Start Puppeteer’s JavaScript and CSS coverage collectors before the page load or interaction you want to measure, then stop them and calculate the proportion of reported source ranges that ran. The percentage describes only the code observed during that particular run—not every route or interaction in an application.
Measure JavaScript and CSS coverage in Puppeteer
This runnable example measures the code observed during an initial page load. It follows Puppeteer’s documented approach of adding the JavaScript and CSS reports together, counting each entry’s source-text length as total bytes, and summing the reported ranges as used bytes. The zero-total check prevents division by zero if the page produces no coverage entries.
const puppeteer = require('puppeteer');
(async () => {
const browser = await puppeteer.launch();
try {
const page = await browser.newPage();
await Promise.all([
page.coverage.startJSCoverage(),
page.coverage.startCSSCoverage(),
]);
await page.goto('https://example.com');
// Perform any interactions you want included in the measurement here.
const [jsCoverage, cssCoverage] = await Promise.all([
page.coverage.stopJSCoverage(),
page.coverage.stopCSSCoverage(),
]);
const entries = [...jsCoverage, ...cssCoverage];
let totalBytes = 0;
let usedBytes = 0;
for (const entry of entries) {
totalBytes += entry.text.length;
for (const range of entry.ranges) {
usedBytes += range.end - range.start - 1;
}
}
const usedPercent = totalBytes === 0 ? 0 : (usedBytes / totalBytes) * 100;
console.log(`Bytes used: ${usedPercent}%`);
} finally {
await browser.close();
}
})();Puppeteer describes the Coverage class as gathering information about JavaScript and CSS parts used by a page. See the Coverage class documentation for its coverage methods.
Choose the measurement window
Initial page load
Start both collectors before calling page.goto(), as in the example. If collection starts after navigation, code that ran during the earlier load is outside the measurement window.
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User interactions
To measure an interaction path, start coverage before the first action, run a repeatable sequence, then stop collection. For example, the sequence might open a menu, submit a form, and navigate to a particular view. State which page and actions were exercised when you report the result; one run cannot establish coverage for actions it did not execute.
Multi-page journeys
Do not rely on resetOnNavigation: false to preserve JavaScript coverage across a navigation. Chrome may discard the previous page’s execution environment and its coverage. The safer approach is to stop collection before leaving each page, start it again on the next page, and combine the returned reports yourself. The behavior and option are documented in JSCoverageOptions.
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Configure what Puppeteer reports
JavaScript options
JavaScript coverage defaults to block-level granularity. Set useBlockCoverage: false when you need function-level coverage instead. The documented defaults are resetOnNavigation: true, reportAnonymousScripts: false, includeRawScriptCoverage: false, and useBlockCoverage: true. Check the API for the Puppeteer version installed in your project because documentation defaults can change.
await page.coverage.startJSCoverage({
resetOnNavigation: true,
reportAnonymousScripts: true,
includeRawScriptCoverage: false,
useBlockCoverage: true,
});With reportAnonymousScripts: true, coverage can include anonymous scripts created by eval or new Function. Their generated URLs commonly begin with debugger://VM, unless a sourceURL comment supplies a URL. For option details, see Coverage.startJSCoverage().
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CSS options and reporting limits
CSS coverage has a resetOnNavigation option that defaults to true. Its API is documented at Coverage.startCSSCoverage(). Puppeteer does not include dynamically injected style tags without sourceURLs; JavaScript anonymous scripts are also omitted by default unless configured otherwise. Consequently, the calculated percentage describes the source represented in the reports Puppeteer returned, not necessarily every runtime-generated style or script.
Understand the percentage
The example calculates usedBytes / totalBytes × 100 across the returned JavaScript and CSS entries. This is a byte-based ratio for the chosen run and configuration, not a published benchmark or a universal application score. Puppeteer’s example calls its result “initially executed code.” Changing the page, action sequence, or coverage options can change the result.
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The example sums every reported range using Puppeteer’s documented range formula. It is an aggregate across entries, not a line-coverage or test-suite metric. Use consistent pages, actions, and settings when comparing runs, and describe what was exercised alongside the figure.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Optional: export coverage for Istanbul
If you already use Istanbul tooling, Puppeteer’s Coverage documentation points to puppeteer-to-istanbul as a way to convert coverage output into a consumable format. Conversion is not required to calculate the byte percentage above.
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Troubleshooting coverage results
- Coverage arrays are empty or unexpectedly small: Start both collectors before navigation or before the target interactions. Check that the page actually loaded and that the actions ran before stopping.
- Anonymous code is missing: Set
reportAnonymousScripts: truewhen starting JavaScript coverage. Anonymous scripts without sourceURLs are excluded by default. - Dynamically added CSS is missing: CSS style tags injected without sourceURLs are not included in Puppeteer’s CSS coverage report.
- Coverage disappears after navigation: Collect each page separately. Stop before navigating away, restart on the next page, and merge the resulting arrays rather than assuming JavaScript coverage survives the transition.
- The percentage is 0 or not a number: The example’s zero-total guard returns 0 when there are no reported source bytes. If the output is unexpected, confirm that coverage was started, the page executed scripts or styles, and the measurement ended after the intended actions.
Or skip the browser setup
For screenshots rather than code-coverage reports, ScreenshotNeo provides a one-request website screenshot API; it does not measure JavaScript or CSS coverage. For example, save a WebP capture of a page with cURL:
curl -G "https://api.screenshotneo.com/v1/shot" -d access_key=YOUR_API_KEY --data-urlencode url=https://stripe.com -o shot.webpSee the ScreenshotNeo API documentation for setup and request options. ScreenshotNeo accepts cookie and consent banners before capture and removes 60+ known consent platforms, newsletter popups, and chat widgets; each cleanup step can be turned off. Bot checks, blank pages, timeouts, failed loads, and cache hits are not billed, and response headers report the page verdict and billing status. Its MCP server offers screenshot tools for AI agents, and every plan includes all features. The free plan includes 1,000 screenshots a month without a card; paid plans start at $5 for 3,000 shots. Sign up for 1,000 free screenshots a month, with no card required.
Frequently Asked Questions
Does Puppeteer coverage tell me whether my whole application is covered?
No. It reports code observed during the pages and actions in the measured run; it does not establish coverage for unvisited routes or untested interactions.
Can I use Puppeteer coverage output with Istanbul?
Yes. Puppeteer’s Coverage documentation points to puppeteer-to-istanbul for converting reports into a format consumable by Istanbul.
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