diff --git a/src/chromium/DOMWorld.ts b/src/chromium/DOMWorld.ts
index f065aa790cbe3..8096b454a6119 100644
--- a/src/chromium/DOMWorld.ts
+++ b/src/chromium/DOMWorld.ts
@@ -25,9 +25,10 @@ import { ElementHandle, JSHandle } from './JSHandle';
 import { LifecycleWatcher } from './LifecycleWatcher';
 import { TimeoutSettings } from '../TimeoutSettings';
 import { ClickOptions, MultiClickOptions, PointerActionOptions, SelectOption } from '../input';
+import * as types from '../types';
 const readFileAsync = helper.promisify(fs.readFile);
 
-export class DOMWorld {
+export class DOMWorld implements types.DOMEvaluationContext<JSHandle> {
   private _frameManager: FrameManager;
   private _frame: Frame;
   private _timeoutSettings: TimeoutSettings;
@@ -79,14 +80,14 @@ export class DOMWorld {
     return this._contextPromise;
   }
 
-  async evaluateHandle(pageFunction: Function | string, ...args: any[]): Promise<JSHandle> {
+  async evaluateHandle<Args extends any[]>(pageFunction: types.Func<Args>, ...args: types.Boxed<Args, JSHandle>): Promise<JSHandle> {
     const context = await this.executionContext();
-    return context.evaluateHandle(pageFunction, ...args);
+    return context.evaluateHandle(pageFunction, ...args as any);
   }
 
-  async evaluate(pageFunction: Function | string, ...args: any[]): Promise<any> {
+  async evaluate<Args extends any[], R>(pageFunction: types.Func<Args, R>, ...args: types.Boxed<Args, JSHandle>): Promise<R> {
     const context = await this.executionContext();
-    return context.evaluate(pageFunction, ...args);
+    return context.evaluate(pageFunction, ...args as any);
   }
 
   async $(selector: string): Promise<ElementHandle | null> {
@@ -111,15 +112,14 @@ export class DOMWorld {
     return value;
   }
 
-  async $eval(selector: string, pageFunction: Function | string, ...args: any[]): Promise<any> {
+  async $eval<Args extends any[], R>(selector: string, pageFunction: types.FuncOn<Element, Args, R>, ...args: types.Boxed<Args, JSHandle>): Promise<R> {
     const document = await this._document();
-    return document.$eval(selector, pageFunction, ...args);
+    return document.$eval(selector, pageFunction, ...args as any);
   }
 
-  async $$eval(selector: string, pageFunction: Function | string, ...args: any[]): Promise<any> {
+  async $$eval<Args extends any[], R>(selector: string, pageFunction: types.FuncOn<Element[], Args, R>, ...args: types.Boxed<Args, JSHandle>): Promise<R> {
     const document = await this._document();
-    const value = await document.$$eval(selector, pageFunction, ...args);
-    return value;
+    return document.$$eval(selector, pageFunction, ...args as any);
   }
 
   async $$(selector: string): Promise<ElementHandle[]> {
@@ -439,7 +439,7 @@ class WaitTask {
   }
 }
 
-async function waitForPredicatePageFunction(predicateBody: string, polling: string, timeout: number, ...args): Promise<any> {
+async function waitForPredicatePageFunction(predicateBody: string, polling: string | number, timeout: number, ...args): Promise<any> {
   const predicate = new Function('...args', predicateBody);
   let timedOut = false;
   if (timeout)
diff --git a/src/chromium/ExecutionContext.ts b/src/chromium/ExecutionContext.ts
index 9f111fef03cc4..c751081f9d76a 100644
--- a/src/chromium/ExecutionContext.ts
+++ b/src/chromium/ExecutionContext.ts
@@ -25,11 +25,12 @@ import { Protocol } from './protocol';
 import * as injectedSource from '../generated/injectedSource';
 import * as cssSelectorEngineSource from '../generated/cssSelectorEngineSource';
 import * as xpathSelectorEngineSource from '../generated/xpathSelectorEngineSource';
+import * as types from '../types';
 
 export const EVALUATION_SCRIPT_URL = '__playwright_evaluation_script__';
 const SOURCE_URL_REGEX = /^[\040\t]*\/\/[@#] sourceURL=\s*(\S*?)\s*$/m;
 
-export class ExecutionContext {
+export class ExecutionContext implements types.EvaluationContext<JSHandle> {
   _client: CDPSession;
   _world: DOMWorld;
   private _injectedPromise: Promise<JSHandle> | null = null;
@@ -45,11 +46,11 @@ export class ExecutionContext {
     return this._world ? this._world.frame() : null;
   }
 
-  async evaluate(pageFunction: Function | string, ...args: any[]): Promise<any> {
-    return await this._evaluateInternal(true /* returnByValue */, pageFunction, ...args);
+  evaluate<Args extends any[], R>(pageFunction: types.Func<Args, R>, ...args: types.Boxed<Args, JSHandle>): Promise<R> {
+    return this._evaluateInternal(true /* returnByValue */, pageFunction, ...args);
   }
 
-  async evaluateHandle(pageFunction: Function | string, ...args: any[]): Promise<JSHandle> {
+  evaluateHandle<Args extends any[]>(pageFunction: types.Func<Args>, ...args: types.Boxed<Args, JSHandle>): Promise<JSHandle> {
     return this._evaluateInternal(false /* returnByValue */, pageFunction, ...args);
   }
 
diff --git a/src/chromium/Frame.ts b/src/chromium/Frame.ts
index 719d0d7eddc8f..08abb5f6b36f7 100644
--- a/src/chromium/Frame.ts
+++ b/src/chromium/Frame.ts
@@ -24,8 +24,9 @@ import { FrameManager } from './FrameManager';
 import { ElementHandle, JSHandle } from './JSHandle';
 import { Response } from './NetworkManager';
 import { Protocol } from './protocol';
+import * as types from '../types';
 
-export class Frame {
+export class Frame implements types.DOMEvaluationContext<JSHandle> {
   _id: string;
   _frameManager: FrameManager;
   private _client: CDPSession;
@@ -68,12 +69,12 @@ export class Frame {
     return this._mainWorld.executionContext();
   }
 
-  async evaluateHandle(pageFunction: Function | string, ...args: any[]): Promise<JSHandle> {
-    return this._mainWorld.evaluateHandle(pageFunction, ...args);
+  evaluateHandle<Args extends any[]>(pageFunction: types.Func<Args>, ...args: types.Boxed<Args, JSHandle>): Promise<JSHandle> {
+    return this._mainWorld.evaluateHandle(pageFunction, ...args as any);
   }
 
-  async evaluate(pageFunction: Function | string, ...args: any[]): Promise<any> {
-    return this._mainWorld.evaluate(pageFunction, ...args);
+  evaluate<Args extends any[], R>(pageFunction: types.Func<Args, R>, ...args: types.Boxed<Args, JSHandle>): Promise<R> {
+    return this._mainWorld.evaluate(pageFunction, ...args as any);
   }
 
   async $(selector: string): Promise<ElementHandle | null> {
@@ -84,12 +85,12 @@ export class Frame {
     return this._mainWorld.$x(expression);
   }
 
-  async $eval(selector: string, pageFunction: Function | string, ...args: any[]): Promise<(object | undefined)> {
-    return this._mainWorld.$eval(selector, pageFunction, ...args);
+  $eval<Args extends any[], R>(selector: string, pageFunction: types.FuncOn<Element, Args, R>, ...args: types.Boxed<Args, JSHandle>): Promise<R> {
+    return this._mainWorld.$eval(selector, pageFunction, ...args as any);
   }
 
-  async $$eval(selector: string, pageFunction: Function | string, ...args: any[]): Promise<(object | undefined)> {
-    return this._mainWorld.$$eval(selector, pageFunction, ...args);
+  $$eval<Args extends any[], R>(selector: string, pageFunction: types.FuncOn<Element[], Args, R>, ...args: types.Boxed<Args, JSHandle>): Promise<R> {
+    return this._mainWorld.$$eval(selector, pageFunction, ...args as any);
   }
 
   async $$(selector: string): Promise<ElementHandle[]> {
diff --git a/src/chromium/JSHandle.ts b/src/chromium/JSHandle.ts
index 370e8691174b4..358ef0acd36ed 100644
--- a/src/chromium/JSHandle.ts
+++ b/src/chromium/JSHandle.ts
@@ -26,6 +26,7 @@ import { Page } from './Page';
 import { Protocol } from './protocol';
 import { releaseObject, valueFromRemoteObject } from './protocolHelper';
 import Injected from '../injected/injected';
+import * as types from '../types';
 
 type SelectorRoot = Element | ShadowRoot | Document;
 
@@ -43,7 +44,7 @@ export function createJSHandle(context: ExecutionContext, remoteObject: Protocol
   return new JSHandle(context, context._client, remoteObject);
 }
 
-export class JSHandle {
+export class JSHandle implements types.HandleEvaluationContext<JSHandle> {
   _context: ExecutionContext;
   protected _client: CDPSession;
   _remoteObject: Protocol.Runtime.RemoteObject;
@@ -59,12 +60,12 @@ export class JSHandle {
     return this._context;
   }
 
-  async evaluate(pageFunction: Function | string, ...args: any[]): Promise<any> {
-    return await this.executionContext().evaluate(pageFunction, this, ...args);
+  evaluate<Args extends any[], R>(pageFunction: types.FuncOn<any, Args, R>, ...args: types.Boxed<Args, JSHandle>): Promise<R> {
+    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<Args extends any[]>(pageFunction: types.FuncOn<any, Args>, ...args: types.Boxed<Args, JSHandle>): Promise<JSHandle> {
+    return this.executionContext().evaluateHandle(pageFunction, this, ...args);
   }
 
   async getProperty(propertyName: string): Promise<JSHandle | null> {
@@ -432,22 +433,22 @@ export class ElementHandle extends JSHandle {
     return result;
   }
 
-  async $eval(selector: string, pageFunction: Function | string, ...args: any[]): Promise<object | undefined> {
+  async $eval<Args extends any[], R>(selector: string, pageFunction: types.FuncOn<Element, Args, R>, ...args: types.Boxed<Args, JSHandle>): Promise<R> {
     const elementHandle = await this.$(selector);
     if (!elementHandle)
       throw new Error(`Error: failed to find element matching selector "${selector}"`);
-    const result = await elementHandle.evaluate(pageFunction, ...args);
+    const result = await elementHandle.evaluate(pageFunction, ...args as any);
     await elementHandle.dispose();
     return result;
   }
 
-  async $$eval(selector: string, pageFunction: Function | string, ...args: any[]): Promise<object | undefined> {
+  async $$eval<Args extends any[], R>(selector: string, pageFunction: types.FuncOn<Element[], Args, R>, ...args: types.Boxed<Args, JSHandle>): Promise<R> {
     const arrayHandle = await this.evaluateHandle(
       (root: SelectorRoot, selector: string, injected: Injected) => injected.querySelectorAll('css=' + selector, root),
       selector, await this._context._injected()
     );
 
-    const result = await arrayHandle.evaluate(pageFunction, ...args);
+    const result = await arrayHandle.evaluate(pageFunction, ...args as any);
     await arrayHandle.dispose();
     return result;
   }
diff --git a/src/chromium/Page.ts b/src/chromium/Page.ts
index 8256942129fbf..8347dc75f5d6f 100644
--- a/src/chromium/Page.ts
+++ b/src/chromium/Page.ts
@@ -43,6 +43,7 @@ import { Protocol } from './protocol';
 import { getExceptionMessage, releaseObject, valueFromRemoteObject } from './protocolHelper';
 import { Target } from './Target';
 import { TaskQueue } from './TaskQueue';
+import * as types from '../types';
 
 const writeFileAsync = helper.promisify(fs.writeFile);
 
@@ -55,7 +56,7 @@ export type Viewport = {
   hasTouch?: boolean;
 }
 
-export class Page extends EventEmitter {
+export class Page extends EventEmitter implements types.DOMEvaluationContext<JSHandle> {
   private _closed = false;
   _client: CDPSession;
   private _target: Target;
@@ -230,17 +231,16 @@ export class Page extends EventEmitter {
     return this.mainFrame().$(selector);
   }
 
-  async evaluateHandle(pageFunction: Function | string, ...args: any[]): Promise<JSHandle> {
-    const context = await this.mainFrame().executionContext();
-    return context.evaluateHandle(pageFunction, ...args);
+  evaluateHandle<Args extends any[]>(pageFunction: types.Func<Args>, ...args: types.Boxed<Args, JSHandle>): Promise<JSHandle> {
+    return this.mainFrame().evaluateHandle(pageFunction, ...args as any);
   }
 
-  async $eval(selector: string, pageFunction: Function | string, ...args: any[]): Promise<(object | undefined)> {
-    return this.mainFrame().$eval(selector, pageFunction, ...args);
+  $eval<Args extends any[], R>(selector: string, pageFunction: types.FuncOn<Element, Args, R>, ...args: types.Boxed<Args, JSHandle>): Promise<R> {
+    return this.mainFrame().$eval(selector, pageFunction, ...args as any);
   }
 
-  async $$eval(selector: string, pageFunction: Function | string, ...args: any[]): Promise<(object | undefined)> {
-    return this.mainFrame().$$eval(selector, pageFunction, ...args);
+  $$eval<Args extends any[], R>(selector: string, pageFunction: types.FuncOn<Element[], Args, R>, ...args: types.Boxed<Args, JSHandle>): Promise<R> {
+    return this.mainFrame().$$eval(selector, pageFunction, ...args as any);
   }
 
   async $$(selector: string): Promise<ElementHandle[]> {
@@ -565,8 +565,8 @@ export class Page extends EventEmitter {
     return this._viewport;
   }
 
-  async evaluate(pageFunction: Function | string, ...args: any[]): Promise<any> {
-    return this._frameManager.mainFrame().evaluate(pageFunction, ...args);
+  evaluate<Args extends any[], R>(pageFunction: types.Func<Args, R>, ...args: types.Boxed<Args, JSHandle>): Promise<R> {
+    return this._frameManager.mainFrame().evaluate(pageFunction, ...args as any);
   }
 
   async evaluateOnNewDocument(pageFunction: Function | string, ...args: any[]) {
diff --git a/src/chromium/features/workers.ts b/src/chromium/features/workers.ts
index e5de3d7276ce3..e7de94e88b53e 100644
--- a/src/chromium/features/workers.ts
+++ b/src/chromium/features/workers.ts
@@ -21,6 +21,7 @@ import { debugError } from '../../helper';
 import { JSHandle } from '../JSHandle';
 import { Protocol } from '../protocol';
 import { Events } from '../events';
+import * as types from '../../types';
 
 type AddToConsoleCallback = (type: string, args: JSHandle[], stackTrace: Protocol.Runtime.StackTrace | undefined) => void;
 type HandleExceptionCallback = (exceptionDetails: Protocol.Runtime.ExceptionDetails) => void;
@@ -53,7 +54,7 @@ export class Workers extends EventEmitter {
   }
 }
 
-export class Worker extends EventEmitter {
+export class Worker extends EventEmitter implements types.EvaluationContext<JSHandle> {
   private _client: CDPSession;
   private _url: string;
   private _executionContextPromise: Promise<ExecutionContext>;
@@ -85,11 +86,11 @@ export class Worker extends EventEmitter {
     return this._executionContextPromise;
   }
 
-  async evaluate(pageFunction: Function | string, ...args: any[]): Promise<any> {
-    return (await this._executionContextPromise).evaluate(pageFunction, ...args);
+  async evaluate<Args extends any[], R>(pageFunction: string | ((...args: Args) => R | Promise<R>), ...args: types.Boxed<Args, JSHandle>): Promise<R> {
+    return (await this._executionContextPromise).evaluate(pageFunction, ...args as any);
   }
 
-  async evaluateHandle(pageFunction: Function | string, ...args: any[]): Promise<JSHandle> {
-    return (await this._executionContextPromise).evaluateHandle(pageFunction, ...args);
+  async evaluateHandle<Args extends any[]>(pageFunction: string | ((...args: Args) => any), ...args: types.Boxed<Args, JSHandle>): Promise<JSHandle> {
+    return (await this._executionContextPromise).evaluateHandle(pageFunction, ...args as any);
   }
 }
diff --git a/src/firefox/DOMWorld.ts b/src/firefox/DOMWorld.ts
index 2df99d64a8ab9..cfffd2855278a 100644
--- a/src/firefox/DOMWorld.ts
+++ b/src/firefox/DOMWorld.ts
@@ -4,10 +4,11 @@ import * as fs from 'fs';
 import * as util from 'util';
 import {ElementHandle, JSHandle} from './JSHandle';
 import { ExecutionContext } from './ExecutionContext';
+import * as types from '../types';
 
 const readFileAsync = util.promisify(fs.readFile);
 
-export class DOMWorld {
+export class DOMWorld implements types.DOMEvaluationContext<JSHandle> {
   _frame: any;
   _timeoutSettings: any;
   _documentPromise: any;
@@ -61,14 +62,14 @@ export class DOMWorld {
     throw new Error('Method not implemented.');
   }
 
-  async evaluateHandle(pageFunction, ...args): Promise<JSHandle> {
+  async evaluateHandle<Args extends any[]>(pageFunction: types.Func<Args>, ...args: types.Boxed<Args, JSHandle>): Promise<JSHandle> {
     const context = await this.executionContext();
-    return context.evaluateHandle(pageFunction, ...args);
+    return context.evaluateHandle(pageFunction, ...args as any);
   }
 
-  async evaluate(pageFunction, ...args): Promise<any> {
+  async evaluate<Args extends any[], R>(pageFunction: types.Func<Args, R>, ...args: types.Boxed<Args, JSHandle>): Promise<R> {
     const context = await this.executionContext();
-    return context.evaluate(pageFunction, ...args);
+    return context.evaluate(pageFunction, ...args as any);
   }
 
   async $(selector: string): Promise<ElementHandle | null> {
@@ -87,14 +88,14 @@ export class DOMWorld {
     return document.$x(expression);
   }
 
-  async $eval(selector: string, pageFunction: Function | string, ...args: Array<any>): Promise<(object | undefined)> {
+  async $eval<Args extends any[], R>(selector: string, pageFunction: types.FuncOn<Element, Args, R>, ...args: types.Boxed<Args, JSHandle>): Promise<R> {
     const document = await this._document();
-    return document.$eval(selector, pageFunction, ...args);
+    return document.$eval(selector, pageFunction, ...args as any);
   }
 
-  async $$eval(selector: string, pageFunction: Function | string, ...args: Array<any>): Promise<(object | undefined)> {
+  async $$eval<Args extends any[], R>(selector: string, pageFunction: types.FuncOn<Element[], Args, R>, ...args: types.Boxed<Args, JSHandle>): Promise<R> {
     const document = await this._document();
-    return document.$$eval(selector, pageFunction, ...args);
+    return document.$$eval(selector, pageFunction, ...args as any);
   }
 
   async $$(selector: string): Promise<Array<ElementHandle>> {
diff --git a/src/firefox/ExecutionContext.ts b/src/firefox/ExecutionContext.ts
index 1d55c04cc908e..0cf9d8bf98fdf 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> {
+  async evaluateHandle<Args extends any[]>(pageFunction: types.Func<Args>, ...args: types.Boxed<Args, JSHandle>): Promise<JSHandle> {
     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> {
+  async evaluate<Args extends any[], R>(pageFunction: types.Func<Args, R>, ...args: types.Boxed<Args, JSHandle>): Promise<R> {
     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 0a91468262bb3..96fda6029e4c6 100644
--- a/src/firefox/FrameManager.ts
+++ b/src/firefox/FrameManager.ts
@@ -11,6 +11,7 @@ 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'),
@@ -139,7 +140,7 @@ export class FrameManager extends EventEmitter {
   }
 }
 
-export class Frame {
+export class Frame implements types.DOMEvaluationContext<JSHandle> {
   _parentFrame: Frame|null = null;
   private _session: JugglerSession;
   _page: Page;
@@ -359,8 +360,8 @@ export class Frame {
     return this._mainWorld.setContent(html);
   }
 
-  async evaluate(pageFunction, ...args): Promise<any> {
-    return this._mainWorld.evaluate(pageFunction, ...args);
+  evaluate<Args extends any[], R>(pageFunction: types.Func<Args, R>, ...args: types.Boxed<Args, JSHandle>): Promise<R> {
+    return this._mainWorld.evaluate(pageFunction, ...args as any);
   }
 
   async $(selector: string): Promise<ElementHandle | null> {
@@ -371,20 +372,20 @@ 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);
+  $eval<Args extends any[], R>(selector: string, pageFunction: types.FuncOn<Element, Args, R>, ...args: types.Boxed<Args, JSHandle>): Promise<R> {
+    return this._mainWorld.$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);
+  $$eval<Args extends any[], R>(selector: string, pageFunction: types.FuncOn<Element[], Args, R>, ...args: types.Boxed<Args, JSHandle>): Promise<R> {
+    return this._mainWorld.$$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<Args extends any[]>(pageFunction: types.Func<Args>, ...args: types.Boxed<Args, JSHandle>): Promise<JSHandle> {
+    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 4962273aeadf6..72157e02f4067 100644
--- a/src/firefox/JSHandle.ts
+++ b/src/firefox/JSHandle.ts
@@ -20,12 +20,13 @@ import { assert, debugError, helper } from '../helper';
 import { ClickOptions, fillFunction, MultiClickOptions, selectFunction, SelectOption } from '../input';
 import { JugglerSession } from './Connection';
 import Injected from '../injected/injected';
-
-type SelectorRoot = Element | ShadowRoot | Document;
 import { ExecutionContext } from './ExecutionContext';
 import { Frame } from './FrameManager';
+import * as types from '../types';
+
+type SelectorRoot = Element | ShadowRoot | Document;
 
-export class JSHandle {
+export class JSHandle implements types.HandleEvaluationContext<JSHandle> {
   _context: ExecutionContext;
   protected _session: JugglerSession;
   private _executionContextId: string;
@@ -54,12 +55,12 @@ export class JSHandle {
     return this._context;
   }
 
-  async evaluate(pageFunction: Function | string, ...args: any[]): Promise<(any)> {
-    return await this.executionContext().evaluate(pageFunction, this, ...args);
+  evaluate<Args extends any[], R>(pageFunction: types.FuncOn<any, Args, R>, ...args: types.Boxed<Args, JSHandle>): Promise<R> {
+    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<Args extends any[]>(pageFunction: types.FuncOn<any, Args>, ...args: types.Boxed<Args, JSHandle>): Promise<JSHandle> {
+    return this.executionContext().evaluateHandle(pageFunction, this, ...args);
   }
 
   toString(): string {
@@ -188,7 +189,7 @@ 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);
@@ -231,22 +232,22 @@ export class ElementHandle extends JSHandle {
     return result;
   }
 
-  async $eval(selector: string, pageFunction: Function | string, ...args: Array<any>): Promise<object | undefined> {
+  async $eval<Args extends any[], R>(selector: string, pageFunction: types.FuncOn<Element, Args, R>, ...args: types.Boxed<Args, JSHandle>): Promise<R> {
     const elementHandle = await this.$(selector);
     if (!elementHandle)
       throw new Error(`Error: failed to find element matching selector "${selector}"`);
-    const result = await this._frame.evaluate(pageFunction, elementHandle, ...args);
+    const result = await this._frame.evaluate(pageFunction, elementHandle, ...args as any);
     await elementHandle.dispose();
     return result;
   }
 
-  async $$eval(selector: string, pageFunction: Function | string, ...args: Array<any>): Promise<object | undefined> {
+  async $$eval<Args extends any[], R>(selector: string, pageFunction: types.FuncOn<Element[], Args, R>, ...args: types.Boxed<Args, JSHandle>): Promise<R> {
     const arrayHandle = await this._frame.evaluateHandle(
       (root: SelectorRoot, selector: string, injected: Injected) => injected.querySelectorAll('css=' + selector, root),
       this, selector, await this._context._injected()
     );
 
-    const result = await this._frame.evaluate(pageFunction, arrayHandle, ...args);
+    const result = await this._frame.evaluate(pageFunction, arrayHandle, ...args as any);
     await arrayHandle.dispose();
     return result;
   }
@@ -268,7 +269,7 @@ 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)
@@ -284,7 +285,7 @@ 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)
@@ -325,7 +326,7 @@ export class ElementHandle extends JSHandle {
   }
 
   async focus() {
-    await this._frame.evaluate(element => element.focus(), this);
+    await this._frame.evaluate((element: HTMLElement) => element.focus(), this);
   }
 
   async type(text: string, options: { delay: (number | undefined); } | undefined) {
diff --git a/src/firefox/Page.ts b/src/firefox/Page.ts
index cb2490d492239..0a202fe1880d4 100644
--- a/src/firefox/Page.ts
+++ b/src/firefox/Page.ts
@@ -16,11 +16,12 @@ import { createHandle, ElementHandle, JSHandle } from './JSHandle';
 import { NavigationWatchdog } from './NavigationWatchdog';
 import { NetworkManager, NetworkManagerEvents, Request, Response } from './NetworkManager';
 import { ClickOptions, MultiClickOptions } from '../input';
+import * as types from '../types';
 
 
 const writeFileAsync = helper.promisify(fs.writeFile);
 
-export class Page extends EventEmitter {
+export class Page extends EventEmitter implements types.DOMEvaluationContext<JSHandle> {
   private _timeoutSettings: TimeoutSettings;
   private _session: JugglerSession;
   private _target: Target;
@@ -460,8 +461,8 @@ export class Page extends EventEmitter {
     }
   }
 
-  evaluate(pageFunction, ...args) {
-    return this.mainFrame().evaluate(pageFunction, ...args);
+  evaluate<Args extends any[], R>(pageFunction: types.Func<Args, R>, ...args: types.Boxed<Args, JSHandle>): Promise<R> {
+    return this.mainFrame().evaluate(pageFunction, ...args as any);
   }
 
   addScriptTag(options: { content?: string; path?: string; type?: string; url?: string; }): Promise<ElementHandle> {
@@ -532,20 +533,20 @@ 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);
+  $eval<Args extends any[], R>(selector: string, pageFunction: types.FuncOn<Element, Args, R>, ...args: types.Boxed<Args, JSHandle>): Promise<R> {
+    return this._frameManager.mainFrame().$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);
+  $$eval<Args extends any[], R>(selector: string, pageFunction: types.FuncOn<Element[], Args, R>, ...args: types.Boxed<Args, JSHandle>): Promise<R> {
+    return this._frameManager.mainFrame().$$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<Args extends any[]>(pageFunction: types.Func<Args>, ...args: types.Boxed<Args, JSHandle>): Promise<JSHandle> {
+    return this._frameManager.mainFrame().evaluateHandle(pageFunction, ...args as any);
   }
 
   async close(options: any = {}) {
diff --git a/src/types.ts b/src/types.ts
new file mode 100644
index 0000000000000..4b304710141e1
--- /dev/null
+++ b/src/types.ts
@@ -0,0 +1,21 @@
+// Copyright (c) Microsoft Corporation.
+// Licensed under the MIT license.
+
+export type Boxed<Args extends any[], Handle> = { [Index in keyof Args]: Args[Index] | Handle };
+export type Func<Args extends any[], R = any> = string | ((...args: Args) => R | Promise<R>);
+export type FuncOn<On, Args extends any[], R = any> = string | ((on: On, ...args: Args) => R | Promise<R>);
+
+export interface EvaluationContext<Handle> {
+  evaluate<Args extends any[], R>(fn: Func<Args, R>, ...args: Boxed<Args, Handle>): Promise<R>;
+  evaluateHandle<Args extends any[]>(fn: Func<Args>, ...args: Boxed<Args, Handle>): Promise<Handle>;
+}
+
+export interface DOMEvaluationContext<Handle> extends EvaluationContext<Handle> {
+  $eval<Args extends any[], R>(selector: string, fn: FuncOn<Element, Args, R>, ...args: Boxed<Args, Handle>): Promise<R>;
+  $$eval<Args extends any[], R>(selector: string, fn: FuncOn<Element[], Args, R>, ...args: Boxed<Args, Handle>): Promise<R>;
+}
+
+export interface HandleEvaluationContext<Handle> {
+  evaluate<Args extends any[], R>(fn: FuncOn<any, Args, R>, ...args: Boxed<Args, Handle>): Promise<R>;
+  evaluateHandle<Args extends any[]>(fn: FuncOn<any, Args>, ...args: Boxed<Args, Handle>): Promise<Handle>;
+}
diff --git a/src/webkit/Browser.ts b/src/webkit/Browser.ts
index 316232b9952fe..e5e63eded91cb 100644
--- a/src/webkit/Browser.ts
+++ b/src/webkit/Browser.ts
@@ -75,7 +75,7 @@ export class Browser extends EventEmitter {
   async userAgent(): Promise<string> {
     const context = await this.createIncognitoBrowserContext();
     const page = await context.newPage();
-    const userAgent = await page.evaluate('navigator.userAgent');
+    const userAgent = await page.evaluate(() => navigator.userAgent);
     context.close();
     return userAgent;
   }
diff --git a/src/webkit/ExecutionContext.ts b/src/webkit/ExecutionContext.ts
index 627454343746f..5ec33f7a63a33 100644
--- a/src/webkit/ExecutionContext.ts
+++ b/src/webkit/ExecutionContext.ts
@@ -24,11 +24,12 @@ import { Protocol } from './protocol';
 import * as injectedSource from '../generated/injectedSource';
 import * as cssSelectorEngineSource from '../generated/cssSelectorEngineSource';
 import * as xpathSelectorEngineSource from '../generated/xpathSelectorEngineSource';
+import * as types from '../types';
 
 export const EVALUATION_SCRIPT_URL = '__playwright_evaluation_script__';
 const SOURCE_URL_REGEX = /^[\040\t]*\/\/[@#] sourceURL=\s*(\S*?)\s*$/m;
 
-export class ExecutionContext {
+export class ExecutionContext implements types.EvaluationContext<JSHandle> {
   _globalObjectId?: string;
   _session: TargetSession;
   _frame: Frame;
@@ -55,11 +56,11 @@ export class ExecutionContext {
     return this._frame;
   }
 
-  async evaluate(pageFunction: Function | string, ...args: any[]): Promise<any> {
-    return await this._evaluateInternal(true /* returnByValue */, pageFunction, ...args);
+  evaluate<Args extends any[], R>(pageFunction: types.Func<Args, R>, ...args: types.Boxed<Args, JSHandle>): Promise<R> {
+    return this._evaluateInternal(true /* returnByValue */, pageFunction, ...args);
   }
 
-  async evaluateHandle(pageFunction: Function | string, ...args: any[]): Promise<JSHandle> {
+  evaluateHandle<Args extends any[]>(pageFunction: types.Func<Args>, ...args: types.Boxed<Args, JSHandle>): Promise<JSHandle> {
     return this._evaluateInternal(false /* returnByValue */, pageFunction, ...args);
   }
 
diff --git a/src/webkit/FrameManager.ts b/src/webkit/FrameManager.ts
index 2026c911d0e18..a7b8daec167b1 100644
--- a/src/webkit/FrameManager.ts
+++ b/src/webkit/FrameManager.ts
@@ -27,6 +27,7 @@ import { NetworkManager, NetworkManagerEvents, Request, Response } from './Netwo
 import { Page } from './Page';
 import { Protocol } from './protocol';
 import { MultiClickOptions, ClickOptions, SelectOption } from '../input';
+import * as types from '../types';
 const readFileAsync = helper.promisify(fs.readFile);
 
 export const FrameManagerEvents = {
@@ -229,7 +230,7 @@ export class FrameManager extends EventEmitter {
   }
 }
 
-export class Frame {
+export class Frame implements types.DOMEvaluationContext<JSHandle> {
   _id: string;
   _frameManager: FrameManager;
   _session: any;
@@ -309,14 +310,14 @@ export class Frame {
     return this._contextPromise;
   }
 
-  async evaluateHandle(pageFunction: Function | string, ...args: Array<any>): Promise<JSHandle> {
+  async evaluateHandle<Args extends any[]>(pageFunction: types.Func<Args>, ...args: types.Boxed<Args, JSHandle>): Promise<JSHandle> {
     const context = await this.executionContext();
-    return context.evaluateHandle(pageFunction, ...args);
+    return context.evaluateHandle(pageFunction, ...args as any);
   }
 
-  async evaluate(pageFunction: Function | string, ...args: Array<any>): Promise<any> {
+  async evaluate<Args extends any[], R>(pageFunction: types.Func<Args, R>, ...args: types.Boxed<Args, JSHandle>): Promise<R> {
     const context = await this.executionContext();
-    return context.evaluate(pageFunction, ...args);
+    return context.evaluate(pageFunction, ...args as any);
   }
 
   async $(selector: string): Promise<ElementHandle | null> {
@@ -341,14 +342,14 @@ export class Frame {
     return value;
   }
 
-  async $eval(selector: string, pageFunction: Function | string, ...args: Array<any>): Promise<(any)> {
+  async $eval<Args extends any[], R>(selector: string, pageFunction: types.FuncOn<Element, Args, R>, ...args: types.Boxed<Args, JSHandle>): Promise<R> {
     const document = await this._document();
-    return document.$eval(selector, pageFunction, ...args);
+    return document.$eval(selector, pageFunction, ...args as any);
   }
 
-  async $$eval(selector: string, pageFunction: Function | string, ...args: Array<any>): Promise<(any)> {
+  async $$eval<Args extends any[], R>(selector: string, pageFunction: types.FuncOn<Element[], Args, R>, ...args: types.Boxed<Args, JSHandle>): Promise<R> {
     const document = await this._document();
-    const value = await document.$$eval(selector, pageFunction, ...args);
+    const value = await document.$$eval(selector, pageFunction, ...args as any);
     return value;
   }
 
diff --git a/src/webkit/JSHandle.ts b/src/webkit/JSHandle.ts
index f4af598f8298d..510238e755e3f 100644
--- a/src/webkit/JSHandle.ts
+++ b/src/webkit/JSHandle.ts
@@ -24,6 +24,7 @@ import { Page } from './Page';
 import { Protocol } from './protocol';
 import { releaseObject, valueFromRemoteObject } from './protocolHelper';
 import Injected from '../injected/injected';
+import * as types from '../types';
 
 type SelectorRoot = Element | ShadowRoot | Document;
 
@@ -38,7 +39,7 @@ export function createJSHandle(context: ExecutionContext, remoteObject: Protocol
   return new JSHandle(context, context._session, remoteObject);
 }
 
-export class JSHandle {
+export class JSHandle implements types.HandleEvaluationContext<JSHandle> {
   _context: ExecutionContext;
   protected _client: TargetSession;
   _remoteObject: Protocol.Runtime.RemoteObject;
@@ -54,12 +55,12 @@ export class JSHandle {
     return this._context;
   }
 
-  async evaluate(pageFunction: Function | string, ...args: any[]): Promise<any> {
-    return await this.executionContext().evaluate(pageFunction, this, ...args);
+  evaluate<Args extends any[], R>(pageFunction: types.FuncOn<any, Args, R>, ...args: types.Boxed<Args, JSHandle>): Promise<R> {
+    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<Args extends any[]>(pageFunction: types.FuncOn<any, Args>, ...args: types.Boxed<Args, JSHandle>): Promise<JSHandle> {
+    return this.executionContext().evaluateHandle(pageFunction, this, ...args);
   }
 
   async getProperty(propertyName: string): Promise<JSHandle | null> {
@@ -311,22 +312,22 @@ export class ElementHandle extends JSHandle {
     return result;
   }
 
-  async $eval(selector: string, pageFunction: Function | string, ...args: any[]): Promise<object | undefined> {
+  async $eval<Args extends any[], R>(selector: string, pageFunction: types.FuncOn<Element, Args, R>, ...args: types.Boxed<Args, JSHandle>): Promise<R> {
     const elementHandle = await this.$(selector);
     if (!elementHandle)
       throw new Error(`Error: failed to find element matching selector "${selector}"`);
-    const result = await elementHandle.evaluate(pageFunction, ...args);
+    const result = await elementHandle.evaluate(pageFunction, ...args as any);
     await elementHandle.dispose();
     return result;
   }
 
-  async $$eval(selector: string, pageFunction: Function | string, ...args: any[]): Promise<object | undefined> {
+  async $$eval<Args extends any[], R>(selector: string, pageFunction: types.FuncOn<Element[], Args, R>, ...args: types.Boxed<Args, JSHandle>): Promise<R> {
     const arrayHandle = await this.evaluateHandle(
       (root: SelectorRoot, selector: string, injected: Injected) => injected.querySelectorAll('css=' + selector, root),
       selector, await this._context._injected()
     );
 
-    const result = await arrayHandle.evaluate(pageFunction, ...args);
+    const result = await arrayHandle.evaluate(pageFunction, ...args as any);
     await arrayHandle.dispose();
     return result;
   }
diff --git a/src/webkit/Page.ts b/src/webkit/Page.ts
index 5c87b76f2bda9..271cb19b54861 100644
--- a/src/webkit/Page.ts
+++ b/src/webkit/Page.ts
@@ -32,6 +32,7 @@ import { Protocol } from './protocol';
 import { valueFromRemoteObject } from './protocolHelper';
 import { Target } from './Target';
 import { TaskQueue } from './TaskQueue';
+import * as types from '../types';
 
 const writeFileAsync = helper.promisify(fs.writeFile);
 
@@ -40,7 +41,7 @@ export type Viewport = {
   height: number;
 }
 
-export class Page extends EventEmitter {
+export class Page extends EventEmitter implements types.DOMEvaluationContext<JSHandle> {
   private _closed = false;
   private _session: TargetSession;
   private _target: Target;
@@ -198,17 +199,16 @@ export class Page extends EventEmitter {
     return this.mainFrame().$(selector);
   }
 
-  async evaluateHandle(pageFunction: Function | string, ...args: any[]): Promise<JSHandle> {
-    const context = await this.mainFrame().executionContext();
-    return context.evaluateHandle(pageFunction, ...args);
+  evaluateHandle<Args extends any[]>(pageFunction: types.Func<Args>, ...args: types.Boxed<Args, JSHandle>): Promise<JSHandle> {
+    return this.mainFrame().evaluateHandle(pageFunction, ...args as any);
   }
 
-  async $eval(selector: string, pageFunction: Function | string, ...args: any[]): Promise<(object | undefined)> {
-    return this.mainFrame().$eval(selector, pageFunction, ...args);
+  $eval<Args extends any[], R>(selector: string, pageFunction: types.FuncOn<Element, Args, R>, ...args: types.Boxed<Args, JSHandle>): Promise<R> {
+    return this.mainFrame().$eval(selector, pageFunction, ...args as any);
   }
 
-  async $$eval(selector: string, pageFunction: Function | string, ...args: any[]): Promise<(object | undefined)> {
-    return this.mainFrame().$$eval(selector, pageFunction, ...args);
+  $$eval<Args extends any[], R>(selector: string, pageFunction: types.FuncOn<Element[], Args, R>, ...args: types.Boxed<Args, JSHandle>): Promise<R> {
+    return this.mainFrame().$$eval(selector, pageFunction, ...args as any);
   }
 
   async $$(selector: string): Promise<ElementHandle[]> {
@@ -342,8 +342,8 @@ export class Page extends EventEmitter {
     return this._viewport;
   }
 
-  async evaluate(pageFunction: Function | string, ...args: any[]): Promise<any> {
-    return this._frameManager.mainFrame().evaluate(pageFunction, ...args);
+  evaluate<Args extends any[], R>(pageFunction: types.Func<Args, R>, ...args: types.Boxed<Args, JSHandle>): Promise<R> {
+    return this._frameManager.mainFrame().evaluate(pageFunction, ...args as any);
   }
 
   async evaluateOnNewDocument(pageFunction: Function | string, ...args: Array<any>) {