The Tool Desk
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Capture the failed test in Mocha’s afterEach() hook
Mocha runs afterEach() after each test and before the suite-level after() teardown hook. That makes it the right place to inspect the test result and capture the page before quitting the browser. See Mocha’s hook documentation and Selenium’s JavaScript WebDriver API.
The example below is for a suite that creates and owns one driver. It assumes the project uses Mocha’s BDD interface and a Selenium JavaScript WebDriver version with takeScreenshot(). Add the hook to the suite where driver is defined, or adapt it to your project’s shared driver fixture.
import fs from 'node:fs/promises';
import path from 'node:path';
const screenshotDir = 'artifacts/screenshots';
let driver;
function safeName(title) {
return title.replace(/[^a-z0-9-_]+/gi, '_').slice(0, 120) || 'unnamed-test';
}
afterEach(async function () {
const test = this.currentTest;
if (test?.state !== 'failed' || !driver) return;
await fs.mkdir(screenshotDir, { recursive: true });
const image = await driver.takeScreenshot();
const filename = `${safeName(test.fullTitle())}.png`;
await fs.writeFile(path.join(screenshotDir, filename), image, 'base64');
});
after(async function () {
if (driver) await driver.quit();
});This is an implementation pattern, not a claim that the snippet has been run against your installed versions. Make sure the file uses a module format supported by your Node.js project, and that afterEach and after are available through your chosen Mocha setup. The regular function declaration matters: Mocha supplies the current test through the hook context, while an arrow function does not receive Mocha’s own this.
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Why the hook checks the test state
this.currentTest identifies the test associated with the hook, and its state is checked for failed before any file work occurs. Without that guard, the hook would save an image after successful tests too, which can generate unnecessary artifacts and obscure the failures you need to investigate. Confirm the hook context and state behavior with the Mocha version and configuration in your project.
Why the browser must still be open
The screenshot call needs a live WebDriver session. Selenium documents that quit() ends the session, so put the capture in afterEach() and leave driver.quit() for the later suite teardown. A screenshot attempt after quitting cannot retrieve the failed page. Selenium’s API describes screenshot capture as best effort and returns a base64-encoded PNG; browser and driver behavior can affect what appears in the image.
Save and identify screenshot files reliably
The example creates artifacts/screenshots recursively so the first failure does not fail simply because the output directory is missing. It sanitizes the test’s full title for use in a filename and limits its length. The 'base64' argument tells Node’s file-writing API to decode the string as image bytes rather than saving the base64 text itself. Selenium’s browser-interaction documentation demonstrates writing a screenshot this way: Working with windows and tabs.
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Avoid collisions with retries and parallel workers
A test title alone may not uniquely identify an artifact. A retried test can fail more than once, and separate workers can run tests with identical full titles. In those setups, add a retry number, worker identifier, timestamp, or other unique value to the filename. For example, the naming policy can include test.currentRetry() where appropriate for your Mocha version, plus a worker-specific value supplied by your test runner. Ensure the added values are safe for filenames. This is practical artifact-management guidance; the cited framework documentation does not prescribe a naming scheme.
If your CI system collects artifacts, configure it to retain the screenshot directory when a job fails. Keep the path stable across local runs and CI so developers know where to find the image. Consider retention and storage limits before keeping every artifact indefinitely.
What this screenshot does—and does not—show
A WebDriver screenshot records the browser’s current page view at the time takeScreenshot() runs. It is useful for checking visible layout, overlays, navigation state, and other visual clues around a failed assertion. It does not automatically explain why the test failed, and it may not include the entire document: full-page behavior depends on the browser and driver implementation. If a failure changes the page immediately, capture in the hook before other teardown code navigates away or alters the browser state.
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For a more complete failure artifact, you can save additional diagnostics in the same hook, such as the current URL or test title, subject to your project’s privacy and logging rules. Screenshots may contain account details, personal information, tokens, or other sensitive content visible in the browser. Restrict access to test artifacts and avoid capturing production user data.
Choose a custom hook or an existing package
| Approach | When it fits | Trade-offs to check |
|---|---|---|
Custom afterEach() hook |
You want a small implementation that fits the driver and artifact conventions already in your test suite. | You control filenames, output paths, and which diagnostics to collect, but you must handle directory creation, unique names, cleanup, and any extra logs yourself. |
| Package already used by the suite | Your project already uses mocha-webdriver and its debug-capture configuration is relevant. |
The npm listing describes automatic logs and screenshots after failed test cases when debug capture and MOCHA_WEBDRIVER_LOGDIR are configured. Check the current package documentation, maintenance, configuration, and compatibility with your installed Mocha and Selenium versions before relying on it: mocha-webdriver on npm. |
Do not add a package solely to avoid a short hook without first checking how it fits your existing driver lifecycle, retry strategy, parallel workers, output directory, and CI artifact collection. The package listing is a lead to investigate, not proof of compatibility with a particular project or current release.
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Troubleshoot missing or unusable screenshots
No file appears
- Confirm the test actually failed and that the hook runs for the suite in question. A hook in one suite does not automatically apply to every unrelated suite.
- Check that
driveris defined in the hook’s scope and still refers to the active session. - Inspect the test output for errors from
mkdir,takeScreenshot(), orwriteFile(). The hook can itself fail if the session has ended or the path is not writable. - Verify the working directory used by the test runner. The relative path
artifacts/screenshotsis relative to the process working directory, which may differ between a local shell and CI.
The hook says the test state is unavailable
Use a regular function () {} for the hook so Mocha can provide its context, and verify that the project uses the BDD interface assumed by the example. Check the installed Mocha API and test organization if hooks are wrapped by a custom runner or shared fixture.
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The PNG is invalid or contains encoded text
Write the returned string with the 'base64' encoding argument, as shown. Omitting it can save the base64 representation as text rather than decoding it into PNG bytes. Do not add a data-URL prefix unless your own code has introduced one; Selenium’s documented return value is the base64 image data.
The screenshot shows the wrong page or is blank
Check whether another teardown step navigates, closes a tab, or quits the driver before capture. Keep screenshot capture early in the failure teardown sequence, then inspect how the test leaves the browser and whether the active window is the expected one. Selenium describes this operation as best effort, so a screenshot is not guaranteed to resolve every browser or driver issue.
Artifacts overwrite one another
Add a unique component to the filename when retries or parallel workers are enabled. A sanitized test title prevents many unsafe filename characters, but it does not prevent duplicate titles from colliding.
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Or skip the browser setup
ScreenshotNeo is a website screenshot API and MCP server, useful when you need a screenshot of a URL without setting up Selenium for that capture. It is not a substitute for the live browser session in a failing end-to-end test: use the hook above when you need the precise state produced by that test. ScreenshotNeo’s URL-based capture can be called like this; see the ScreenshotNeo documentation for API details.
curl -G "https://api.screenshotneo.com/v1/shot" -d access_key=YOUR_API_KEY --data-urlencode url=https://stripe.com -o shot.webpScreenshotNeo removes cookie banners, newsletter popups, and chat widgets before the shot; each cleanup step can be turned off. Bot checks, blank pages, failed loads, timeouts, and cache hits are not billed, and response headers identify the page verdict and billing status. Its MCP server lets AI agents use screenshot tools, and the free plan includes 1,000 screenshots a month with no card; paid plans start at $5 for 3,000 shots.
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Keep the failure artifact useful
Start with the smallest reliable change: add a failure-only afterEach() hook, save the base64 PNG before teardown, and ensure filenames remain unique under the way your suite actually runs. If you are diagnosing a test, preserve that test’s Selenium session state; a separate URL screenshot service is for captures where a fresh page load is what you want.
Frequently Asked Questions
Does Selenium’s JavaScript screenshot API return JPEG or PNG?
The documented takeScreenshot() result is base64-encoded PNG data.
Can I capture screenshots after every test instead of only failures?
Yes. Remove the failed-state guard in the hook, but expect a file for each test and plan artifact volume and retention accordingly.
Can I use the hook if each test creates its own WebDriver?
Yes, but reference the driver associated with the current test rather than a suite-level shared variable, and capture it before that test’s teardown quits the session.
Quick Recap
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