To run JavaScript before a page’s own scripts execute, register it with page.evaluateOnNewDocument() before calling page.goto(). Puppeteer runs the registered function after a document is created but before its scripts run.
Run code before the page’s scripts
Here is a complete Node.js example using Puppeteer. Replace the URL with the page you want to visit.
const puppeteer = require('puppeteer');
(async () => {
const browser = await puppeteer.launch();
try {
const page = await browser.newPage();
await page.evaluateOnNewDocument(() => {
// This executes in the browser page context before the page's scripts.
Object.defineProperty(navigator, 'languages', {
get: () => ['en-US', 'en'],
});
});
const response = await page.goto('https://example.com');
console.log('Status:', response?.status() ?? 'No main-resource response');
} finally {
await browser.close();
}
})();The important ordering is the registration first, then navigation. The callback runs in the page context, not in Node.js. It is invoked for navigations and when child frames attach or navigate. Puppeteer API: Page.evaluateOnNewDocument()
Choose the right Puppeteer API
| Need | API | Timing and scope |
|---|---|---|
| Set up code in newly created documents before their scripts run | page.evaluateOnNewDocument(fn, ...args) |
Register before navigation; runs on navigation and child-frame attach/navigation. |
| Evaluate code in the current page context | page.evaluate(fn, ...args) |
Runs when called in the current page context; it is not the document-start registration hook. |
| Add a script element to the page | page.addScriptTag({content}) or a URL option |
Inserts a script tag; Puppeteer documents this as a shortcut for the main frame. |
Use evaluateOnNewDocument() when page scripts must see the setup from the start. Use evaluate() for work that should happen after the page context is available, and addScriptTag() when inserting a script element is what you need. See the evaluate API and addScriptTag API.
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Pass values from Node.js explicitly
Puppeteer serializes the function passed for page-context evaluation. It cannot see Node.js lexical variables or functions unless you pass values as arguments. For example:
const languageList = ['en-US', 'en'];
await page.evaluateOnNewDocument((languages) => {
Object.defineProperty(navigator, 'languages', {
get: () => languages,
});
}, languageList);
await page.goto('https://example.com');Return values are serialized back to Node.js. If you need to retain a DOM object by reference rather than receive a serialized value, use a handle. Consult the JavaScript execution guide and evaluate API for the execution and return-value details.
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Handle frames and later removal
The registration runs for new documents as they are navigated and for child frames when they attach or navigate. However, evaluation in one frame does not affect nested child frames. If a site depends on iframe behavior, verify the required frame’s behavior instead of assuming that code evaluated in the main frame changes every nested frame. The Frame API documents this frame-scope distinction.
evaluateOnNewDocument() returns a NewDocumentScriptEvaluation containing an identifier. Keep that identifier if you may want to stop injecting the script:
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const registration = await page.evaluateOnNewDocument(() => {
window.exampleSetup = true;
});
await page.goto('https://example.com');
await page.removeScriptToEvaluateOnNewDocument(registration.identifier);Removal prevents that registered script from being injected in the future; it does not undo changes already made in documents where it has run. See removeScriptToEvaluateOnNewDocument().
Navigation details to account for
page.goto(url) navigates to the given URL and accepts options that control when its navigation wait resolves. Its return value is the main resource response, but it can be null for about:blank and same-URL hash navigation. Do not treat a null response in those cases as proof that the pre-navigation registration failed. See the goto API.
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Troubleshoot scripts that do not run early enough
- The site’s script runs first: Register the callback before the relevant
page.goto(). Callingevaluate()after navigation evaluates in the current page context and is not a substitute for document-start registration. - The callback cannot find a Node.js variable: Keep the callback self-contained or pass required data as arguments. Node.js lexical scope is not carried into the page context.
- An iframe does not show the expected change: Frame evaluation is scoped; one frame’s evaluation does not affect nested child frames. Check the target frame and the behavior required by the site.
- You receive no response from
goto(): Puppeteer documentsnullforabout:blankand same-URL hash navigation. Check the navigation target and do not assume every navigation returns a main-resource response. - The setup runs on later navigations when you no longer want it: Save the registration identifier and pass it to
removeScriptToEvaluateOnNewDocument().
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Frequently Asked Questions
Does evaluateOnNewDocument run before the browser creates the document?
No. Puppeteer runs the callback after document creation but before that document’s scripts execute.
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Can I use evaluateOnNewDocument to change a DOM element before it appears?
It runs before the page’s scripts, but DOM elements may not exist yet at that point. Use it for document-start setup, and arrange any DOM-dependent work for an appropriate later point.
Quick Recap
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