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To measure JavaScript code coverage in Puppeteer, start collection with page.coverage.startJSCoverage() before the navigation or interaction you want to observe, exercise the page, then call page.coverage.stopJSCoverage(). Puppeteer returns script text and executed ranges; dividing used bytes by total script-text bytes gives a byte-based coverage percentage for that captured session.
Collect and calculate JavaScript coverage
This runnable example follows Puppeteer’s documented byte-based approach. Replace the example URL and add the interactions that represent the behavior you want to measure.
import puppeteer from 'puppeteer';
const browser = await puppeteer.launch();
try {
const page = await browser.newPage();
// Begin before the navigation and activity you want to capture.
await page.coverage.startJSCoverage();
await page.goto('https://example.com');
// Exercise the application: click controls, submit forms, and visit
// the states whose JavaScript execution you want included.
const entries = await page.coverage.stopJSCoverage();
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 percent = totalBytes === 0 ? 0 : (usedBytes / totalBytes) * 100;
console.log(`Bytes used: ${percent}%`);
} finally {
await browser.close();
}The zero-total guard handles a capture that returns no script text. Puppeteer’s Coverage API also has CSS collection methods, but this calculation is for JavaScript.
Understand what the percentage measures
The percentage compares executed JavaScript bytes with the total bytes of script text returned for the captured session. It is not a percentage of tests passed, branches defined, or all code that could theoretically run. Coverage records runtime activity, so the result depends on which routes, interactions, and states your test actually exercised.
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A low percentage can indicate untested behavior, but the number alone cannot tell whether code is dead, intentionally conditional, or simply outside the scenario. Use it to find scripts and ranges your scenario did not execute, then investigate those areas rather than treating a single score as a general quality rating.
Choose collection options deliberately
The current Puppeteer API reference lists these defaults for startJSCoverage():
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| Option | Default | When to change it |
|---|---|---|
resetOnNavigation |
true |
Do not rely on changing this to retain data across page transitions; Chrome may discard the previous execution environment. |
reportAnonymousScripts |
false |
Set to true when dynamically generated scripts matter. They may be named like debugger://VM, unless a //# sourceURL comment supplies a URL. |
includeRawScriptCoverage |
false |
Enable only if a downstream workflow needs V8’s raw script coverage entries. |
useBlockCoverage |
true |
Set to false to request function-level rather than block-level coverage. |
These are API defaults, not universal recommendations. Include anonymous scripts if your application relies on generated JavaScript; choose function-level data if that granularity better suits your reporting.
Preserve coverage across a multi-page journey
Coverage can be reset at navigation, and setting resetOnNavigation: false does not guarantee that data survives: the browser can discard the old page’s execution environment. For a journey that crosses pages, collect each page explicitly:
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- Start JavaScript coverage before the activity on the current page.
- Exercise the current page, then call
stopJSCoverage()before leaving it. - Navigate to the next page and start a new coverage collection.
- Merge or process the separate reports in your downstream reporting workflow.
Use the results in an Istanbul workflow
Puppeteer returns its own coverage entries. If your reporting pipeline consumes Istanbul-compatible coverage, the Puppeteer guide points to puppeteer-to-istanbul as a conversion option. The Puppeteer API reference and guide are versioned; check the reference for the Puppeteer release installed in your project before relying on option behavior.
Troubleshoot common coverage problems
- The result is zero or unexpectedly small: Confirm that coverage starts before the target navigation or interaction, that the application activity occurs before stopping, and that the flow exercises the intended state.
- A navigation seems to erase earlier results: This is consistent with the default reset behavior. Stop collection before navigating, start again on the next page, and merge reports rather than depending on
resetOnNavigation: false. - Dynamically generated code is absent: Anonymous scripts are excluded by default. Try
reportAnonymousScripts: true; generated scripts may have VM-style names unless they declare asourceURL. - Your report granularity is not what you expect: Block-level coverage is the default. Set
useBlockCoverage: falsewhen you need function-level data. - Your downstream tool cannot read the returned entries: Convert Puppeteer’s output to the format your reporting pipeline expects; the Puppeteer guide identifies
puppeteer-to-istanbulfor Istanbul-consumable output.
Or skip the browser setup
ScreenshotNeo captures screenshots or PDFs; it does not collect JavaScript code coverage, so it is not a replacement for Puppeteer’s coverage API. It can be useful when the deliverable is a clean page image rather than execution data. One GET request captures a URL:
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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. Cookie banners, newsletter popups, and chat widgets are removed before the shot; bot checks, blank pages, and failed loads are never billed. An MCP server provides screenshot tools for AI agents, and the free plan includes 1,000 screenshots a month with no card; paid plans start at $5 for 3,000.
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