Add more explanation for page.evaluateHandle()

docs/api.md

@@ -695,22 +695,21 @@ List of all available devices is available in the source code: [DeviceDescriptor
 #### page.evaluate(pageFunction, ...args)
 - `pageFunction` <[function]|[string]> Function to be evaluated in the page context
 - `...args` <...[Serializable]|[JSHandle]> Arguments to pass to `pageFunction`
-- returns: <[Promise]<[Serializable]>> Resolves to the return value of `pageFunction`
+- returns: <[Promise]<[Serializable]>> Promise which resolves to the return value of `pageFunction` as a javascript-object
 
-If the function, passed to the `page.evaluate`, returns a [Promise], then `page.evaluate` would wait for the promise to resolve and return its value.
+If the function passed to the `page.evaluate` returns a [Promise], then `page.evaluate` would wait for the promise to resolve and return its value.
 
 If the function passed into `page.evaluate` returns a non-[Serializable] value, then `page.evaluate` resolves to `undefined`.
-Passing arguments to ```pageFunction```.
 
+Passing arguments to `pageFunction`:
 ```js
 const result = await page.evaluate(x => {
   return Promise.resolve(8 * x);
 }, 7);
 console.log(result); // prints "56"
 ```
 
-A string can also be passed in instead of a function.
-
+A string can also be passed in instead of a function:
 ```js
 console.log(await page.evaluate('1 + 2')); // prints "3"
 const x = 10;
@@ -729,19 +728,19 @@ Shortcut for [page.mainFrame().evaluate(pageFunction, ...args)](#frameevaluatepa
 #### page.evaluateHandle(pageFunction, ...args)
 - `pageFunction` <[function]|[string]> Function to be evaluated in the page context
 - `...args` <...[Serializable]|[JSHandle]> Arguments to pass to `pageFunction`
-- returns: <[Promise]<[JSHandle]>> Resolves to the return value of `pageFunction`
+- returns: <[Promise]<[JSHandle]>> Promise which resolves to the return value of `pageFunction` as in-page object (JSHandle)
 
-If the function, passed to the `page.evaluateHandle`, returns a [Promise], then `page.evaluateHandle` would wait for the promise to resolve and return its value.
+If the function passed to the `page.evaluateHandle` returns a [Promise], then `page.evaluateHandle` would wait for the promise to resolve and return its value.
 
+The only difference between `page.evaluate` and `page.evaluateHandler` is that the former returns JSON representation of the object and the latter returns in-page object (JSHandle):
 ```js
-const aWindowHandle = await page.evaluateHandle(() => Promise.resolve(window));
-aWindowHandle; // Handle for the window object.
+const aWindowHandle = await page.evaluateHandle(() => Promise.resolve(window)); 
+aWindowHandle; // Handle for the window object
 ```
 
-A string can also be passed in instead of a function.
-
+A string can also be passed in instead of a function:
 ```js
-const aHandle = await page.evaluateHandle('document'); // Handle for the 'document'.
+const aHandle = await page.evaluateHandle('document'); // Handle for the 'document'
 ```
 
 [JSHandle] instances can be passed as arguments to the `page.evaluateHandle`:

examples/element-html.js

@@ -0,0 +1,46 @@
+/**
+ * Copyright 2017 Google Inc. All rights reserved.
+ *
+ * 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.
+ */
+
+/**
+ * @fileoverview Gets HTML of the top search element from developers.google.com/web
+ * using page.evaluate() and page.evaluateHandle() to show the difference between these two methods
+ **/
+
+'use strict';
+
+const puppeteer = require('puppeteer');
+
+(async() => {
+  const browser = await puppeteer.launch();
+  const page = await browser.newPage();
+
+  await page.goto('https://developers.google.com/web/');
+
+  const elementHandle = await page.$('#top-search');
+  const pageFunction = element => element.innerHTML;
+
+  // page.evaluate() evaluates pageFunction in page context and returns javascript-object
+  const jsObject = await page.evaluate(pageFunction, elementHandle);
+
+  // page.evaluateHandler() evaluates pageFunction in page context and returns in-page object (JSHandle)
+  const jsHandle = await page.evaluateHandle(pageFunction, elementHandle);
+
+  // to show the difference between two methods
+  console.log(`jsObject is NOT equal to jsHandle: ${jsObject !== jsHandle}`); // prints true
+  console.log(`jsObject is equal to jsHandle.jsonValue(): ${jsObject === (await jsHandle.jsonValue())}`); // prints true
+
+  await browser.close();
+})();

lib/Page.js

@@ -643,7 +643,7 @@ class Page extends EventEmitter {
   }
 
   /**
-   * @param {function()} pageFunction
+   * @param {function()|string} pageFunction
    * @param {!Array<*>} args
    * @return {!Promise<*>}
    */