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Use page.evaluateOnNewDocument() and register the function before calling page.goto(). Puppeteer runs it after the new document is created but before that document’s scripts, so it is the right hook for setting page-visible values before site code executes.
Register code before navigating
Here is a complete minimal example using Puppeteer. The script sets navigator.languages in each newly created document before the page’s own scripts run:
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const puppeteer = require('puppeteer');
(async () => {
const browser = await puppeteer.launch();
try {
const page = await browser.newPage();
await page.evaluateOnNewDocument(() => {
Object.defineProperty(navigator, 'languages', {
get: () => ['en-US', 'en'],
});
});
const response = await page.goto('https://example.com');
console.log('HTTP status:', response?.status() ?? 'no main-resource response');
} finally {
await browser.close();
}
})();The order matters: create or obtain the Page, register the new-document script, then navigate to the URL of interest. If navigation has already happened, registering the hook does not retroactively run it in that existing document.
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What runs when, and where
page.evaluateOnNewDocument(fn, ...args) registers a function or string for future document creation. It runs after the document is created but before its scripts, including on navigation and when child frames attach or navigate. This timing makes it suitable for initialization that the site’s own JavaScript must see from its earliest execution.
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It is not the same as evaluating code in the page that is already open, or adding a script element:
| Need | API | Timing and scope |
|---|---|---|
| Run setup before scripts in newly created documents | page.evaluateOnNewDocument(fn, ...args) |
Registered for navigations and child-frame attach/navigation. |
| Run code in the current page context | page.evaluate(fn, ...args) |
Executes when called in the current page context; it is not the document-start registration hook. |
| Insert a script element | page.addScriptTag({ content }) or a URL option |
Adds a <script> element; the API describes this as a shortcut for the main frame. |
Pass data from Node.js explicitly
Puppeteer serializes the function supplied for page-context execution. It does not carry over the Node.js caller’s lexical variables or functions. Pass values as arguments instead, and keep browser-side code self-contained.
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const languageList = ['en-US', 'en'];
await page.evaluateOnNewDocument((languages) => {
Object.defineProperty(navigator, 'languages', {
get: () => languages,
});
}, languageList);
await page.goto('https://example.com');Page-context return values are serialized. If you need to retain a DOM object by reference, use a handle rather than expecting the object itself to cross back to Node.js as a live object.
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Frame coverage
Puppeteer documents the new-document callback as running on navigation and when child frames attach or navigate. However, evaluation in one frame does not affect nested child frames. Do not assume a main-frame operation changes every nested frame; for iframe-heavy pages, verify the frame behavior the task requires.
Remove a registered script
evaluateOnNewDocument() returns a NewDocumentScriptEvaluation that includes an identifier. Keep that identifier if you may need to stop injecting the script:
const registration = await page.evaluateOnNewDocument(() => {
// New-document setup
});
// Later, stop injecting this registered script.
await page.removeScriptToEvaluateOnNewDocument(registration.identifier);Removing the registration prevents its future injection; it is not a way to undo changes already made in a document.
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Navigation response and waiting behavior
page.goto(url) navigates to the requested URL and accepts options that control when its navigation wait resolves. It returns the main resource response, or null for about:blank and same-URL hash navigation. The example uses optional chaining because a response is not guaranteed for those cases.
Troubleshooting
- The site’s first script sees the original value. Register the hook before the navigation that creates the document. A call made after
goto()is too late for that document. - A Node.js variable is undefined in the callback. The callback is serialized for the page context and cannot close over Node.js lexical scope. Pass the needed value through
...args. - An iframe does not show the change. A frame’s evaluation does not alter nested child frames. Check which frame is being initialized and verify the target iframe’s own attach/navigation behavior.
- You expected a return value or live DOM node. Page-context values are serialized; use a handle when you need a DOM object by reference.
goto()produced no response. A null main-resource response is documented forabout:blankand same-URL hash navigation. Do not treat that return value alone as proof that a normal document request failed.
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For example, this cURL call saves a WebP screenshot of the target URL; see the ScreenshotNeo API documentation for request options:
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