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docs(verification): nits and typos
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@pavelfeldman
pavelfeldman committed Apr 21, 2020
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9 changes: 4 additions & 5 deletions docs/README.md
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- [Modify requests](./network.md#modify-requests)
- [Abort requests](./network.md#abort-requests)
1. [Scraping and verification](./verification.md)
- [Screenshots](./verification.md#capturing-screenshot)
- [Evaluation](./verification.md#evaluating-javascript)
- [Console messages](./verification.md#listening-console-messages)
- [Uncaghut exceptions](./verification.md#uncaghut-exceptions)
- [Page crashes](./verification.md#page-crashes)
- [Evaluating JavaScript](./verification.md#evaluating-javascript)
- [Capturing screenshot](./verification.md#capturing-screenshot)
- [Page events](./verification.md#page-events)
- [Handling exceptions](./verification.md#handling-exceptions)
1. [Navigation and loading](./loading.md)
- [Common scenarios](./loading.md#common-scenarios)
- [Loading a popup](./loading.md#loading-a-popup)
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178 changes: 87 additions & 91 deletions docs/verification.md
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# Scraping and verification

Playwright allows verifiying state of the page and catching abnormal behavior by:
- evaluating JavaScript code snippets in the page
- capturing screenshots (png or jpeg)
- listening console messages
- observing uncaghut exceptions in the page
- observing page crashes
- etc

#### Contents
- [Evaluating JavaScript](#evaluating-javascript)
- [Capturing screenshot](#capturing-screenshot)
- [Listening console messages](#listening-console-messages)
- [Uncaghut exceptions](#uncaghut-exceptions)
- [Page crashes](#page-crashes)
- [Page events](#page-events)
- [Handling exceptions](#handling-exceptions)

<br/>

## Evaluating JavaScript

Execute JavaScript function in the page:
```js
const href = await page.evaluate(() => document.location.href);
const href = await page.evaluate(() => document.location.href);
```

If the result is a Promise or if the function is asynchronouse eveluate will automatically wait until it's resolved:
If the result is a Promise or if the function is asynchronous evaluate will automatically wait until it's resolved:
```js
const status = await page.evaluate(async () => {
const response = await fetch(location.href);
return response.status;
});
const status = await page.evaluate(async () => {
const response = await fetch(location.href);
return response.status;
});
```

Get object handle and use it in multiple evaluations:
```js
// Create a new array in the page, wriate a reference to it in
// window.myArray and get a handle to it.
const myArrayHandle = await page.evaluateHandle(() => {
window.myArray = [1];
return myArray;
});

// Get current length of the array using the handle.
const length = await page.evaluate(
(arg) => arg.myArray.length,
{ myArray: myArrayHandle }
);

// Add one more element to the array using the handle
await page.evaluate((arg) => arg.myArray.push(arg.newElement), {
myArray:myArrayHandle,
newElement: 2
});

// Get current length of the array using window.myArray reference.
const newLength = await page.evaluate(() => window.myArray.length);

// Release the object when it's no longer needed.
await myArrayHandle.dispose();
// Create a new array in the page, write a reference to it in
// window.myArray and get a handle to it.
const myArrayHandle = await page.evaluateHandle(() => {
window.myArray = [1];
return myArray;
});

// Get current length of the array using the handle.
const length = await page.evaluate(
(arg) => arg.myArray.length,
{ myArray: myArrayHandle }
);

// Add one more element to the array using the handle
await page.evaluate((arg) => arg.myArray.push(arg.newElement), {
myArray: myArrayHandle,
newElement: 2
});

// Get current length of the array using window.myArray reference.
const newLength = await page.evaluate(() => window.myArray.length);

// Release the object when it's no longer needed.
await myArrayHandle.dispose();
```

#### API reference
Expand All @@ -75,90 +66,95 @@ Get object handle and use it in multiple evaluations:

Take screenshot of the page's viewport and save it in a png file:
```js
await page.screenshot({path: 'screenshot.png'});
await page.screenshot({path: 'screenshot.png'});
```
Capture entire scrollable area of the page:

#### Variations

Capture particular element:
```js
await page.screenshot({path: 'screenshot.png', fullPage: true});
const elementHandle = await page.$('.header');
await elementHandle.screenshot({ path: 'screenshot.png' });
```
Save screenshot in an in-memory buffer (the content is base64-encoded image bytes):

Capture full page screenshot:
```js
await page.screenshot({path: 'screenshot.png', fullPage: true});
```

Capture screenshot into a Node [Buffer](https://nodejs.org/api/buffer.html#buffer_class_buffer).
```js
const buffer = await page.screenshot();
// Log the length.
console.log(buffer.length);
const buffer = await page.screenshot();
// Log the length.
console.log(buffer.toString('base64'));
```


#### API reference

- [page.screenshot([options])](./api.md#pagescreenshotoptions)
- [Node.js Buffer](https://nodejs.org/api/buffer.html)
- [elementHandle.screenshot([options])](./api.md#elementhandlescreenshotoptions)

<br/>

## Listening console messages
## Page events

Listen all console messages in a page and dump _errors_ in to the terminal:
You can listen for various events on the `page` object. Following are just some of the examples of the events you can assert and handle:

#### "`console`" - get all console messages from the page

```js
// Get all console messages from the page.
page.on('console', msg => {
// Handle only errors.
if (msg.type() !== 'error')
return;
console.log(`text: "${msg.text()}"`);
});

await page.evaluate(() => console.error('Page error message'));
```
Get access to the console message arguments:
```js
page.on('console', msg => {
for (let i = 0; i < msg.args().length; ++i)
console.log(`${i}: ${msg.args()[i]}`);
});
page.on('console', msg => {
// Handle only errors.
if (msg.type() !== 'error')
return;
console.log(`text: "${msg.text()}"`);
});
```

#### API reference

- [class: ConsoleMessage](./api.md#class-consolemessage)
- [class `JSHandle`](./api.md#class-jshandle)
- [event: 'console'](./api.md#event-console)

<br/>
#### "`dialog`" - Handle assert, confirm, prompt

```js
page.on('dialog', dialog => {
dialog.accept();
});
```

## Uncaghut exceptions
#### "`popup`" - Handle assert, confirm, prompt

Listen uncaghut exceptions in the page:
```js
// Log all uncaught errors to the terminal
page.on('pageerror', exception => {
console.log(`Uncaught exception: "${exception}"`);
});

// Navigate to a page with an exception.
await page.goto('data:text/html,<script>throw new Error("Test")</script>');
const [popup] = await Promise.all([
page.waitForEvent('popup'),
page.click('#open')
]);
```

#### API reference

- [event: 'pageerror'](./api.md#event-pageerror)
- [class: Page](./api.md#class-page)
- [event: 'console'](./api.md#event-console)
- [event: 'dialog'](./api.md#event-dialog)
- [event: 'popup'](./api.md#event-popup)
- [class: ConsoleMessage](./api.md#class-consolemessage)

<br/>

## Page crashes

Listen to page crashes:
## Handling exceptions

Listen uncaught exceptions in the page:
```js
page.on('crash', exception => {
console.log(`Page crashed`);
});
// Log all uncaught errors to the terminal
page.on('pageerror', exception => {
console.log(`Uncaught exception: "${exception}"`);
});

// Navigate to a page with an exception.
await page.goto('data:text/html,<script>throw new Error("Test")</script>');
```
It's very unusual for page to crash but might happen if a page allocates too much memory or due to a bug in a browser.

#### API reference

- [event: 'crash'](./api.md#event-crash)
- [event: 'pageerror'](./api.md#event-pageerror)

<br/>

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