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What Loki tests—and what it does not
Loki runs visual comparisons against your Storybook stories. Each story should represent a meaningful UI state—such as a default button, an error message, or a populated form—that you want to check for appearance changes. Loki captures the rendered stories and compares the images with reference screenshots you have saved.
This workflow detects visual differences; it does not establish that a component behaves correctly, is accessible, or passes unit tests. Keep those checks in your broader test suite.
Install Loki and initialize the project
The Loki getting-started guide lists Node 16+ and uses Yarn in its examples. Its setup and compatibility notes were last updated in 2024, so treat them as documentation guidance rather than a guarantee for every current Loki, Storybook, Node, or browser combination. Check the requirements for the Loki version and CI image you actually use.
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- From the project root, add Loki as a development dependency:
yarn add loki --dev. - Initialize its configuration:
yarn loki init. The initializer detects the project and writes a default configuration. - Review the generated browser target, dimensions, and other settings before relying on the defaults. Loki documents targets for Chrome in Docker, Chrome in AWS Lambda, local Chrome, and iOS and Android simulators or emulators. The README recommends Chrome in Docker; the available target can depend on your setup.
Documented prerequisites vary by target: the guide lists GraphicsMagick as optional for the gm diff engine, Docker for chrome.docker, and Chrome 59+ for chrome.app. Because these notes are dated, verify them against your installed Loki version and execution environment.
Run the baseline-and-review workflow
Start Storybook yourself
Start the Storybook app using your project’s existing script, for example yarn storybook if that script is configured. Loki does not start Storybook or a simulator for you; its README explicitly says to ensure Storybook and any simulator or emulator are running before tests. Keep this server running while you execute Loki.
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Create the initial reference screenshots
With Storybook available, run yarn loki update. This creates the reference images that future test runs will use. Review the resulting files and commit them to version control so the baseline is intentional and changes can be reviewed alongside the UI changes. The Loki guide says Git LFS can be used optionally for storing references.
Compare after a UI or story change
After changing a component or story, run yarn loki test. Loki captures the stories again, compares the images with the references, and writes current captures and visual differences to its output folders. Use the configured output directories to find the artifacts; the CLI reference documents separate locations for reference images, current output, and difference images.
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Inspect and approve deliberately
Open each diff image and decide whether the change is expected. If it is, approve the new image with yarn loki approve, or use the command Loki suggests to update only failed tests. Commit approved reference changes with the implementation. Do not update baselines automatically just to make a failing run green: that can conceal an unintended change.
Configure targets and comparisons
Loki’s settings let you choose how Storybook is reached, where images are stored, and how image differences are calculated. Check the generated configuration and Loki CLI reference for option names supported by your installed version.
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- Storybook address and ports: use the configured Storybook URI and ports to point Loki at the server or static build you intend to test.
- Browser or device target: choose among the documented local, containerized, Lambda, iOS simulator, and Android emulator options according to the environment you can reliably run.
- Image directories: identify the reference-image, current-capture, and difference-image directories so reviewers can inspect the right artifacts.
- Diff engine: the CLI reference lists
pixelmatch,looks-same, andgm. The getting-started guide identifies GraphicsMagick as optional forgm. - Named configurations: the getting-started guide describes selecting configurations by regex; for example, a name containing
laptopcan be targeted with a matching expression. Confirm exact CLI/config syntax for your Loki version.
When tuning a target, keep the conditions consistent between baseline and test runs: browser, viewport or device, Storybook content, and runtime environment all affect the rendered image. A difference is useful only when it reflects a meaningful change rather than an accidental mismatch in capture setup.
Run Loki in CI with a static Storybook
The official Loki CI guide documents a static-build pattern: build Storybook, then point Loki to the generated directory and require a reference for every tested story. Adapt the build command and output path to your package scripts.
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- Build Storybook using the project’s configured build script.
- Run Loki against the resulting static directory:
yarn loki test --requireReference --reactUri file:./storybook-static. - Keep approved reference images under version control so CI checks changes against an intentional baseline.
--requireReference makes CI fail when a story has no baseline. That is useful because CI should report missing coverage or unexpected changes—not create or approve reference images automatically. Ensure the path after file: matches the actual static output location in your workflow.
Common failures and practical fixes
- Loki cannot connect to Storybook: start the Storybook server before running Loki, or use the static Storybook path in CI. Check that the URI and port in the Loki configuration match the server you started.
- A simulator or emulator target fails: launch the required simulator or emulator separately before the Loki command; Loki does not start it for you.
- CI reports a missing reference: with
--requireReference, this means a tested story has no committed baseline. Run the baseline workflow deliberately, review the image, and commit the approved reference. - A test reports visual differences: inspect the current capture and diff image. If the UI change is intended, approve the new reference; otherwise fix the UI or the inconsistency in the capture environment.
- The
gmdiff engine cannot run: check whether GraphicsMagick is installed and available in the environment, since the guide lists it as an optional prerequisite for that engine. - Documented browser or Node versions do not fit the project: the cited setup guide’s version notes are from 2024, not a current compatibility matrix. Verify requirements against the Loki version, browser target, and CI image actually in use rather than assuming old version statements guarantee compatibility.
Performance, reliability, and baseline maintenance
Loki’s documented workflow depends on a running Storybook or a built static Storybook plus the selected browser or device target. The sources do not establish a universal run time or performance benchmark, so measure it in your own CI environment. For a dependable comparison, use repeatable capture conditions and keep baseline updates reviewable; investigate whether a change is in the UI or environment before approving it.
The project documentation describes repository-based reference images and optional Git LFS storage. That keeps baseline changes tied to code review, but teams should account for the storage and review process their own story set requires. Loki’s documentation does not provide a complete current feature-by-feature comparison with hosted visual-review services.
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