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# Extensibility

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- [Custom selector engines](#custom-selector-engines)
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## Custom selector engines

Playwright supports custom selector engines, registered with
[selectors.register(name, script[, options])](./api.md#selectorsregistername-script-options).

Selector engine should have the following properties:
- `create` function to create a relative selector from `root` (root is either
  a `Document`, `ShadowRoot` or `Element`) to a `target` element.
- `query` function to query first element matching `selector` relative to the
  `root`.
- `queryAll` function to query all elements matching `selector` relative to
  the `root`.

By default the engine is run directly in the frame's JavaScript context and, for
example, can call an application-defined function. To isolate the engine from
any JavaScript in the frame, but leave access to the DOM, register the engine
with `{contentScript: true}` option. Content script engine is safer because it
is protected from any tampering with the global objects, for example altering
`Node.prototype` methods. All built-in selector engines run as content scripts.
Note that running as a content script is not guaranteed when the engine is used
together with other custom engines.

An example of registering selector engine that queries elements based on a tag
name:

```js
// Must be a function that evaluates to a selector engine instance.
const createTagNameEngine = () => ({
  // Returns the first element matching given selector in the root's subtree.
  query(root, selector) {
    return root.querySelector(selector);
  },

  // Returns all elements matching given selector in the root's subtree.
  queryAll(root, selector) {
    return Array.from(root.querySelectorAll(selector));
  }
});

// Register the engine. Selectors will be prefixed with "tag=".
await selectors.register('tag', createTagNameEngine);

// Now we can use 'tag=' selectors.
const button = await page.$('tag=button');

// We can combine it with other selector engines using `>>` combinator.
await page.click('tag=div >> span >> "Click me"');

// We can use it in any methods supporting selectors.
const buttonCount = await page.$$eval('tag=button', buttons => buttons.length);
```

[Playwright]: api.md#class-playwright "Playwright"
[Browser]: api.md#class-browser "Browser"
[BrowserContext]: api.md#class-browsercontext "BrowserContext"
[Page]: api.md#class-page "Page"
[Frame]: api.md#class-frame "Frame"
[ElementHandle]: api.md#class-elementhandle "ElementHandle"
[JSHandle]: api.md#class-jshandle "JSHandle"
[ConsoleMessage]: api.md#class-consolemessage "ConsoleMessage"
[Dialog]: api.md#class-dialog "Dialog"
[Download]: api.md#class-download "Download"
[Video]: api.md#class-video "Video"
[FileChooser]: api.md#class-filechooser "FileChooser"
[Keyboard]: api.md#class-keyboard "Keyboard"
[Mouse]: api.md#class-mouse "Mouse"
[Touchscreen]: api.md#class-touchscreen "Touchscreen"
[Request]: api.md#class-request "Request"
[Response]: api.md#class-response "Response"
[Selectors]: api.md#class-selectors "Selectors"
[Route]: api.md#class-route "Route"
[WebSocket]: api.md#class-websocket "WebSocket"
[TimeoutError]: api.md#class-timeouterror "TimeoutError"
[Accessibility]: api.md#class-accessibility "Accessibility"
[Worker]: api.md#class-worker "Worker"
[BrowserServer]: api.md#class-browserserver "BrowserServer"
[BrowserType]: api.md#class-browsertype "BrowserType"
[Logger]: api.md#class-logger "Logger"
[ChromiumBrowser]: api.md#class-chromiumbrowser "ChromiumBrowser"
[ChromiumBrowserContext]: api.md#class-chromiumbrowsercontext "ChromiumBrowserContext"
[ChromiumCoverage]: api.md#class-chromiumcoverage "ChromiumCoverage"
[CDPSession]: api.md#class-cdpsession "CDPSession"
[FirefoxBrowser]: api.md#class-firefoxbrowser "FirefoxBrowser"
[WebKitBrowser]: api.md#class-webkitbrowser "WebKitBrowser"
[Array]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array "Array"
[Buffer]: https://nodejs.org/api/buffer.html#buffer_class_buffer "Buffer"
[ChildProcess]: https://nodejs.org/api/child_process.html "ChildProcess"
[Element]: https://developer.mozilla.org/en-US/docs/Web/API/element "Element"
[Error]: https://nodejs.org/api/errors.html#errors_class_error "Error"
[EvaluationArgument]: #evaluationargument "Evaluation Argument"
[Map]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Map "Map"
[Object]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object "Object"
[Promise]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Promise "Promise"
[RegExp]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/RegExp "RegExp"
[Serializable]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/JSON/stringify#Description "Serializable"
[UIEvent.detail]: https://developer.mozilla.org/en-US/docs/Web/API/UIEvent/detail "UIEvent.detail"
[URL]: https://nodejs.org/api/url.html "URL"
[USKeyboardLayout]: ../src/usKeyboardLayout.ts "USKeyboardLayout"
[UnixTime]: https://en.wikipedia.org/wiki/Unix_time "Unix Time"
[boolean]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Boolean_type "Boolean"
[function]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Function "Function"
[iterator]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Iteration_protocols "Iterator"
[null]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/null "null"
[number]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Number_type "Number"
[origin]: https://developer.mozilla.org/en-US/docs/Glossary/Origin "Origin"
[selector]: https://developer.mozilla.org/en-US/docs/Web/CSS/CSS_Selectors "selector"
[Readable]: https://nodejs.org/api/stream.html#stream_class_stream_readable "Readable"
[string]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#String_type "string"
[xpath]: https://developer.mozilla.org/en-US/docs/Web/XPath "xpath"