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A free scan shows the junk files, broken settings and background clutter dragging Windows down - then fixes them in one click.Free scan · Windows 10 & 11Start Puppeteer’s JavaScript and CSS coverage collectors before the page load or interaction you want to measure, stop them afterward, then compare the returned used ranges with the reported source text. The resulting percentage describes only the code observed during that particular run—not universal coverage of the whole application.
Measure JavaScript and CSS coverage in Puppeteer
This runnable example follows Puppeteer’s documented combined calculation. It starts both collectors before navigation, stops them after the page load and any actions you choose, then calculates the share of reported source-text bytes covered by the returned ranges. The zero-total check avoids dividing by zero.
const page = await browser.newPage();
await Promise.all([
page.coverage.startJSCoverage(),
page.coverage.startCSSCoverage(),
]);
await page.goto('https://example.com');
// Perform the interactions that define the measurement window 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}%`);Puppeteer describes its Coverage class as gathering information about parts of JavaScript and CSS used by a page. The byte calculation is a derived percentage, not a benchmark or a claim that the application is fully covered. See the Puppeteer Coverage API.
Choose the measurement window
Initial page-load coverage
Start both collectors before page.goto(), as in the example. Stop them after navigation has completed and before closing the page. This measures code observed during that load. Puppeteer’s documentation labels its example result “initially executed code.”
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Coverage during interactions
For interaction coverage, start the collectors before the actions that matter, run the same defined sequence each time, and stop after it finishes. For example, exercise a menu, submit a form, and open a dialog only if those actions reflect the behavior you intend to measure. Record the pages and actions alongside the percentage; changing the sequence changes what code can be observed.
Journeys across multiple pages
Do not rely on resetOnNavigation: false to preserve all JavaScript coverage when navigating. Chrome may discard the previous page’s execution environment and coverage data. For a multi-page journey, stop and collect coverage before leaving each page, start new collectors on the next page, and combine the returned reports for your later calculation. Puppeteer documents this caveat in its JavaScript coverage options.
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Configure what Puppeteer reports
Options control the detail and scripts included in the results. Defaults listed below are those in Puppeteer’s API documentation; check the API for the version installed in your project, since documentation can change.
| Collector or option | Documented default | What to change it for |
|---|---|---|
JavaScript resetOnNavigation |
true |
Controls reset behavior on navigation; setting it to false does not guarantee data survives a page change. |
JavaScript reportAnonymousScripts |
false |
Set to true to include anonymous scripts created by eval or new Function. Generated URLs ordinarily begin debugger://VM, unless a sourceURL comment supplies a URL. |
JavaScript includeRawScriptCoverage |
false |
Set to true when you need raw V8 script coverage in addition to the regular report. |
JavaScript useBlockCoverage |
true |
Set to false for function-level rather than block-level coverage. |
CSS resetOnNavigation |
true |
Controls CSS coverage reset behavior on navigation. |
For the available JavaScript settings, see startJSCoverage() and the JSCoverageOptions interface. CSS collector options are documented at startCSSCoverage().
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Understand omissions and interpret the percentage
- Anonymous JavaScript scripts are omitted by default. Enable
reportAnonymousScriptsif they matter to your measurement. - CSS coverage omits dynamically injected style tags without
sourceURLcomments. - The result depends on the pages, actions, settings, and reports included in the run. It does not prove that every runtime-generated script or style was accounted for.
The percentage is calculated as used bytes divided by total reported source-text bytes, multiplied by 100. Puppeteer’s documented range calculation uses range.end - range.start - 1 for each range and entry.text.length for the entry’s total. That is the documented method; do not describe its output as a general application coverage guarantee.
Use coverage output with Istanbul
If your workflow already uses Istanbul tooling, Puppeteer’s Coverage documentation points to puppeteer-to-istanbul for converting reports into an Istanbul-consumable form. Conversion is optional; it is not needed to calculate the basic used-byte percentage above. See the Coverage documentation.
Troubleshoot common coverage problems
- The report is empty or the percentage is zero: Check that both collectors started before the relevant page activity and stopped afterward. Confirm navigation succeeded and your chosen actions ran before stopping.
- Coverage disappears between pages: Collect and stop coverage before navigating away, then start a fresh collection on the next page. Merge those reports instead of assuming
resetOnNavigation: falseretains the previous JavaScript environment. - Code created with
evalis missing: Start JavaScript coverage withreportAnonymousScripts: true; anonymous scripts are excluded by default. - Dynamically added CSS is missing: CSS coverage omits dynamically injected style tags without
sourceURL. Treat the result as coverage of returned entries, not a complete inventory of runtime styles. - The number changes between runs: Make navigation and interactions repeatable, and keep the same collector options. A changed action sequence can execute different code.
Or skip the browser setup:
For screenshots rather than coverage reports, ScreenshotNeo is a website screenshot API and MCP server. It does not measure JavaScript or CSS coverage; Puppeteer is the method above for that task. ScreenshotNeo offers a one-request screenshot capture:
curl -G "https://api.screenshotneo.com/v1/shot" -d access_key=YOUR_API_KEY --data-urlencode url=https://example.com -o shot.webpSee the ScreenshotNeo API documentation. It accepts cookie and consent banners like a visitor and removes more than 60 known consent platforms, newsletter popups, and chat widgets before capture. Bot checks, blank pages, timeouts, failed loads, and cache hits are not billed; response headers identify the page verdict and billing status. Its MCP server lets AI agents using Claude, Cursor, or any MCP client use screenshot tools. The Free plan includes 1,000 screenshots per month without a card; paid plans start at $5 for 3,000 shots.
Sign up for ScreenshotNeo’s free plan: 1,000 screenshots a month, no card required.
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Frequently Asked Questions
Can I measure coverage for only one interaction?
Yes. Start both collectors immediately before that interaction, run it, then stop them; the returned report reflects the measured window.
Can Puppeteer export coverage for Istanbul?
Puppeteer’s Coverage documentation points to puppeteer-to-istanbul as an optional conversion workflow.
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