The Tool Desk
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What Loki tests
Loki is an open-source development dependency for visual regression testing of Storybook. Each story is rendered in a selected browser or device target and captured as an image. A later run compares the current capture with the committed reference. A mismatch is evidence for review, not an automatic verdict about correctness.
The Loki project states aims of easy setup, low maintenance, reproducible tests across operating systems, CI execution, and support for Storybook platforms. Those are project aims rather than independently measured guarantees. Compatibility can vary with Loki, Node, Storybook, browser, Docker, and simulator versions.
How the baseline workflow works
1. Install and initialize Loki
Use your JavaScript package manager to add Loki as a development dependency, then initialize its configuration. A typical Yarn-based project follows the setup documented by Loki:
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yarn add --dev loki
yarn loki initNode 16 or newer is listed as a prerequisite in the surfaced project documentation. Target-specific setup can also require Docker, Chrome, or GraphicsMagick. Confirm the exact requirements for the Loki release and target you select; the setup page was last updated on 2024-08-27.
2. Start Storybook
Run Storybook in another terminal and leave it available while Loki captures stories:
yarn storybookUse the command your project defines if the script has a different name. Make sure the server has finished compiling before starting a capture.
3. Create initial references
Generate the first set of screenshots:
yarn loki updateLoki stores reference files in a loki directory by default. Commit these files to Git so every developer and CI run compares against the same review artifact. Git LFS is an optional choice when the image volume is large.
4. Change code, then test
After modifying a component, style, font, or story, run:
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yarn loki testLoki captures the current stories and reports differences from the references. Inspect the current images and the diff images. Look for genuine regressions such as shifted spacing, wrong colors, missing states, clipped content, or an unintended responsive change. Also investigate environmental causes such as a missing font, animation, nondeterministic data, or a different browser version.
5. Approve intentional changes
If the visual change is deliberate and reviewed, update the references:
yarn loki approveCommit the changed files with the component or story change. Do not use approval as a way to make a failing build green without examining the diff. Loki supplies capture, comparison, and approval commands; it does not determine whether a design change is acceptable.
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The project README lists these documented targets:
| Target | What it represents | Operational consideration |
|---|---|---|
| Chrome in Docker | Browser capture in a container | Requires a compatible Docker and Chrome setup. |
| Chrome in AWS Lambda | Browser capture in a Lambda runtime | Validate the Loki release and runtime compatibility before standardizing it. |
| Local Chrome | Capture on a developer or CI host | Browser and operating-system differences can affect rendering. |
| iOS simulator | Storybook capture on a simulated iOS device | Requires the simulator toolchain and a configured simulator. |
| Android emulator | Storybook capture on a simulated Android device | Requires an Android emulator environment. |
These are supported target options documented by the project, not a promise that every current version combination works. Pin and regularly review the versions of Node, Storybook, Loki, browsers, containers, and simulators used by your team.
Making visual tests reviewable
Keep references with the code
Store the default loki directory in the repository. A pull request then contains both the implementation change and any proposed image change. Git LFS can help with repository size, but it does not replace visual review.
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Separate intended and accidental changes
- Read the story name and component change before opening the image diff.
- Check whether all changed pixels correspond to the intended design or content change.
- Investigate broad, unexpected changes first; they often indicate fonts, viewport, browser, or data instability.
- Approve only the references that have been reviewed.
Control nondeterminism
Use stable fixture data, deterministic dates, fixed viewport settings, and disabled or completed animations. Ensure web fonts are available before capture. If a story depends on network content, provide a predictable mock. Otherwise a test can fail even when the component code is unchanged.
Running Loki in CI
CI should compare against committed references rather than silently creating new ones. Loki’s CI guidance uses --requireReference so a missing baseline fails the run. The guide also demonstrates building a static Storybook and passing a file URI to Loki, avoiding the need to run Storybook in server mode in that setup.
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- Build the static Storybook with your project’s build command.
- Run Loki against the generated Storybook file URI and require references.
- Publish the current screenshots and diffs as CI artifacts so a reviewer can inspect failures.
- After approval, commit updated references and rerun the job.
Because the surfaced CI page is dated 2024-08-27, verify the current release’s flag names and file-URI syntax before copying them into a pipeline. A representative command shape is:
yarn loki test --requireReference --storybookFile=file:///absolute/path/to/storybookUse the exact option spelling supported by your installed release. If your CI starts Storybook as a server instead, wait for its health check before invoking Loki and shut it down after capture.
Diagnosing failures
Every story changes
Check browser and operating-system versions, device scale, fonts, timezone, and animation state. A containerized target can improve consistency, but it still needs pinned images and dependencies.
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A reference is missing
Confirm that the loki directory is checked out and that Git LFS objects, if used, were downloaded. In CI, --requireReference should expose this as a failure rather than creating an unreviewed baseline. Generate a reference locally only after deciding that the story is new and the output is correct.
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Storybook cannot be reached
Wait for compilation, verify the host and port, and ensure the CI process remains alive for the entire capture. For static mode, verify the file URI points to the built Storybook directory and uses an absolute path.
Captures are blank or incomplete
Look for JavaScript errors, resources blocked by the test environment, lazy content that has not loaded, and stories that depend on a missing browser API. Add deterministic fixtures and a readiness strategy rather than repeatedly approving blank output.
Only text differs
Compare installed fonts and font-loading timing first. A fallback font changes line breaks and can produce a large diff from a small environment change.
Mobile targets fail to start
Verify that the iOS simulator or Android emulator is installed, booted, and accessible to the process running Loki. Treat simulator images and SDK versions as part of the CI environment, not as an implicit dependency.
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Performance, maintenance, and cost decisions
Loki runs where you configure it: on a developer workstation, in a browser container, in Lambda, or on a mobile simulator. Local execution avoids a hosted visual-testing service but makes environment consistency your responsibility. Hosted or containerized runners can centralize dependencies while adding setup and runtime administration. The project materials do not provide independent speed, accuracy, adoption, or cost benchmarks, so choose based on your own story count and pipeline limits.
Maintenance usually comes from intentional UI evolution, browser updates, font changes, and unstable stories. Keep stories focused, remove redundant states, and review reference updates in the same pull request as the code that caused them. When comparing Loki with another approach, evaluate execution location, browser and mobile coverage, baseline storage, approval workflow, missing-baseline behavior, CI integration, and compatibility burden rather than assuming one tool is universally better.
Or skip the browser setup
For one-off captures, documentation images, or an automation pipeline that does not need Storybook baselines, ScreenshotNeo provides a website screenshot API and MCP server. A single request returns PNG, JPEG, WebP, or PDF. Its cleanup steps accept cookie and consent banners and remove more than 60 known consent platforms, newsletter popups, and chat widgets before capture; each step can be disabled. Bot checks, CAPTCHAs, blank pages, timeouts, failed loads, and cache hits are not billed, and response headers identify the page verdict and billing status.
cURL:
curl -G "https://api.screenshotneo.com/v1/shot" -d access_key=YOUR_API_KEY --data-urlencode url=https://stripe.com -o shot.webpPython:
import requests
r = requests.get("https://api.screenshotneo.com/v1/shot", params={"access_key": "YOUR_API_KEY", "url": "https://stripe.com"}, timeout=90)
open("shot.webp", "wb").write(r.content)Node.js:
const q = new URLSearchParams({ access_key: 'YOUR_API_KEY', url: 'https://stripe.com' });
const res = await fetch(`https://api.screenshotneo.com/v1/shot?${q}`);See the ScreenshotNeo API documentation for options including full-page and element capture, device presets, retina scale, PDF settings, custom CSS and JavaScript, waits, request blocking, cookies and headers, geolocation, caching, signed links, asynchronous jobs, bulk capture, and usage reporting. Its MCP server exposes take_screenshot, get_page_info, and capture_pdf to Claude, Cursor, and other MCP clients.
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Frequently Asked Questions
Does Loki replace a unit or accessibility test?
No. Loki compares rendered images. Keep functional, interaction, accessibility, and performance tests alongside it.
Should baseline images be committed to Git?
Yes. Loki’s default workflow stores references in the loki directory, which the guide says should be checked into Git; Git LFS is optional for large files.
Can Loki decide whether a visual difference is correct?
No. A developer or reviewer must inspect the diff and approve an intentional change.
What should a new story do in CI?
Add and review its baseline deliberately. With --requireReference, CI fails when the reference is absent instead of silently creating one.
Quick Recap
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