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Register the Node.js callback with page.exposeFunction(name, callback), then call it from browser-side code as window[name](...). The callback runs in Node.js, and the browser receives a Promise for its result. Use page.evaluate() to run the calling code in the page context.
How the Node-to-page bridge works
Puppeteer’s Page.exposeFunction() method “adds a function called name on the page’s window object,” according to the Puppeteer API reference for Page.exposeFunction(). The function is registered by your Node.js program, but page code calls it through window. Puppeteer invokes the supplied callback in Node.js and returns its value to the page asynchronously as a Promise.
page.evaluate() has the opposite role: it runs the function you pass in the browser page context and returns that function’s result to Node.js. If the evaluated page function returns a Promise, Puppeteer waits for it. See the Puppeteer API reference for Page.evaluate().
Expose and call a Node.js function
This complete example registers a Node-side MD5 function and calls it from the page. It follows Puppeteer’s documented pattern; the code is an example, not an independently reported test.
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const puppeteer = require('puppeteer');
const crypto = require('node:crypto');
async function main() {
const browser = await puppeteer.launch();
try {
const page = await browser.newPage();
// Register this Node.js callback on this page's window.
await page.exposeFunction('md5', text =>
crypto.createHash('md5').update(text).digest('hex')
);
// This function runs in the browser context.
const digest = await page.evaluate(async () => {
return await window.md5('PUPPETEER');
});
console.log(digest);
} finally {
await browser.close();
}
}
main().catch(error => {
console.error(error);
process.exitCode = 1;
});- Launch Puppeteer and create the
Pagethat needs access to the function. - Call and await
page.exposeFunction('md5', callback)from Node.js before calling the exposed name. - Run page-context code with
page.evaluate(), invokewindow.md5(...), and await its Promise. - Close the browser when the work is complete.
The official API reference also demonstrates exposing a readfile function backed by Node.js fs.readFile. In both cases, the file or hashing work happens in the Node callback, not inside page.evaluate().
Choose the right Puppeteer method
| Method | Where it runs | Use it for | Result |
|---|---|---|---|
page.exposeFunction(name, callback) |
Registers a callback that Puppeteer executes in Node.js when page code calls it. | Letting page code request work handled by your Node program. | The page receives a Promise for the callback’s return value. |
page.evaluate(pageFunction, ...args) |
The supplied function runs in the browser page context. | Reading or changing page state, including calling a function exposed on window. |
The Node caller receives the evaluated function’s result; Puppeteer waits if it is a Promise. |
Register the exposed function on the specific Page whose page context needs it. A Puppeteer browser can contain multiple pages, so exposing a function on one page does not mean you registered it on every page. The Puppeteer Page reference describes a page as a tab or extension background page.
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Keep the bridge available or remove it
Exposed functions survive navigations, according to the Page.exposeFunction() reference. To remove a previously exposed name from the page’s window, call page.removeExposedFunction(name) from Node.js:
await page.removeExposedFunction('md5');Check the API reference matching the Puppeteer version installed in your project before relying on a method signature or behavior. The linked exposeFunction() and evaluate() references identify version 25.12.0; the linked removal reference identifies version 25.3.0.
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Troubleshooting
window.md5is not a function: Confirm thatpage.exposeFunction('md5', ...)completed before the page calls it, that the spelling matches, and that you exposed it on the samePageyou are evaluating.- The Node callback does not run: Check that the call to
window.md5(...)is inside page-context code, such as the function passed topage.evaluate(). Writing a function in the Node script does not by itself make it available to page code. - The evaluated result arrives too soon or is missing: Await the exposed function in the page context and await
page.evaluate()in Node. Both calls can involve Promises. - The exposed function is unavailable after cleanup: Check whether your code called
page.removeExposedFunction(name). Exposed functions otherwise survive navigation according to Puppeteer’s API reference. - You need the function on another tab: Register it separately on the other page. Exposure is attached to a
Page.
Or skip the browser setup
If your goal is simply to capture a website rather than run custom browser-side logic, ScreenshotNeo offers a screenshot API and MCP server. A single GET request can return a PNG, JPEG, WebP, or PDF, without launching and configuring Puppeteer yourself. Its capture options include device and viewport settings, full-page shots, CSS selectors, custom CSS and JavaScript, cookies, and wait conditions.
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