How to Run JavaScript Before Printing a PDF with Headless Chromium

Run the page’s JavaScript, wait for a condition that proves the printable content is ready, and only then call Puppeteer’s page.pdf(). A practical baseline is page.goto() with waitUntil: 'networkidle2', followed by an application-specific check such as page.waitForFunction() or an awaited function exposed by the page. This avoids PDFs that capture a loading shell, stale data, or an unfinished chart.

The reliable sequence

Headless Chromium executes JavaScript during navigation. Your code must distinguish “the document loaded” from “the report is ready to print.” Use this sequence:

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1. Launch Chromium and create a page.
2. Navigate with a suitable lifecycle condition.
3. Await the page operation or readiness predicate that represents completed data and layout work.
4. Select print or screen media and configure PDF options.
5. Call await page.pdf().
6. Close the browser in a finally block.

Puppeteer’s PDF API renders with print CSS media by default. The navigation and readiness waits are separate concerns: network idle describes requests, while your application state describes whether the report is actually usable.

Prerequisites and a complete JavaScript example

Install Puppeteer

npm install puppeteer

The API names and defaults below correspond to current Puppeteer documentation around versions 25.10.0–25.12.0. Check the documentation for the version installed in your project before relying on a changed default.

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Wait for a page-defined readiness flag

import puppeteer from 'puppeteer';

const browser = await puppeteer.launch();
try {
  const page = await browser.newPage();
  await page.goto('https://example.com/report', {
    waitUntil: 'networkidle2',
  });

  // The site must set this only after data and print layout are ready.
  await page.waitForFunction(
    () => window.reportReady === true,
    { timeout: 30_000, polling: 'mutation' }
  );

  await page.pdf({
    path: 'report.pdf',
    printBackground: true,
    format: 'A4',
    margin: { top: '16mm', right: '16mm', bottom: '16mm', left: '16mm' },
  });
} finally {
  await browser.close();
}

window.reportReady is illustrative, not a Chromium or Puppeteer signal. The application must set it after its own fetches, chart rendering, fonts, and print-only layout work have completed. If the page cannot expose a flag, wait for a selector that appears only in the completed state or use an awaited page function.

Ways to wait for JavaScript work

Await a known asynchronous operation with page.evaluate()

When the page supplies a callable function that returns a Promise, run it in the page context and await the result:

await page.evaluate(async () => {
  await window.prepareReportForPrint();
});

await page.pdf({ path: 'report.pdf', printBackground: true });

This assumes the site defines prepareReportForPrint. It is not a built-in Puppeteer method. The Promise must resolve only after the operation needed for printing is complete.

Wait for an application predicate with page.waitForFunction()

Use this when readiness is represented by a value in the page:

await page.waitForFunction(
  () => document.querySelector('[data-print-ready="true"]') !== null,
  { timeout: 30_000, polling: 100 }
);

The predicate is evaluated in the page context until it becomes truthy or the timeout expires. Keep the predicate tied to a real completion signal, such as a data-loaded attribute, a completed render state, or a page-owned flag.

Use navigation and network-idle waits as milestones

await page.goto(url, { waitUntil: 'networkidle2' });

The official PDF example uses networkidle2 before generating the file. It is useful when the page settles after navigation, but it does not prove that application work is complete. Analytics, polling, WebSockets, long-lived requests, or delayed rendering can make network activity a poor proxy for print readiness.

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Puppeteer also provides page.waitForNetworkIdle(). It waits for network activity to remain idle for the configured idle period and always waits at least that idle time. Pair it with a page-specific condition when the report has a meaningful completion state.

Why an arbitrary sleep is a weak default

await new Promise(resolve => setTimeout(resolve, 5000));

A fixed delay may be too short on a slow run and unnecessarily long on a fast one. Prefer a known Promise, readiness predicate, selector, or network-idle condition. A delay can still be a deliberate supplement when an animation or debounce has no observable completion signal; document why it is needed and keep it bounded.

Make the PDF match the intended design

Print media versus screen media

page.pdf() uses the print CSS media type by default. If the page’s screen stylesheet is the desired output, switch media first:

await page.emulateMediaType('screen');
await page.pdf({ path: 'screen-styled.pdf' });

Otherwise, leave the default in place and maintain print-specific rules with @media print and @page.

Important PDF options

Chromium modifies PDF colors for print by default. If exact colors matter, use the documented CSS property -webkit-print-color-adjust in the page’s print styles.

A production-oriented pattern

import puppeteer from 'puppeteer';

export async function renderReport(url, outputPath) {
  const browser = await puppeteer.launch();
  try {
    const page = await browser.newPage();
    page.setDefaultTimeout(30_000);

    await page.goto(url, { waitUntil: 'networkidle2' });

    await page.waitForFunction(
      () => window.reportReady === true,
      { timeout: 30_000 }
    );

    // Keep print CSS. Use 'screen' here only if screen styling is required.
    await page.pdf({
      path: outputPath,
      format: 'A4',
      printBackground: true,
      preferCSSPageSize: true,
      waitForFonts: true,
    });
  } finally {
    await browser.close();
  }
}

await renderReport('https://example.com/report', 'report.pdf');

Set the readiness flag only after asynchronous data has arrived, components have rendered, and any print preparation has run. If you control the page, expose a small, deterministic contract rather than making the PDF worker guess from timing.

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Troubleshooting

The PDF contains a spinner or empty chart

Cause: navigation completed before application rendering. Fix: add a real readiness flag, selector, or awaited function. Keep networkidle2 as a milestone, not the sole assertion.

waitForFunction times out

Cause: the predicate is never true, the flag is set in a different frame, or the page failed earlier. Fix: verify the flag in the page, increase the timeout only when the workload genuinely needs it, and capture console and page-error events while diagnosing:

page.on('console', message => console.log('[browser]', message.text()));
page.on('pageerror', error => console.error('[page error]', error));

Network idle never arrives

Cause: polling, analytics, streaming, or another long-lived request keeps activity alive. Fix: navigate with a less restrictive lifecycle milestone and rely on an application-specific readiness predicate, or use page.waitForNetworkIdle() with an appropriate bounded setting.

Colors or backgrounds are missing

Cause: printBackground defaults to false, and print CSS may intentionally alter colors. Fix: set printBackground: true and adjust print styles, including -webkit-print-color-adjust when exact colors are required.

The PDF uses the wrong layout

Cause: print media is active by default. Fix: inspect @media print rules, or call page.emulateMediaType('screen') before page.pdf() when screen CSS is the requirement.

Fonts change pagination

Cause: the intended webfont was not available when layout was captured. Fix: keep waitForFonts: true, ensure the font request succeeds, and make the readiness signal occur after font-dependent rendering.

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The browser remains running after an error

Cause: cleanup is not guaranteed on exceptions. Fix: put PDF generation inside try and close the browser in finally, as in the examples.

Performance, reliability, and operational choices

• Use the narrowest meaningful wait. A page-owned completion signal avoids both premature output and needless delay.
• Bound every wait. Timeouts turn a stuck page into a diagnosable failure instead of an indefinitely running job.
• Keep rendering deterministic. Disable or account for animations, make data snapshots consistent, and ensure resources needed for the report are reachable from the Chromium process.
• Choose output settings deliberately. Background graphics, paper size, margins, scale, orientation, page ranges, and CSS page-size preference all affect pagination and file appearance.
• Measure the stages you own. Record navigation, readiness, and PDF-generation durations separately; a slow readiness check and a slow PDF encode have different remedies.
• Pin and review Puppeteer versions. API behavior and defaults can change, so align implementation and documentation with the installed version.

Or skip the browser setup

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cURL

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Python

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Node.js

const q = new URLSearchParams({ access_key: 'YOUR_API_KEY', url: 'https://stripe.com' });
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Frequently Asked Questions

Does Chromium wait for every JavaScript timer automatically?

No. It executes JavaScript, but your script must await the operation or condition that defines readiness before calling page.pdf().

Can I print only selected PDF pages?

Yes. Puppeteer’s page.pdf() supports pageRanges for selecting output pages.

Is networkidle2 a guarantee that a report is complete?

No. It is a network-activity milestone. Pair it with an application-specific readiness check when rendering has additional work.

Quick Recap

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$239.99

SaleBestseller No. 2

Epson EcoTank Photo ET-8550 Wireless Wide-Format All-in-One Tank Printer

$669.00

Bestseller No. 3

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