {void}
      */
-    startListeningForStats(conference) {
+    startListeningForStats(conference: Object) {
         const { connectionQuality } = JitsiMeetJS.events;
 
         conference.on(connectionQuality.LOCAL_STATS_UPDATED,
@@ -44,7 +46,7 @@ const statsEmitter = {
      * user have been updated.
      * @returns {void}
      */
-    subscribeToClientStats(id, callback) {
+    subscribeToClientStats(id: ?string, callback: Function) {
         if (!id) {
             return;
         }
@@ -66,7 +68,7 @@ const statsEmitter = {
      * stat updates for the specified user id.
      * @returns {void}
      */
-    unsubscribeToClientStats(id, callback) {
+    unsubscribeToClientStats(id: string, callback: Function) {
         if (!subscribers[id]) {
             return;
         }
@@ -89,7 +91,7 @@ const statsEmitter = {
      * @param {Object} stats - New connection stats for the user.
      * @returns {void}
      */
-    _emitStatsUpdate(id, stats = {}) {
+    _emitStatsUpdate(id: string, stats: Object = {}) {
         const callbacks = subscribers[id] || [];
 
         callbacks.forEach(callback => {
@@ -107,7 +109,7 @@ const statsEmitter = {
      * by the library.
      * @returns {void}
      */
-    _onStatsUpdated(currentUserId, stats) {
+    _onStatsUpdated(currentUserId: string, stats: Object) {
         const allUserFramerates = stats.framerate || {};
         const allUserResolutions = stats.resolution || {};
 
diff --git a/react/features/device-selection/popup.js b/react/features/device-selection/popup.js
index 09d8729c37..99621e0c61 100644
--- a/react/features/device-selection/popup.js
+++ b/react/features/device-selection/popup.js
@@ -1,17 +1,16 @@
+/* global JitsiMeetJS */
+
 import 'aui-css';
 import 'aui-experimental-css';
 
 import DeviceSelectionPopup from './DeviceSelectionPopup';
 
-declare var JitsiMeetJS: Object;
-
 let deviceSelectionPopup;
 
-window.init = function(i18next) {
+window.init = i18next => {
     JitsiMeetJS.init({}).then(() => {
         deviceSelectionPopup = new DeviceSelectionPopup(i18next);
     });
 };
 
-window.addEventListener('beforeunload', () =>
-    deviceSelectionPopup.close());
+window.addEventListener('beforeunload', () => deviceSelectionPopup.close());
diff --git a/react/features/dial-out/actions.js b/react/features/dial-out/actions.js
index e359509efb..5249d65562 100644
--- a/react/features/dial-out/actions.js
+++ b/react/features/dial-out/actions.js
@@ -1,3 +1,5 @@
+// @flow
+
 import {
     DIAL_OUT_CANCELED,
     DIAL_OUT_CODES_UPDATED,
@@ -25,8 +27,8 @@ export function cancel() {
  * @param {string} dialNumber - The number to dial.
  * @returns {Function}
  */
-export function dial(dialNumber) {
-    return (dispatch, getState) => {
+export function dial(dialNumber: string) {
+    return (dispatch: Dispatch<*>, getState: Function) => {
         const { conference } = getState()['features/base/conference'];
 
         conference.dial(dialNumber);
@@ -39,8 +41,8 @@ export function dial(dialNumber) {
  * @param {string} dialNumber - The dial number to check for validity.
  * @returns {Function}
  */
-export function checkDialNumber(dialNumber) {
-    return (dispatch, getState) => {
+export function checkDialNumber(dialNumber: string) {
+    return (dispatch: Dispatch<*>, getState: Function) => {
         const { dialOutAuthUrl } = getState()['features/base/config'];
 
         if (!dialOutAuthUrl) {
@@ -78,7 +80,7 @@ export function checkDialNumber(dialNumber) {
  * @returns {Function}
  */
 export function updateDialOutCodes() {
-    return (dispatch, getState) => {
+    return (dispatch: Dispatch<*>, getState: Function) => {
         const { dialOutCodesUrl } = getState()['features/base/config'];
 
         if (!dialOutCodesUrl) {
diff --git a/react/features/feedback/actions.js b/react/features/feedback/actions.js
index 4e9a171638..57164a3d58 100644
--- a/react/features/feedback/actions.js
+++ b/react/features/feedback/actions.js
@@ -1,11 +1,9 @@
-import { FEEDBACK_REQUEST_IN_PROGRESS } from '../../../modules/UI/UIErrors';
+// @flow
 
 import { openDialog } from '../../features/base/dialog';
+import { FEEDBACK_REQUEST_IN_PROGRESS } from '../../../modules/UI/UIErrors';
 
-import {
-    CANCEL_FEEDBACK,
-    SUBMIT_FEEDBACK
-} from './actionTypes';
+import { CANCEL_FEEDBACK, SUBMIT_FEEDBACK } from './actionTypes';
 import { FeedbackDialog } from './components';
 
 declare var config: Object;
@@ -23,7 +21,7 @@ declare var interfaceConfig: Object;
  *     score: number
  * }}
  */
-export function cancelFeedback(score, message) {
+export function cancelFeedback(score: number, message: string) {
     return {
         type: CANCEL_FEEDBACK,
         message,
@@ -42,8 +40,8 @@ export function cancelFeedback(score, message) {
  * resolved with true if the dialog is disabled or the feedback was already
  * submitted. Rejected if another dialog is already displayed.
  */
-export function maybeOpenFeedbackDialog(conference) {
-    return (dispatch, getState) => {
+export function maybeOpenFeedbackDialog(conference: Object) {
+    return (dispatch: Dispatch<*>, getState: Function) => {
         const state = getState();
 
         if (interfaceConfig.filmStripOnly || config.iAmRecorder) {
@@ -93,7 +91,7 @@ export function maybeOpenFeedbackDialog(conference) {
  * is closed.
  * @returns {Object}
  */
-export function openFeedbackDialog(conference, onClose) {
+export function openFeedbackDialog(conference: Object, onClose: ?Function) {
     return openDialog(FeedbackDialog, {
         conference,
         onClose
@@ -113,7 +111,10 @@ export function openFeedbackDialog(conference, onClose) {
  *     type: SUBMIT_FEEDBACK
  * }}
  */
-export function submitFeedback(score, message, conference) {
+export function submitFeedback(
+        score: number,
+        message: string,
+        conference: Object) {
     conference.sendFeedback(score, message);
 
     return {
diff --git a/react/features/invite/actions.js b/react/features/invite/actions.js
index 59a4ee0d1b..704a6aec0c 100644
--- a/react/features/invite/actions.js
+++ b/react/features/invite/actions.js
@@ -1,3 +1,5 @@
+// @flow
+
 import { openDialog } from '../../features/base/dialog';
 
 import {
@@ -27,7 +29,7 @@ export function openInviteDialog() {
  *     visible: boolean
  * }}
  */
-export function setInfoDialogVisibility(visible) {
+export function setInfoDialogVisibility(visible: boolean) {
     return {
         type: SET_INFO_DIALOG_VISIBILITY,
         visible
@@ -40,7 +42,7 @@ export function setInfoDialogVisibility(visible) {
  * @returns {Function}
  */
 export function updateDialInNumbers() {
-    return (dispatch, getState) => {
+    return (dispatch: Dispatch<*>, getState: Function) => {
         const state = getState();
         const { dialInConfCodeUrl, dialInNumbersUrl, hosts }
             = state['features/base/config'];
diff --git a/react/features/invite/functions.js b/react/features/invite/functions.js
index 78a87c380f..2ed4d956f0 100644
--- a/react/features/invite/functions.js
+++ b/react/features/invite/functions.js
@@ -1,31 +1,18 @@
+// @flow
+
 declare var $: Function;
 declare var interfaceConfig: Object;
 
 /**
- * Sends an ajax request to a directory service.
+ * Get the position of the invite option in the interfaceConfig.INVITE_OPTIONS
+ * list.
  *
- * @param {string} serviceUrl - The service to query.
- * @param {string} jwt - The jwt token to pass to the search service.
- * @param {string} text - Text to search.
- * @param {Array<string>} queryTypes - Array with the query types that will be
- * executed - "conferenceRooms" | "user" | "room".
- * @returns {Promise} - The promise created by the request.
+ * @param {string} name - The invite option name.
+ * @private
+ * @returns {number} - The position of the option in the list.
  */
-export function searchPeople( // eslint-disable-line max-params
-        serviceUrl,
-        jwt,
-        text,
-        queryTypes = [ 'conferenceRooms', 'user', 'room' ]) {
-    const queryTypesString = JSON.stringify(queryTypes);
-
-    return new Promise((resolve, reject) => {
-        $.getJSON(`${serviceUrl}?query=${encodeURIComponent(text)}`
-            + `&queryTypes=${queryTypesString}&jwt=${jwt}`,
-        response => resolve(response)
-        ).fail((jqxhr, textStatus, error) =>
-            reject(error)
-        );
-    });
+export function getInviteOptionPosition(name: string): number {
+    return interfaceConfig.INVITE_OPTIONS.indexOf(name);
 }
 
 /**
@@ -38,17 +25,21 @@ export function searchPeople( // eslint-disable-line max-params
  * @param {Immutable.List} people - The list of the "user" type items to invite.
  * @returns {Promise} - The promise created by the request.
  */
-export function invitePeople(inviteServiceUrl, inviteUrl, jwt, people) { // eslint-disable-line max-params, max-len
+export function invitePeople( // eslint-disable-line max-params
+        inviteServiceUrl: string,
+        inviteUrl: string,
+        jwt: string,
+        people: Object) {
     return new Promise((resolve, reject) => {
-        $.post(`${inviteServiceUrl}?token=${jwt}`,
-            JSON.stringify({
-                'invited': people,
-                'url': inviteUrl }),
-            response => resolve(response),
-            'json')
-            .fail((jqxhr, textStatus, error) =>
-                reject(error)
-            );
+        $.post(
+                `${inviteServiceUrl}?token=${jwt}`,
+                JSON.stringify({
+                    'invited': people,
+                    'url': inviteUrl
+                }),
+                resolve,
+                'json')
+            .fail((jqxhr, textStatus, error) => reject(error));
     });
 }
 
@@ -61,10 +52,11 @@ export function invitePeople(inviteServiceUrl, inviteUrl, jwt, people) { // esli
  * invite.
  * @returns {void}
  */
-export function inviteRooms(conference, rooms) {
+export function inviteRooms(
+        conference: { createVideoSIPGWSession: Function },
+        rooms: Object) {
     for (const room of rooms) {
-        const sipAddress = room.id;
-        const displayName = room.name;
+        const { id: sipAddress, name: displayName } = room;
 
         if (sipAddress && displayName) {
             const newSession
@@ -86,18 +78,32 @@ export function inviteRooms(conference, rooms) {
  * @returns {boolean} - True to indicate that the given invite option is
  * enabled, false - otherwise.
  */
-export function isInviteOptionEnabled(name) {
-    return interfaceConfig.INVITE_OPTIONS.indexOf(name) !== -1;
+export function isInviteOptionEnabled(name: string) {
+    return getInviteOptionPosition(name) !== -1;
 }
 
 /**
- * Get the position of the invite option in the interfaceConfig.INVITE_OPTIONS
- * list.
+ * Sends an ajax request to a directory service.
  *
- * @param {string} optionName - The invite option name.
- * @private
- * @returns {number} - The position of the option in the list.
+ * @param {string} serviceUrl - The service to query.
+ * @param {string} jwt - The jwt token to pass to the search service.
+ * @param {string} text - Text to search.
+ * @param {Array<string>} queryTypes - Array with the query types that will be
+ * executed - "conferenceRooms" | "user" | "room".
+ * @returns {Promise} - The promise created by the request.
  */
-export function getInviteOptionPosition(optionName) {
-    return interfaceConfig.INVITE_OPTIONS.indexOf(optionName);
+export function searchPeople( // eslint-disable-line max-params
+        serviceUrl: string,
+        jwt: string,
+        text: string,
+        queryTypes: Array<string> = [ 'conferenceRooms', 'user', 'room' ]) {
+    const queryTypesString = JSON.stringify(queryTypes);
+
+    return new Promise((resolve, reject) => {
+        $.getJSON(
+                `${serviceUrl}?query=${encodeURIComponent(text)}&queryTypes=${
+                    queryTypesString}&jwt=${jwt}`,
+                resolve)
+            .fail((jqxhr, textStatus, error) => reject(error));
+    });
 }
diff --git a/react/features/toolbox/functions.web.js b/react/features/toolbox/functions.web.js
index 784c0a1418..f055e49197 100644
--- a/react/features/toolbox/functions.web.js
+++ b/react/features/toolbox/functions.web.js
@@ -1,3 +1,5 @@
+// @flow
+
 import SideContainerToggler
     from '../../../modules/UI/side_pannels/SideContainerToggler';
 
@@ -101,7 +103,7 @@ export function getToolbarClassNames(props: Object) {
  * @returns {boolean} - True to indicate that the given toolbar button
  * is enabled, false - otherwise.
  */
-export function isButtonEnabled(name) {
+export function isButtonEnabled(name: string) {
     return interfaceConfig.TOOLBAR_BUTTONS.indexOf(name) !== -1
             || interfaceConfig.MAIN_TOOLBAR_BUTTONS.indexOf(name) !== -1;
 }