diff --git a/lib/firebase.js b/lib/firebase.js
index 65685af2..c6662227 100644
--- a/lib/firebase.js
+++ b/lib/firebase.js
@@ -15,7 +15,7 @@ import Messaging, { statics as MessagingStatics } from './modules/messaging';
 import Analytics from './modules/analytics';
 import Crash from './modules/crash';
 
-const instances = { default: null };
+const instances: Object = { default: null };
 const FirebaseModule = NativeModules.RNFirebase;
 
 /**
@@ -34,6 +34,19 @@ export default class Firebase {
   _remoteConfig: ?Object;
   _crash: ?Object;
 
+  auth: Function;
+  storage: Function;
+  database: Function;
+  messaging: Function;
+
+  eventHandlers: Object;
+  debug: boolean;
+  options: {
+    errorOnMissingPlayServices: boolean,
+    debug?: boolean,
+    persistence?: boolean
+  };
+
   /**
    *
    * @param options
@@ -127,14 +140,17 @@ export default class Firebase {
    * @returns {function()}
    * @private
    */
-  _staticsOrInstance(name, statics, InstanceClass) {
+  _staticsOrInstance(name, statics, InstanceClass): Function {
     const getInstance = () => {
       const internalPropName = `_${name}`;
 
+      // $FlowFixMe
       if (!this[internalPropName]) {
+        // $FlowFixMe
         this[internalPropName] = new InstanceClass(this);
       }
 
+      // $FlowFixMe
       return this[internalPropName];
     };
 
diff --git a/lib/flow.js b/lib/flow.js
index ec6ada1f..ab295056 100644
--- a/lib/flow.js
+++ b/lib/flow.js
@@ -21,3 +21,13 @@ declare type GoogleApiAvailabilityType = {
   isUserResolvableError?: boolean,
   error?: string
 };
+
+declare class FirebaseError {
+  message: string,
+  name: string,
+  code: string,
+  stack: string,
+  path: string,
+  details: string,
+  modifiers: string
+};
diff --git a/lib/modules/auth/index.js b/lib/modules/auth/index.js
index bd00d445..b04cd89a 100644
--- a/lib/modules/auth/index.js
+++ b/lib/modules/auth/index.js
@@ -42,7 +42,12 @@ export default class Auth extends Base {
     if (auth && auth.user && !this._user) this._user = new User(this, auth);
     else if ((!auth || !auth.user) && this._user) this._user = null;
     else if (this._user) this._user._updateValues(auth);
-    if (emit) this.emit('onAuthStateChanged', this._authResult.user || null);
+    if (emit) {
+      this.emit(
+        'onAuthStateChanged',
+        this._authResult && this._authResult.user || null
+      );
+    }
     return auth ? this._user : null;
   }
 
diff --git a/lib/modules/crash/index.js b/lib/modules/crash/index.js
index 436952e9..7c4f7664 100644
--- a/lib/modules/crash/index.js
+++ b/lib/modules/crash/index.js
@@ -33,7 +33,7 @@ export default class Analytics extends Base {
    * @param {Error} error
    * @param maxStackSize
    */
-  report(error: Error, maxStackSize: Number = 10): void {
+  report(error: FirebaseError, maxStackSize: number = 10): void {
     if (!error || !error.code || !error.message) return;
 
     let errorMessage = `Message: ${error.message}\r\n`;
diff --git a/lib/modules/database/disconnect.js b/lib/modules/database/disconnect.js
index f79795eb..8b5c1f93 100644
--- a/lib/modules/database/disconnect.js
+++ b/lib/modules/database/disconnect.js
@@ -12,6 +12,7 @@ const FirebaseDatabase = NativeModules.RNFirebaseDatabase;
  */
 export default class Disconnect {
   ref: Reference;
+  path: string;
 
   /**
    *
diff --git a/lib/modules/database/index.js b/lib/modules/database/index.js
index df6df3b6..e5baf473 100644
--- a/lib/modules/database/index.js
+++ b/lib/modules/database/index.js
@@ -218,7 +218,8 @@ export default class Database extends Base {
       firebaseMessage = `${firebaseMessage} at /${path}\r\n`;
     }
 
-    const firebaseError = new Error(firebaseMessage);
+    // $FlowFixMe
+    const firebaseError: FirebaseError = new Error(firebaseMessage);
 
     firebaseError.code = code;
     firebaseError.path = path;
diff --git a/lib/modules/database/reference.js b/lib/modules/database/reference.js
index cc17a609..6fb43b1b 100644
--- a/lib/modules/database/reference.js
+++ b/lib/modules/database/reference.js
@@ -157,19 +157,25 @@ export default class Reference extends ReferenceBase {
    * @param onComplete
    * @param applyLocally
    */
-  transaction(transactionUpdate, onComplete?: () => any, applyLocally: boolean = false) {
+  transaction(
+    transactionUpdate: Function,
+    onComplete?: (?Error, any, ?Snapshot) => any,
+    applyLocally: boolean = false
+  ) {
     if (!isFunction(transactionUpdate)) return Promise.reject(new Error('Missing transactionUpdate function argument.'));
 
     return new Promise((resolve, reject) => {
       const onCompleteWrapper = (error, committed, snapshotData) => {
         if (error) {
-          if (isFunction(onComplete)) onComplete(error, committed, null);
+          if (typeof onComplete === 'function') {
+            onComplete(error, committed, null);
+          }
           return reject(error);
         }
 
         const snapshot = new Snapshot(this, snapshotData);
 
-        if (isFunction(onComplete)) {
+        if (typeof onComplete === 'function') {
           onComplete(null, committed, snapshot);
         }
 
diff --git a/lib/modules/database/snapshot.js b/lib/modules/database/snapshot.js
index d81ae220..97c28546 100644
--- a/lib/modules/database/snapshot.js
+++ b/lib/modules/database/snapshot.js
@@ -62,7 +62,7 @@ export default class Snapshot {
    * @link https://firebase.google.com/docs/reference/js/firebase.database.DataSnapshot#exists
    * @returns {boolean}
    */
-  exists(): Boolean {
+  exists(): boolean {
     return this._value !== null;
   }
 
@@ -71,7 +71,7 @@ export default class Snapshot {
    * @link https://firebase.google.com/docs/reference/js/firebase.database.DataSnapshot#forEach
    * @param action
    */
-  forEach(action: (key: any) => any): Boolean {
+  forEach(action: (key: any) => any): boolean {
     if (!this._childKeys.length) return false;
     let cancelled = false;
 
@@ -94,7 +94,7 @@ export default class Snapshot {
    * @link https://firebase.google.com/docs/reference/js/firebase.database.DataSnapshot#getPriority
    * @returns {String|Number|null}
    */
-  getPriority(): String|Number|null {
+  getPriority(): string | number | null {
     return this._priority;
   }
 
@@ -104,7 +104,7 @@ export default class Snapshot {
    * @param path
    * @returns {Boolean}
    */
-  hasChild(path: string): Boolean {
+  hasChild(path: string): boolean {
     return deepExists(this._value, path);
   }
 
@@ -113,7 +113,7 @@ export default class Snapshot {
    * @link https://firebase.google.com/docs/reference/js/firebase.database.DataSnapshot#hasChildren
    * @returns {boolean}
    */
-  hasChildren(): Boolean {
+  hasChildren(): boolean {
     return this.numChildren() > 0;
   }
 
@@ -122,7 +122,7 @@ export default class Snapshot {
    * @link https://firebase.google.com/docs/reference/js/firebase.database.DataSnapshot#numChildren
    * @returns {Number}
    */
-  numChildren(): Number {
+  numChildren(): number {
     if (!isObject(this._value)) return 0;
     return Object.keys(this._value).length;
   }
diff --git a/lib/modules/database/transaction.js b/lib/modules/database/transaction.js
index f4a2fdaa..8a4b529a 100644
--- a/lib/modules/database/transaction.js
+++ b/lib/modules/database/transaction.js
@@ -13,7 +13,7 @@ const FirebaseDatabase = NativeModules.RNFirebaseDatabase;
  * @class Database
  */
 export default class TransactionHandler extends Base {
-  constructor(firebase: Object, database: Object, FirebaseDatabaseEvt) {
+  constructor(firebase: Object, database: Object, FirebaseDatabaseEvt: Object) {
     super(firebase, {});
     this.transactions = {};
     this.database = database;
@@ -32,7 +32,12 @@ export default class TransactionHandler extends Base {
    * @param onComplete
    * @param applyLocally
    */
-  add(reference, transactionUpdater, onComplete, applyLocally = false) {
+  add(
+    reference: Object,
+    transactionUpdater: Function,
+    onComplete?: Function,
+    applyLocally?: boolean = false
+  ) {
     const id = this._generateTransactionId();
 
     this.transactions[id] = {
@@ -45,7 +50,7 @@ export default class TransactionHandler extends Base {
       started: true,
     };
 
-    FirebaseDatabase.startTransaction(reference.path, id, applyLocally || false);
+    FirebaseDatabase.startTransaction(reference.path, id, applyLocally);
   }
 
   /**
@@ -57,7 +62,7 @@ export default class TransactionHandler extends Base {
    * @returns {string}
    * @private
    */
-  _generateTransactionId() {
+  _generateTransactionId(): string {
     return generatePushID(this.database.serverTimeOffset);
   }
 
@@ -72,7 +77,7 @@ export default class TransactionHandler extends Base {
       case 'update':
         return this._handleUpdate(event);
       case 'error':
-        return this._handleError(error);
+        return this._handleError(event);
       case 'complete':
         return this._handleComplete(event);
       default:
diff --git a/lib/utils/index.js b/lib/utils/index.js
index 72a75693..bedb5a48 100644
--- a/lib/utils/index.js
+++ b/lib/utils/index.js
@@ -1,3 +1,6 @@
+/**
+ * @flow
+ */
 // modeled after base64 web-safe chars, but ordered by ASCII
 const PUSH_CHARS = '-0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ_abcdefghijklmnopqrstuvwxyz';
 const hasOwnProperty = Object.hasOwnProperty;
@@ -8,7 +11,8 @@ const _handler = (resolve, reject, errorPrefix, err, resp) => {
   // resolve / reject after events etc
   setImmediate(() => {
     if (err) {
-      const firebaseError = new Error(err.message);
+      // $FlowFixMe
+      const firebaseError: FirebaseError = new Error(err.message);
 
       if (isObject(err)) {
         Object.keys(err).forEach(key => Object.defineProperty(firebaseError, key, { value: err[key] }));
@@ -23,7 +27,7 @@ const _handler = (resolve, reject, errorPrefix, err, resp) => {
   });
 };
 
-export function toWebSDKErrorCode(code, prefix) {
+export function toWebSDKErrorCode(code: any, prefix: string): string {
   if (!code || typeof code !== 'string') return '';
   return code.toLowerCase().replace('error_', prefix).replace(/_/g, '-');
 }
@@ -36,7 +40,11 @@ export function toWebSDKErrorCode(code, prefix) {
  * @param joiner
  * @returns {*}
  */
-export function deepGet(object, path, joiner = '/') {
+export function deepGet(
+  object: Object,
+  path: string,
+  joiner?: string = '/'
+): any {
   const keys = path.split(joiner);
 
   let i = 0;
@@ -60,7 +68,11 @@ export function deepGet(object, path, joiner = '/') {
  * @param joiner
  * @returns {*}
  */
-export function deepExists(object, path, joiner = '/') {
+export function deepExists(
+  object: Object,
+  path: string,
+  joiner?: string = '/'
+): boolean {
   const keys = path.split(joiner);
 
   let i = 0;
@@ -81,7 +93,7 @@ export function deepExists(object, path, joiner = '/') {
  * @param item
  * @returns {boolean}
  */
-export function isObject(item) {
+export function isObject(item: any): boolean {
   return (item && typeof item === 'object' && !Array.isArray(item) && item !== null);
 }
 
@@ -90,8 +102,8 @@ export function isObject(item) {
  * @param item
  * @returns {*|boolean}
  */
-export function isFunction(item) {
-  return (item && typeof item === 'function');
+export function isFunction(item?: any): boolean {
+  return Boolean(item && typeof item === 'function');
 }
 
 /**
@@ -99,9 +111,9 @@ export function isFunction(item) {
  * @param string
  * @returns {*}
  */
-export function tryJSONParse(string) {
+export function tryJSONParse(string: string | null): any {
   try {
-    return JSON.parse(string);
+    return string && JSON.parse(string);
   } catch (jsonError) {
     return string;
   }
@@ -112,7 +124,7 @@ export function tryJSONParse(string) {
  * @param data
  * @returns {*}
  */
-export function tryJSONStringify(data) {
+export function tryJSONStringify(data: any): string | null {
   try {
     return JSON.stringify(data);
   } catch (jsonError) {
@@ -149,7 +161,11 @@ export function noop(): void {
  * @param NativeModule
  * @param errorPrefix
  */
-export function promisify(fn: Function, NativeModule: Object, errorPrefix): Function<Promise> {
+export function promisify(
+  fn: Function | string,
+  NativeModule: Object,
+  errorPrefix?: string
+): (any) => Promise<> {
   return (...args) => {
     return new Promise((resolve, reject) => {
       const _fn = typeof fn === 'function' ? fn : NativeModule[fn];
@@ -168,7 +184,12 @@ export function promisify(fn: Function, NativeModule: Object, errorPrefix): Func
  * @param callback
  * @private
  */
-function _delayChunk(collection, chunkSize, operation, callback): void {
+function _delayChunk(
+  collection: Array<*>,
+  chunkSize: number,
+  operation: Function,
+  callback: Function
+): void {
   const length = collection.length;
   const iterations = Math.ceil(length / chunkSize);
 
@@ -196,21 +217,28 @@ function _delayChunk(collection, chunkSize, operation, callback): void {
  * @param iterator
  * @param cb
  */
-export function each(array: Array, chunkSize?: number, iterator: Function, cb: Function): void {
+export function each(
+  array: Array<*>,
+  chunkSize: number | Function,
+  iterator: Function,
+  cb?: Function
+): void {
   if (typeof chunkSize === 'function') {
     cb = iterator;
     iterator = chunkSize;
     chunkSize = DEFAULT_CHUNK_SIZE;
   }
 
-  _delayChunk(array, chunkSize, (slice, start) => {
-    for (let ii = 0, jj = slice.length; ii < jj; ii += 1) {
-      iterator(slice[ii], start + ii);
-    }
-  }, cb);
+  if (cb) {
+    _delayChunk(array, chunkSize, (slice, start) => {
+      for (let ii = 0, jj = slice.length; ii < jj; ii += 1) {
+        iterator(slice[ii], start + ii);
+      }
+    }, cb);
+  }
 }
 
-export function typeOf(value) {
+export function typeOf(value: any): string {
   if (value === null) return 'null';
   if (Array.isArray(value)) return 'array';
   return typeof value;
@@ -224,7 +252,12 @@ export function typeOf(value) {
  * @param cb
  * @returns {*}
  */
-export function map(array: Array, chunkSize?: number, iterator: Function, cb: Function): void {
+export function map(
+  array: Array<*>,
+  chunkSize: number | Function,
+  iterator: Function,
+  cb?: Function
+): void {
   if (typeof chunkSize === 'function') {
     cb = iterator;
     iterator = chunkSize;
@@ -237,7 +270,7 @@ export function map(array: Array, chunkSize?: number, iterator: Function, cb: Fu
       result.push(iterator(slice[ii], start + ii, array));
     }
     return result;
-  }, () => cb(result));
+  }, () => cb && cb(result));
 }