Start JavaScript coverage before the navigation or interaction you want to measure, then stop it to retrieve the results: call await page.coverage.startJSCoverage(), exercise the page, and call await page.coverage.stopJSCoverage(). The returned entries include script text and execution ranges that you can use to estimate how much code ran.
Start coverage before the activity you want to measure
Create a page, enable coverage, and only then navigate or interact. This example measures the initial load of a page and prints the percentage of captured JavaScript bytes marked as used:
const page = await browser.newPage();
await page.coverage.startJSCoverage();
await page.goto('https://example.com');
// Exercise any additional routes or interactions you want included.
const jsCoverage = await page.coverage.stopJSCoverage();
let totalBytes = 0;
let usedBytes = 0;
for (const entry of jsCoverage) {
totalBytes += entry.text.length;
for (const range of entry.ranges) {
usedBytes += range.end - range.start - 1;
}
}
console.log(`Bytes used: ${(usedBytes / totalBytes) * 100}%`);The calls resolve asynchronously: startJSCoverage() resolves when collection has started, and stopJSCoverage() resolves with an array of JavaScript coverage entries. The byte calculation follows the method in the Puppeteer Coverage class API documentation.
The percentage describes only the scripts and activity included in this collection. A route or interaction you never exercised cannot contribute evidence of whether its code runs. Treat the result as a measure of your test journey, not a universal completeness score for the application.
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Choose the coverage options you need
The live Puppeteer options reference identifies these defaults. Check the reference for the Puppeteer version pinned in your project, because the surfaced documentation versions differ: the options page labels itself 25.12.0, while the start-method page labels itself 25.10.0.
| Option | Documented default | What it changes |
|---|---|---|
resetOnNavigation |
true |
Coverage is reset on navigation by default. Setting it to false does not guarantee that data survives: Chrome can discard the previous page’s JavaScript execution environment. |
reportAnonymousScripts |
false |
When enabled, reports anonymous scripts such as dynamically created code from eval or new Function. They are identified with a debugger://VM URL unless the script has a //# sourceURL comment. |
includeRawScriptCoverage |
false |
Controls whether raw script coverage is included in the results. |
useBlockCoverage |
true |
Controls block-level rather than function-level coverage collection. |
These defaults and option descriptions are from Puppeteer’s JSCoverageOptions interface reference. For example, opt in to anonymous scripts when dynamically generated code is part of the behavior under test:
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await page.coverage.startJSCoverage({
reportAnonymousScripts: true,
});Handle navigation as separate collections when necessary
Starting coverage before the first navigation matters if initial page execution is in scope. The Chrome DevTools Protocol’s Profiler reference warns that “Coverage data for JavaScript executed before enabling precise code coverage may be incomplete.” Enabling precise coverage also prevents optimized code and resets execution counters; see the Profiler domain reference.
Because navigation can reset coverage—and Chrome may discard the earlier execution environment—do not rely on a single collection to retain data across multiple page loads. Stop coverage before leaving a page, start a new collection for the next page, and combine the reports in your reporting step if you need a multi-page view.
The Tool Desk
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Choose Puppeteer or DevTools for the job
Use Puppeteer when coverage collection needs to run as part of a repeatable script or automated test. Use Chrome DevTools Coverage when you want to explore a page interactively and inspect JavaScript and CSS resources while reloading and using the site. DevTools describes finding unused code as a first step; deciding what to refactor still depends on the application’s technology stack. See Chrome DevTools Coverage documentation.
The Puppeteer Coverage API documentation also identifies puppeteer-to-istanbul as an option for producing Istanbul-consumable output. The cited API documentation does not establish a package URL or other availability details.
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Or skip the browser setup
For screenshots rather than JavaScript execution coverage, ScreenshotNeo is a website screenshot API and MCP server. It does not replace Puppeteer coverage: it captures a page as an image or PDF. A single GET request can save a screenshot:
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 for request options. Before capture, it can accept cookie or consent banners and remove more than 60 known consent platforms, newsletter popups, and chat widgets; each step can be turned off. Bot checks, blank pages, timeouts, failed loads, and cache hits are not billed, and response headers identify the page verdict and billing status. Its MCP server provides take_screenshot, get_page_info, and capture_pdf tools for AI agents and MCP clients.
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Troubleshoot common coverage surprises
- Coverage misses code from the initial load: start collection before
page.goto(). Code executed before precise coverage is enabled may be incomplete. - Scripts created with
evaldo not appear: enablereportAnonymousScripts. Anonymous scripts are excluded by default; asourceURLcomment can give one a more meaningful URL. - Earlier-page data is missing after navigation: navigation resets coverage by default, and disabling that reset is not a guarantee that Chrome preserves the previous execution environment. Stop and collect before navigating away, then start a separate collection.
- The usage percentage seems unexpectedly low: add representative routes and user actions to the measured run. Coverage only reflects the scripts and activity collected.
- You need to compare manual JavaScript and CSS use: Puppeteer’s workflow here is JavaScript coverage; use the DevTools Coverage panel for interactive inspection of JavaScript and CSS.
Frequently Asked Questions
Does JavaScript coverage include anonymous scripts by default?
No. Puppeteer’s documentation says anonymous scripts are excluded by default; enable reportAnonymousScripts to include them.
Does Puppeteer coverage tell me which CSS is unused?
The workflow described here retrieves JavaScript coverage. Chrome DevTools Coverage can inspect both JavaScript and CSS interactively.
Quick Recap
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