diff --git a/src/firefox/DOMWorld.ts b/src/firefox/DOMWorld.ts
index 2df99d64a8..d2c523222d 100644
--- a/src/firefox/DOMWorld.ts
+++ b/src/firefox/DOMWorld.ts
@@ -1,7 +1,25 @@
+/**
+ * Copyright 2019 Google Inc. All rights reserved.
+ * Modifications copyright (c) Microsoft Corporation.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
 import {helper, assert} from '../helper';
 import {TimeoutError} from '../Errors';
 import * as fs from 'fs';
 import * as util from 'util';
+import * as types from '../types';
 import {ElementHandle, JSHandle} from './JSHandle';
 import { ExecutionContext } from './ExecutionContext';
 
@@ -61,14 +79,14 @@ export class DOMWorld {
     throw new Error('Method not implemented.');
   }
 
-  async evaluateHandle(pageFunction, ...args): Promise<JSHandle> {
+  evaluateHandle: types.EvaluateHandle<JSHandle> = async (pageFunction, ...args) => {
     const context = await this.executionContext();
-    return context.evaluateHandle(pageFunction, ...args);
+    return context.evaluateHandle(pageFunction, ...args as any);
   }
 
-  async evaluate(pageFunction, ...args): Promise<any> {
+  evaluate: types.Evaluate<JSHandle> = async (pageFunction, ...args) => {
     const context = await this.executionContext();
-    return context.evaluate(pageFunction, ...args);
+    return context.evaluate(pageFunction, ...args as any);
   }
 
   async $(selector: string): Promise<ElementHandle | null> {
@@ -87,12 +105,12 @@ export class DOMWorld {
     return document.$x(expression);
   }
 
-  async $eval(selector: string, pageFunction: Function | string, ...args: Array<any>): Promise<(object | undefined)> {
+  $eval: types.$Eval<JSHandle> = async (selector, pageFunction, ...args) => {
     const document = await this._document();
     return document.$eval(selector, pageFunction, ...args);
   }
 
-  async $$eval(selector: string, pageFunction: Function | string, ...args: Array<any>): Promise<(object | undefined)> {
+  $$eval: types.$$Eval<JSHandle> = async (selector, pageFunction, ...args) => {
     const document = await this._document();
     return document.$$eval(selector, pageFunction, ...args);
   }
diff --git a/src/firefox/ExecutionContext.ts b/src/firefox/ExecutionContext.ts
index 1d55c04cc9..ecff55c952 100644
--- a/src/firefox/ExecutionContext.ts
+++ b/src/firefox/ExecutionContext.ts
@@ -21,8 +21,9 @@ import { Frame } from './FrameManager';
 import * as injectedSource from '../generated/injectedSource';
 import * as cssSelectorEngineSource from '../generated/cssSelectorEngineSource';
 import * as xpathSelectorEngineSource from '../generated/xpathSelectorEngineSource';
+import * as types from '../types';
 
-export class ExecutionContext {
+export class ExecutionContext implements types.EvaluationContext<JSHandle> {
   _session: any;
   _frame: Frame;
   _executionContextId: string;
@@ -34,7 +35,7 @@ export class ExecutionContext {
     this._executionContextId = executionContextId;
   }
 
-  async evaluateHandle(pageFunction, ...args): Promise<JSHandle> {
+  evaluateHandle: types.EvaluateHandle<JSHandle> = async (pageFunction, ...args) => {
     if (helper.isString(pageFunction)) {
       const payload = await this._session.send('Runtime.evaluate', {
         expression: pageFunction.trim(),
@@ -62,7 +63,7 @@ export class ExecutionContext {
         throw new Error('Passed function is not well-serializable!');
       }
     }
-    args = args.map(arg => {
+    const protocolArgs = args.map(arg => {
       if (arg instanceof JSHandle) {
         if (arg._context !== this)
           throw new Error('JSHandles can be evaluated only in the context they were created!');
@@ -84,7 +85,7 @@ export class ExecutionContext {
     try {
       callFunctionPromise = this._session.send('Runtime.callFunction', {
         functionDeclaration: functionText,
-        args,
+        args: protocolArgs,
         executionContextId: this._executionContextId
       });
     } catch (err) {
@@ -106,9 +107,9 @@ export class ExecutionContext {
     return this._frame;
   }
 
-  async evaluate(pageFunction, ...args): Promise<any> {
+  evaluate: types.Evaluate<JSHandle> = async (pageFunction, ...args) => {
     try {
-      const handle = await this.evaluateHandle(pageFunction, ...args);
+      const handle = await this.evaluateHandle(pageFunction, ...args as any);
       const result = await handle.jsonValue();
       await handle.dispose();
       return result;
diff --git a/src/firefox/FrameManager.ts b/src/firefox/FrameManager.ts
index 0a91468262..448c094744 100644
--- a/src/firefox/FrameManager.ts
+++ b/src/firefox/FrameManager.ts
@@ -1,3 +1,20 @@
+/**
+ * Copyright 2019 Google Inc. All rights reserved.
+ * Modifications copyright (c) Microsoft Corporation.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
 import { JugglerSession } from './Connection';
 import { Page } from './Page';
 
@@ -11,6 +28,7 @@ import { JSHandle, ElementHandle } from './JSHandle';
 import { TimeoutSettings } from '../TimeoutSettings';
 import { NetworkManager } from './NetworkManager';
 import { MultiClickOptions, ClickOptions, SelectOption } from '../input';
+import * as types from '../types';
 
 export const FrameManagerEvents = {
   FrameNavigated: Symbol('FrameManagerEvents.FrameNavigated'),
@@ -359,8 +377,8 @@ export class Frame {
     return this._mainWorld.setContent(html);
   }
 
-  async evaluate(pageFunction, ...args): Promise<any> {
-    return this._mainWorld.evaluate(pageFunction, ...args);
+  evaluate: types.Evaluate<JSHandle> = (pageFunction, ...args) => {
+    return this._mainWorld.evaluate(pageFunction, ...args as any);
   }
 
   async $(selector: string): Promise<ElementHandle | null> {
@@ -371,20 +389,20 @@ export class Frame {
     return this._mainWorld.$$(selector);
   }
 
-  async $eval(selector: string, pageFunction: Function | string, ...args: Array<any>): Promise<(object | undefined)> {
-    return this._mainWorld.$eval(selector, pageFunction, ...args);
+  $eval: types.$Eval<JSHandle> = (selector, pageFunction, ...args) => {
+    return this._mainWorld.$eval(selector, pageFunction, ...args as any);
   }
 
-  async $$eval(selector: string, pageFunction: Function | string, ...args: Array<any>): Promise<(object | undefined)> {
-    return this._mainWorld.$$eval(selector, pageFunction, ...args);
+  $$eval: types.$$Eval<JSHandle> = (selector, pageFunction, ...args) => {
+    return this._mainWorld.$$eval(selector, pageFunction, ...args as any);
   }
 
   async $x(expression: string): Promise<Array<ElementHandle>> {
     return this._mainWorld.$x(expression);
   }
 
-  async evaluateHandle(pageFunction, ...args): Promise<JSHandle> {
-    return this._mainWorld.evaluateHandle(pageFunction, ...args);
+  evaluateHandle: types.EvaluateHandle<JSHandle> = (pageFunction, ...args) => {
+    return this._mainWorld.evaluateHandle(pageFunction, ...args as any);
   }
 
   async addScriptTag(options: { content?: string; path?: string; type?: string; url?: string; }): Promise<ElementHandle> {
diff --git a/src/firefox/JSHandle.ts b/src/firefox/JSHandle.ts
index cf411bbae4..5fcd1d337b 100644
--- a/src/firefox/JSHandle.ts
+++ b/src/firefox/JSHandle.ts
@@ -17,6 +17,7 @@
 
 import * as fs from 'fs';
 import * as path from 'path';
+import * as types from '../types';
 import { assert, debugError, helper } from '../helper';
 import { ClickOptions, fillFunction, MultiClickOptions, selectFunction, SelectOption } from '../input';
 import { JugglerSession } from './Connection';
@@ -56,12 +57,12 @@ export class JSHandle {
     return this._context;
   }
 
-  async evaluate(pageFunction: Function | string, ...args: any[]): Promise<(any)> {
-    return await this.executionContext().evaluate(pageFunction, this, ...args);
+  evaluate: types.EvaluateOn<JSHandle> = (pageFunction, ...args) => {
+    return this.executionContext().evaluate(pageFunction, this, ...args);
   }
 
-  async evaluateHandle(pageFunction: Function | string, ...args: any[]): Promise<JSHandle> {
-    return await this.executionContext().evaluateHandle(pageFunction, this, ...args);
+  evaluateHandle: types.EvaluateHandleOn<JSHandle> = (pageFunction, ...args) => {
+    return this.executionContext().evaluateHandle(pageFunction, this, ...args);
   }
 
   toString(): string {
@@ -190,7 +191,7 @@ export class ElementHandle extends JSHandle {
   }
 
   isIntersectingViewport(): Promise<boolean> {
-    return this._frame.evaluate(async element => {
+    return this._frame.evaluate(async (element: Element) => {
       const visibleRatio = await new Promise(resolve => {
         const observer = new IntersectionObserver(entries => {
           resolve(entries[0].intersectionRatio);
@@ -233,7 +234,7 @@ export class ElementHandle extends JSHandle {
     return result;
   }
 
-  async $eval(selector: string, pageFunction: Function | string, ...args: Array<any>): Promise<object | undefined> {
+  $eval: types.$Eval<JSHandle> = async (selector, pageFunction, ...args) => {
     const elementHandle = await this.$(selector);
     if (!elementHandle)
       throw new Error(`Error: failed to find element matching selector "${selector}"`);
@@ -242,7 +243,7 @@ export class ElementHandle extends JSHandle {
     return result;
   }
 
-  async $$eval(selector: string, pageFunction: Function | string, ...args: Array<any>): Promise<object | undefined> {
+  $$eval: types.$$Eval<JSHandle> = async (selector, pageFunction, ...args) => {
     const arrayHandle = await this._frame.evaluateHandle(
       (root: SelectorRoot, selector: string, injected: Injected) => injected.querySelectorAll('css=' + selector, root),
       this, selector, await this._context._injected()
@@ -270,7 +271,7 @@ export class ElementHandle extends JSHandle {
   }
 
   async _scrollIntoViewIfNeeded() {
-    const error = await this._frame.evaluate(async element => {
+    const error = await this._frame.evaluate(async (element: Element) => {
       if (!element.isConnected)
         return 'Node is detached from document';
       if (element.nodeType !== Node.ELEMENT_NODE)
@@ -286,7 +287,7 @@ export class ElementHandle extends JSHandle {
         requestAnimationFrame(() => {});
       });
       if (visibleRatio !== 1.0)
-        element.scrollIntoView({block: 'center', inline: 'center', behavior: 'instant'});
+        element.scrollIntoView({block: 'center', inline: 'center', behavior: ('instant' as ScrollBehavior)});
       return false;
     }, this);
     if (error)
diff --git a/src/firefox/Page.ts b/src/firefox/Page.ts
index 6941170792..493a9f21fb 100644
--- a/src/firefox/Page.ts
+++ b/src/firefox/Page.ts
@@ -16,6 +16,7 @@ import { createHandle, ElementHandle, JSHandle } from './JSHandle';
 import { NavigationWatchdog } from './NavigationWatchdog';
 import { NetworkManager, NetworkManagerEvents, Request, Response } from './NetworkManager';
 import * as input from '../input';
+import * as types from '../types';
 
 const writeFileAsync = helper.promisify(fs.writeFile);
 
@@ -471,8 +472,8 @@ export class Page extends EventEmitter {
     }
   }
 
-  evaluate(pageFunction, ...args) {
-    return this.mainFrame().evaluate(pageFunction, ...args);
+  evaluate: types.Evaluate<JSHandle> = (pageFunction, ...args) => {
+    return this.mainFrame().evaluate(pageFunction, ...args as any);
   }
 
   addScriptTag(options: { content?: string; path?: string; type?: string; url?: string; }): Promise<ElementHandle> {
@@ -543,20 +544,20 @@ export class Page extends EventEmitter {
     return this._frameManager.mainFrame().$$(selector);
   }
 
-  $eval(selector: string, pageFunction: Function | string, ...args: Array<any>): Promise<(object | undefined)> {
-    return this._frameManager.mainFrame().$eval(selector, pageFunction, ...args);
+  $eval: types.$Eval<JSHandle> = (selector, pageFunction, ...args) => {
+    return this._frameManager.mainFrame().$eval(selector, pageFunction, ...args as any);
   }
 
-  $$eval(selector: string, pageFunction: Function | string, ...args: Array<any>): Promise<(object | undefined)> {
-    return this._frameManager.mainFrame().$$eval(selector, pageFunction, ...args);
+  $$eval: types.$$Eval<JSHandle> = (selector, pageFunction, ...args) => {
+    return this._frameManager.mainFrame().$$eval(selector, pageFunction, ...args as any);
   }
 
   $x(expression: string): Promise<Array<ElementHandle>> {
     return this._frameManager.mainFrame().$x(expression);
   }
 
-  evaluateHandle(pageFunction, ...args) {
-    return this._frameManager.mainFrame().evaluateHandle(pageFunction, ...args);
+  evaluateHandle: types.EvaluateHandle<JSHandle> = async (pageFunction, ...args) => {
+    return this._frameManager.mainFrame().evaluateHandle(pageFunction, ...args as any);
   }
 
   async close(options: any = {}) {