Implementation:Microsoft Playwright Client Worker

Knowledge Sources	Microsoft_Playwright
Domains	Browser Automation, Web Workers
Last Updated	2026-02-12 00:00 GMT

Overview

Concrete tool for representing and interacting with Web Workers and Service Workers in browser pages provided by the Playwright library.

Description

The Worker class extends ChannelOwner and implements the api.Worker interface. It represents a Web Worker (attached to a Page) or a Service Worker (attached to a BrowserContext). The class maintains optional references to its parent _page or _context.

The constructor sets up console event subscription mapping, handles the 'close' channel event to clean up references from the parent page/context, emits the close event, and closes the _closedScope (a LongStandingScope) with an appropriate error.

Key methods include:

• url(): Returns the worker's script URL from the initializer.
• evaluate(): Executes a function in the worker's execution context and returns the deserialized result.
• evaluateHandle(): Executes a function and returns a JSHandle to the result.
• waitForEvent(): Waits for a named event with timeout and predicate support, using the Waiter class.

Usage

Use the Worker class to interact with Web Workers or Service Workers. Obtain instances from the 'worker' event on a Page or the 'serviceworker' event on a BrowserContext.

Code Reference

Source Location

• Repository: Microsoft_Playwright
• File: packages/playwright-core/src/client/worker.ts

Signature

export class Worker extends ChannelOwner<channels.WorkerChannel> implements api.Worker {
  _page: Page | undefined;
  _context: BrowserContext | undefined;
  readonly _closedScope: LongStandingScope;

  static from(worker: channels.WorkerChannel): Worker;
  static fromNullable(worker: channels.WorkerChannel | undefined): Worker | null;

  constructor(parent: ChannelOwner, type: string, guid: string, initializer: channels.WorkerInitializer);

  url(): string;
  async evaluate<R, Arg>(pageFunction: PageFunction<Arg, R>, arg?: Arg): Promise<R>;
  async evaluateHandle<R, Arg>(pageFunction: PageFunction<Arg, R>, arg?: Arg): Promise<SmartHandle<R>>;
  async waitForEvent(event: string, optionsOrPredicate?: WaitForEventOptions): Promise<any>;
}

Import

import { Worker } from 'playwright-core/src/client/worker';

I/O Contract

Inputs

Name	Type	Required	Description
pageFunction	string	Yes	The function or expression to evaluate in the worker context
arg	any	No	Argument to pass to the evaluation function
event	string	Yes	The event name to wait for
optionsOrPredicate	WaitForEventOptions	No	Timeout and predicate options for waitForEvent

Outputs

Name	Type	Description
url()	string	The URL of the worker's script
evaluate()	Promise<R>	The deserialized return value from the worker evaluation
evaluateHandle()	Promise<SmartHandle<R>>	A JSHandle referencing the return value in the worker context
waitForEvent()	Promise<any>	The event argument when the awaited event fires

Usage Examples

// Listen for new workers
page.on('worker', (worker) => {
  console.log('Worker created:', worker.url());

  worker.on('close', () => {
    console.log('Worker closed:', worker.url());
  });
});

// Evaluate in a worker
const workers = page.workers();
for (const worker of workers) {
  const result = await worker.evaluate(() => {
    return self.location.href;
  });
  console.log('Worker URL:', result);
}

// Wait for a worker event
const worker = await page.waitForEvent('worker');
console.log('New worker:', worker.url());

Related Pages

• Environment:Microsoft_Playwright_Node_Runtime_Environment