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Use --javascript-delay for a fixed wait, or --window-status for a page-controlled readiness signal. wkhtmltopdf enables JavaScript by default, but its documented post-load delay is only 200 milliseconds. That is frequently too short for data fetched after load, chart rendering, lazy images, or client-side layout. The reliable process is to verify your build, choose an explicit readiness strategy, test a representative page, and move to a current browser renderer when the page needs APIs that wkhtmltopdf cannot provide.
What wkhtmltopdf actually waits for
wkhtmltopdf loads the document, runs JavaScript, and then converts the rendered page. JavaScript is enabled by default. Unless you change it, the converter waits only 200 milliseconds after page load before creating the PDF. That short, fixed interval is the usual reason a PDF contains an empty chart, missing API data, or a loading skeleton.
The wait is not a guarantee that every asynchronous operation has completed. Network speed, server response time, JavaScript execution, fonts, images, and layout work vary between runs. Treat the default as a fast baseline, not a readiness check.
Prepare and identify your wkhtmltopdf build
-
Check the executable and version:
wkhtmltopdf --version wkhtmltopdf --help -
Confirm that the options you plan to use are present. The project’s downloads page identifies the 0.12.6 series as stable and dates that release June 11, 2020. Distribution packages can differ from official builds.
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Record whether your build includes patched Qt. The project notes that some features require patched Qt; a package compiled without those patches may behave differently even when the command line looks identical.
Do not assume that increasing a delay repairs a browser-compatibility problem. wkhtmltopdf uses Qt 4, which has been unsupported since 2015, and its bundled WebKit had not been updated since 2012 according to the project status account. Test the exact build deployed in production.
Method 1: wait a fixed number of milliseconds
--javascript-delay <msec> adds a fixed post-load wait. Start with a measured value based on your slowest normal response, then validate under load and on a cold cache.
wkhtmltopdf --enable-javascript --javascript-delay 3000
https://example.com/report.html report.pdfThe number above is an example, not a universal recommendation. A three-second delay may be excessive for a static page and insufficient for a slow API. Increase it only after observing when the required content becomes visible.
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- The page is not under your control.
- You cannot add a readiness marker to the application.
- The asynchronous work has a predictable upper bound.
- A small amount of timing variability is acceptable.
What a delay cannot do
- It cannot make unsupported browser APIs work.
- It cannot distinguish successful data rendering from a failed request.
- It can still capture a loading state when a slow request exceeds the chosen value.
- It adds the full wait to every conversion, including fast pages.
Method 2: let the page signal readiness with window.status
If you control the HTML, expose a deterministic signal after all content required in the PDF has loaded and layout has settled. Set window.status to a known value, then tell wkhtmltopdf to wait for that value.
<script>
(async function () {
try {
const response = await fetch('/api/report');
if (!response.ok) throw new Error(`HTTP ${response.status}`);
const data = await response.json();
renderReport(data);
await document.fonts.ready;
// Set this only after data, images, and layout needed by the PDF are ready.
window.status = 'pdf-ready';
} catch (error) {
console.error(error);
// Always resolve the wait in the failure path.
window.status = 'pdf-error';
}
})();
</script>wkhtmltopdf --enable-javascript
--window-status pdf-ready
https://example.com/report.html report.pdfThe option waits for equality with the supplied status value. Your page must set it on every success path, including branches that render optional sections. Add an application-level timeout or watchdog around the conversion: the manual documents the equality wait but does not define a timeout strategy. Without one, a bug that prevents the assignment can leave a worker waiting indefinitely.
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Make the readiness marker meaningful
- Set it after API data has been inserted, not when the request starts.
- Wait for images, fonts, charts, and any post-render measurements that affect pagination.
- Use a separate error marker and fail the job when it appears; do not silently generate an incomplete PDF.
- Keep the marker out of normal interactive navigation if it could trigger business logic prematurely.
Inject code with –run-script
--run-script <js> runs additional JavaScript after the page is done loading. It can be useful for a small, controlled adjustment, such as setting a class or a status value on legacy content.
wkhtmltopdf --enable-javascript
--run-script "window.status='pdf-ready'"
https://example.com/report.html report.pdfInjection does not turn wkhtmltopdf into a modern browser. It is not a replacement for fixing application readiness or for changing renderers when the site depends on unsupported APIs. Quote and escape the script according to the shell and operating system you use.
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Choosing the right readiness strategy
| Approach | Control | Strength | Main risk |
|---|---|---|---|
| Default 200 ms | None | Fast for genuinely static pages | Captures incomplete asynchronous work |
--javascript-delay |
Converter operator | Works when the page cannot be changed | Either wastes time or remains too short |
--window-status |
Page owner | Waits for an explicit application condition | Missing marker can wait forever without an external timeout |
--run-script |
Converter operator | Applies small post-load changes | Cannot compensate for engine incompatibility |
Use a status marker when you own the page and can define completion precisely. Use a measured delay for third-party pages, while accepting that it is probabilistic. In both cases, inspect the resulting PDF rather than treating a zero exit code as proof that the content is correct.
When wkhtmltopdf is the wrong renderer
The project status recommends Puppeteer for sites that use dynamic JavaScript. Puppeteer’s browser page PDF facility renders through a current browser engine and is generally a better fit for applications that depend on modern JavaScript, browser APIs, or complex client-side frameworks.
For HTML reports that you control and that do not require browser JavaScript, the same status guidance suggests evaluating WeasyPrint or commercial Prince. Those tools are alternatives for document-oriented HTML, not drop-in fixes for an application whose data is still fetched by JavaScript.
Use a representative compatibility test
- Include the slowest API response your production page normally sees.
- Exercise charts, lazy-loaded images, custom fonts, and long tables.
- Compare pagination and computed layout, not only whether text appears.
- Run the exact operating-system package or container image used in production.
Security and isolation
The project explicitly warns not to use wkhtmltopdf with untrusted HTML. User-supplied HTML or JavaScript must be sanitized, and the project suggests considering AppArmor or SELinux containment. These recommendations are defensive controls, not a guarantee that wkhtmltopdf is a complete sandbox.
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- Accept only trusted templates where possible.
- Sanitize user-provided markup and scripts before conversion.
- Run conversion in a restricted account or isolated worker.
- Limit outbound network access if the document does not need it.
- Apply an external process timeout, memory limit, and job-size limit.
Troubleshooting incomplete PDFs
The PDF shows a spinner or empty chart
Increase --javascript-delay temporarily to confirm a timing problem. Then implement --window-status if you control the page, or move to Puppeteer if the page requires APIs unavailable in the old WebKit engine.
The command hangs
A status value was probably never assigned, or an exception bypassed the success path. Add a failure marker, log browser-side errors, and enforce a parent-process timeout. Check that the value passed to --window-status exactly matches the page assignment.
The option is unknown or behaves differently on another machine
Compare wkhtmltopdf --version, package origin, and --help output. Patched-Qt features may be absent from distribution builds.
More delay changes nothing
The problem is likely compatibility, a failed request, authentication, or a JavaScript exception rather than timing. Inspect server logs and browser-console output, then test the same URL in a current browser renderer.
Remote assets are missing
Verify that the conversion environment can resolve the host and access required resources. Check authentication headers, cookies, redirects, certificate validation, and content-security behavior. A longer wait cannot fix a request that is blocked or unauthorized.
The PDF is correct locally but wrong in production
Compare the executable build, fonts, locale, timezone, network path, environment variables, and page data. Reproduce with a cold cache and production-like latency; asynchronous rendering is sensitive to all of these.
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Operational checklist
- Verify the executable and build on every deployment.
- Confirm JavaScript is enabled explicitly when clarity matters.
- Use a measured delay only when a fixed wait is acceptable.
- Prefer a page-owned status marker with an external timeout.
- Fail conversions when the page reports an error marker or missing data.
- Test modern JavaScript pages with Puppeteer before committing to wkhtmltopdf.
- Sanitize untrusted HTML and isolate conversion workers.
- Validate the actual PDF under production latency, fonts, authentication, and pagination.
Frequently Asked Questions
Is JavaScript enabled by default in wkhtmltopdf?
Yes. The documented default enables JavaScript; you can add –enable-javascript to make that intent explicit.
Can I wait for network idle in wkhtmltopdf?
The documented controls are a fixed –javascript-delay, a –window-status equality wait, and –run-script. There is no documented network-idle readiness option in the controls described here.
Why does a longer delay still produce an old or broken page?
Waiting only changes timing. wkhtmltopdf’s Qt 4/WebKit engine may lack browser APIs required by the application; use a current browser renderer such as Puppeteer when compatibility is the issue.
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