Use Puppeteer’s worker.evaluate() method to run JavaScript in a dedicated Web Worker. First identify the right worker with the page’s workercreated event or page.workers(); page.evaluate() runs in the page’s main context instead. The example below waits for a worker, evaluates code inside it, and returns a result to Node.js.
Run code in a dedicated Web Worker
Subscribe to workercreated before the navigation or interaction that starts the worker. Then call worker.evaluate() on the resulting Puppeteer WebWorker object. This example assumes the site creates a dedicated worker after navigation:
import puppeteer from 'puppeteer';
const browser = await puppeteer.launch();
try {
const page = await browser.newPage();
const workerCreated = new Promise(resolve => {
page.once('workercreated', resolve);
});
await page.goto('https://example.com');
const worker = await workerCreated;
console.log('Worker URL:', worker.url());
const result = await worker.evaluate(() => {
// This function runs in the Worker, not the page.
return self.location.href;
});
console.log(result);
} finally {
await browser.close();
}The worker’s URL helps confirm which worker was selected. The relevant APIs are documented in Puppeteer’s WebWorker reference, WebWorker.url(), and Page.workers().
Choose the worker at the right time
When navigation or startup creates it
Set up the event listener before page.goto(), as in the example. If the application starts several workers, collect them and choose by URL or another app-specific property; the first worker is not necessarily the one you want.
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When a user action creates it
Register the listener before performing the action, then await the worker after the action:
const workerCreated = new Promise(resolve => {
page.once('workercreated', resolve);
});
await page.click('#start-worker');
const worker = await workerCreated;
console.log(worker.url());Replace #start-worker with a selector that actually starts the worker on your page. Registering first avoids missing a worker created immediately by the action.
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When the worker is already running
Use page.workers() to inspect active dedicated WebWorkers, then select the intended one. This method does not list ServiceWorkers. Check worker.url() before evaluating if the page can run multiple workers.
Pass arguments and return useful values
Puppeteer serializes the function passed to evaluate() and executes it in the browser’s worker context. The function does not retain access to Node.js variables or helper functions in the surrounding lexical scope. Pass needed values as arguments and include the logic in the callback:
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const factor = 7;
const result = await worker.evaluate(value => value * 6, factor);
console.log(result); // 42Prefer returning primitives or JSON-like data. Complex objects may be truncated or come back as empty objects after protocol serialization. If you need to retain an in-context reference rather than return a serialized value, use evaluateHandle(). See Puppeteer’s JavaScript execution guide and WebWorker.evaluate() reference.
Wait for worker state that changes later
A promise returned by worker.evaluate() is awaited. To wait until a worker condition becomes true later, use worker.waitForFunction() and set a timeout appropriate to the work:
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await worker.evaluate(() => {
self.answer = 42;
});
await worker.waitForFunction(() => self.answer === 42, {
timeout: 5_000
});The API also documents polling and abort-signal options. Consult the installed version’s waitForFunction() reference for the signature available to your project.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Understand which execution context you are using
page.evaluate()runs code in the page context;worker.evaluate()runs it in the selected Web Worker.page.workers()lists dedicated WebWorkers, not ServiceWorkers.page.evaluateOnNewDocument()runs code in a new document before its scripts execute; it is not the method for evaluating code in a Worker. Use the identified worker’s methods instead.
See the official references for Page.evaluate() and Page.evaluateOnNewDocument().
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Troubleshoot common failures
- The worker promise never resolves: the page may not create a worker during navigation, or the listener may have been registered after creation. Register before the triggering action and confirm that the action really starts a dedicated worker.
- You selected the wrong worker: a page may create several. Inspect the candidates from
page.workers()and compare their URLs instead of assuming the first event is the target. - The callback cannot read a Node.js variable: evaluation callbacks run in the browser context and do not capture Node.js lexical scope. Pass the value as an explicit argument.
- The returned object is empty or incomplete: protocol serialization may not preserve a complex object. Return a primitive or JSON-like object, or use
evaluateHandle()when an in-context reference is required. - The code runs in the wrong place: verify that you called
worker.evaluate()on the intendedWebWorker, notpage.evaluate(). - A method signature differs from an example: check the API reference and type definitions for the Puppeteer version installed in your project. The official pages have displayed different documentation version labels; those labels do not establish which package version your project uses.
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cURL:
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