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Use the Puppeteer API that matches your goal: page.addScriptTag() inserts a real <script> element, page.evaluate() runs a function immediately in the page context, and page.evaluateOnNewDocument() installs setup before the page’s own scripts run. For an iframe, call the same method on the relevant Frame object.
Choose the right Puppeteer API
“Add a custom script” can mean three different operations. Decide whether you need a persistent script element, a one-time action, or code that runs before site JavaScript:
| API | What it does | When it runs | Typical use |
|---|---|---|---|
page.addScriptTag() |
Appends a <script> element to the main-frame document |
When you call it in the current document | Load a local file, inline code, or a browser-loadable URL |
page.evaluate() |
Evaluates a function in the page’s JavaScript context; it does not add a script element | Immediately when the evaluation is dispatched | Read or change the DOM, call a page function, or perform a one-off action |
page.evaluateOnNewDocument() |
Registers code for each new document | After document creation but before that document’s scripts | Install globals or browser-environment overrides before application code starts |
Page methods target the main frame. To work in a child frame, obtain its Frame and use the corresponding frame method.
Insert a script element with page.addScriptTag()
addScriptTag() accepts a local path, inline content, or remote url. It resolves to an element handle for the injected script, so you can inspect the resulting element after insertion. The options also include id and type; set type: 'module' for an ES module.
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Load a local JavaScript file
A relative path is resolved from Node.js process.cwd(), not necessarily from the directory containing your source file. Use an absolute path or verify the process working directory when running from a task runner, container, or package script.
import puppeteer from 'puppeteer';
const browser = await puppeteer.launch();
try {
const page = await browser.newPage();
await page.goto('https://example.com', {waitUntil: 'networkidle2'});
const scriptElement = await page.addScriptTag({
path: './custom.js',
id: 'my-custom-script',
});
console.log(await scriptElement.evaluate(element => ({
id: element.id,
src: element.src,
type: element.type,
})));
} finally {
await browser.close();
}The await on both navigation and addScriptTag() matters. Without it, later actions can run before the script has been inserted or loaded.
Inject inline JavaScript
Use content when the code is already available in your Node.js program. This creates a script element in the page; it is different from calling evaluate().
await page.addScriptTag({
content: `
window.myFlag = true;
document.documentElement.dataset.automation = 'enabled';
`,
id: 'inline-setup',
});If the inline code throws, the returned promise rejects. Wrap the call in your normal error handling and inspect the browser console when diagnosing page-side exceptions.
Load a script by URL
await page.addScriptTag({
url: 'https://example.com/custom.js',
type: 'text/javascript',
});The browser must be able to fetch the URL in the page context, and the target site’s environment must permit that load. A URL being syntactically valid does not guarantee that the remote server is reachable or that the page’s policy will allow it.
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Use the other addScriptTag() options
id: assigns anidto the inserted element, making it easier to find or avoid inserting the same script twice.type: sets the script type. Usemodulewhen the file or inline source is an ES module.path,content, andurl: choose the source that matches where your code lives. Keep the source choice explicit rather than constructing ambiguous option combinations.
Run one-off code with page.evaluate()
Use evaluate() when you need to execute a function now, not add a persistent <script> node. Puppeteer serializes the function and evaluates it in the browser. Node.js lexical variables and helper functions are not available inside the function unless you pass their values as arguments.
const pageTitle = await page.evaluate(() => document.title);
console.log(pageTitle);
const label = 'Automation test';
await page.evaluate(text => {
document.body.dataset.testLabel = text;
}, label);Values passed as arguments are serialized. A promise returned by the page function is awaited, and ordinary returned objects are serialized back to Node.js. If you need to keep an in-page object by reference, use evaluateHandle() instead of expecting a normal return value to remain a live DOM object.
const result = await page.evaluate(async () => {
const response = await fetch('/api/status');
return {ok: response.ok, status: response.status};
});
console.log(result);Because this code runs in the page context, browser globals such as window and document are available there, while Node-only APIs such as fs are not.
Run setup before the page’s scripts
Register evaluateOnNewDocument() before navigation when the site’s own scripts must see your setup from the beginning. Puppeteer runs the registered function after a document is created but before that document’s scripts. The registration also applies to child frames when they are attached or navigated.
const preloadId = await page.evaluateOnNewDocument(() => {
Object.defineProperty(navigator, 'languages', {
get: () => ['en-US', 'en'],
});
});
await page.goto('https://example.com', {waitUntil: 'domcontentloaded'});
// Remove the registration when it is no longer needed.
await page.removeScriptToEvaluateOnNewDocument(preloadId);Calling this API after goto() does not retroactively run the code in the document that already exists. Register it first, then navigate or create a new document.
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Use this API for early document setup, not as a replacement for ordinary post-load DOM work. For a button click, text change, or data read after the page is ready, evaluate() is usually clearer.
Inject into an iframe
page.addScriptTag() is a shortcut for the main frame. It does not automatically insert the script into every iframe. Find the intended frame and call addScriptTag() or evaluate() on that frame.
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);
if (!frame) {
throw new Error('Widget frame not found');
}
await frame.addScriptTag({
content: 'window.widgetReady = true;',
});
const ready = await frame.evaluate(() => window.widgetReady === true);
console.log({ready});Frame URLs and document structure are site-specific. A frame may not have its final URL immediately after the parent navigation, so locate it after the relevant frame navigation or wait for a selector that identifies the embedded content. If the frame is cross-origin, use Puppeteer’s frame execution APIs rather than trying to access its DOM through a parent-frame evaluation.
A complete, ordered workflow
- Launch and create a page. Keep the browser in a
try/finallyblock so it closes on failures. - Register pre-document code first. Call
evaluateOnNewDocument()beforegoto()if timing is important. - Navigate and wait for the state your script needs. Select a navigation condition appropriate to the site instead of assuming that the first response means the UI is ready.
- Choose the execution method. Use
addScriptTag()for a script element,evaluate()for a one-off function, or a frame method for an iframe. - Await every asynchronous operation. This includes navigation, script insertion, evaluations, waits, and frame discovery.
- Verify the result in the same context. Read a flag, attribute, or DOM change with
evaluate()so a successful promise also corresponds to the expected page state.
import puppeteer from 'puppeteer';
const browser = await puppeteer.launch();
try {
const page = await browser.newPage();
await page.evaluateOnNewDocument(() => {
window.__customSetup = true;
});
await page.goto('https://example.com', {waitUntil: 'domcontentloaded'});
await page.addScriptTag({
content: `window.customScriptLoaded = true;`,
id: 'custom-script',
});
const state = await page.evaluate(() => ({
setup: window.__customSetup === true,
script: window.customScriptLoaded === true,
title: document.title,
}));
console.log(state);
} finally {
await browser.close();
}Troubleshooting custom script injection
The local file cannot be found
Cause: A relative path is resolved from process.cwd(). Fix: log process.cwd(), start the process from the expected directory, or pass an absolute path. Confirm that the file is readable before launching the browser.
The code runs too late
Cause: addScriptTag() or evaluate() was called after the application had already executed the behavior you needed to influence. Fix: register evaluateOnNewDocument() before goto(), then create a new document.
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The function cannot see a Node.js variable
Cause: evaluate() runs in the page context and does not close over Node.js lexical scope. Fix: pass serializable values as arguments, or expose a deliberate bridge rather than referencing the outer variable directly.
The script URL does not load
Cause: The browser cannot reach the URL, the remote server rejects the request, or the page’s environment prevents the load. Fix: open the URL in the same page context, inspect browser console/network output, and prefer content or a local path when you control the source.
The script appears in the wrong document
Cause: The operation targeted the main frame while the code you are inspecting lives in an iframe. Fix: enumerate page.frames(), identify the intended frame, and call frame.addScriptTag() or frame.evaluate().
A later step sees old state
Cause: A promise was not awaited, or the page navigated after injection and replaced the document. Fix: await the insertion and any navigation, then verify the state after the final navigation.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Version and reliability notes
The referenced Puppeteer API material identifies addScriptTag() options in the 25.10.0 documentation and the Page class in 25.12.0 documentation; the execution and pre-document references use the “next” documentation path. Match these examples to the Puppeteer version installed in your project because APIs can change.
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For repeatable automation, keep injection close to the navigation it belongs to, give each injected element an identifiable id, and make setup functions safe to run on every new document. A page reload creates a new document, so a script element inserted into the old document does not persist unless you register equivalent setup with evaluateOnNewDocument() or inject again.
Or skip the browser setup
If your actual goal is to obtain a clean screenshot rather than execute custom JavaScript in a live Puppeteer session, ScreenshotNeo provides a single HTTP request. Its API accepts a URL and returns PNG, JPEG, WebP, or PDF output; the documentation is at screenshotneo.com/docs/.
curl -G "https://api.screenshotneo.com/v1/shot" -d access_key=YOUR_API_KEY --data-urlencode url=https://stripe.com -o shot.webpimport requests
r = requests.get("https://api.screenshotneo.com/v1/shot", params={"access_key": "YOUR_API_KEY", "url": "https://stripe.com"}, timeout=90)
open("shot.webp", "wb").write(r.content)const q = new URLSearchParams({ access_key: 'YOUR_API_KEY', url: 'https://stripe.com' });
const res = await fetch(`https://api.screenshotneo.com/v1/shot?${q}`);ScreenshotNeo accepts cookie and consent banners as a visitor and removes more than 60 known consent platforms, newsletter popups, and chat widgets before capture; each cleanup step can be disabled. Bot checks and CAPTCHAs, blank pages, timeouts, failed loads, and cache hits are not billed, and each response identifies the result with X-Page-Verdict and X-Billed headers. Its MCP server provides take_screenshot, get_page_info, and capture_pdf tools for Claude, Cursor, and other MCP clients. The Free plan includes 1,000 screenshots per month without a card; paid plans start at $5 for 3,000 screenshots.
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Does adding a script survive a page reload?
No. A script element belongs to the current document. Inject it again after the reload, or register equivalent setup with evaluateOnNewDocument() before the next navigation.
Can I add an ES module with Puppeteer?
Yes. Pass type: 'module' along with the script’s path, content, or url.
How can I confirm which frame received my code?
Inspect the frame’s URL and evaluate a marker in that same frame, such as frame.evaluate(() => window.widgetReady), rather than checking only the parent page.
Quick Recap
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