Loki checks React UI changes by capturing the stories in your Storybook and comparing them with saved reference screenshots. Install and initialize Loki, start Storybook yourself, create and commit a baseline, then run visual tests and review each difference before approving intentional changes. Loki tests appearance; keep unit, behavior, and accessibility tests alongside it.
What Loki does in a React project
Loki is a screenshot-comparison tool for Storybook. Each Storybook story represents a UI state; Loki captures those states and compares the resulting images against reference screenshots. A difference is a signal to inspect, not proof by itself that a change is wrong: expected design changes also produce diffs.
Start by making sure the React states that matter are represented as stories. Loki’s documented workflow operates on Storybook stories, rather than discovering and testing every state in an application automatically. The setup documentation describes the supported configuration and workflow at Loki’s getting-started guide and project README.
Prerequisites and compatibility
- A React project with Storybook configured and the stories you want to check.
- Node.js. Loki’s getting-started documentation lists Node 16+, but the cited documentation was last updated in 2024; verify compatibility against the Loki version, Storybook, Node runtime, and CI image you actually use.
- A browser target. The README recommends Chrome in Docker and also lists Chrome in AWS Lambda, local Chrome, iOS simulator, and Android emulator. The getting-started guide lists Docker for
chrome.dockerand Chrome 59+ forchrome.app; treat these as documentation-era requirements, not a current compatibility guarantee. - GraphicsMagick is optional and applies when using the
gmdiff engine. Other documented diff engines arepixelmatchandlooks-same.
Check the installed Loki version and its configuration before choosing a target. The available documentation does not establish a current compatibility matrix for modern React, Storybook, Node, and browsers.
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Install Loki and initialize it
- From the project root, add Loki as a development dependency:
yarn add loki --dev. Use the package manager already used by your project where appropriate; the documented command is for Yarn. - Initialize the default configuration:
yarn loki init. The initializer detects the project and writes configuration. - Open the generated configuration and review browser targets, dimensions, and other settings before relying on defaults. Loki’s CLI reference documents the Storybook URI and ports, reference-image directory, current-output directory, and difference-image directory: Loki CLI reference.
Do not assume initialization also starts Storybook. Loki’s README explicitly says it will not start servers; the Storybook app (and any required simulator or emulator) must already be running.
Create and maintain the reference screenshots
- Start Storybook using the project’s existing script, for example
yarn storybookif that is the script defined in your project. Keep it running while Loki captures stories. - On the first run, generate reference images with
yarn loki update. - Review the resulting reference files and commit them with the code. The Loki guide says references may optionally be stored using Git LFS; choose storage that makes baseline changes reviewable in your repository workflow.
A reference is the visual expectation for later runs. Generate or update it only when the displayed change is deliberate and reviewed; otherwise a baseline can silently normalize a regression.
Run visual regression tests and review diffs
- With Storybook still available, run
yarn loki testafter a UI or story change. - When Loki reports differences, inspect the current screenshots and diff images in the configured output locations. Determine whether each change is intended, caused by inconsistent capture conditions, or a regression.
- For an intentional design change, approve the reviewed result with
yarn loki approve, or use the command Loki suggests to update only failed tests. - Commit approved reference changes alongside the implementation so reviewers can assess what changed and why.
Do not make approval an automatic response to a failing run. The comparison identifies visual differences; a person needs to decide whether those differences match the intended UI.
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Run Loki in CI
The documented CI pattern builds a static Storybook and then points Loki at that build while requiring reference images. Adapt the build command and output path to the scripts in your repository:
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Choose a capture target and tune the workflow
The target affects what Loki captures and how repeatable the result is. The project README documents these target families:
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| Choice | When it fits | Considerations |
|---|---|---|
| Chrome in Docker | A containerized browser target, including CI workflows. | The README calls it the recommended browser target. Docker is listed as a prerequisite for chrome.docker. |
| Local Chrome or Chrome in AWS Lambda | When the project uses one of these documented Chrome targets. | Confirm the target’s compatibility and runtime setup against your installed Loki version. |
| iOS simulator or Android emulator | When visual coverage needs a mobile simulator or emulator. | Ensure the simulator or emulator is running before Loki, as Loki does not start it. |
| Static Storybook or running Storybook | Use a built Storybook in CI or a local server during development. | For static CI, the documented pattern uses --reactUri file:./storybook-static; set the path to your actual build output. |
The CLI reference documents selecting configuration names by regex. For example, a configuration whose name contains laptop can be selected with a regex, which can help limit a run to a particular target. Review the CLI reference for the syntax supported by the version installed in your project. You can also configure the documented reference, current-output, and diff directories, and choose among pixelmatch, looks-same, and gm diff engines.
Troubleshoot common failures
- Storybook cannot be reached: Loki does not start the server. Start Storybook before running tests, and verify the configured URI and port.
- A simulator or emulator is unavailable: Start it separately before invoking Loki; the README says Loki will not start it.
- CI reports a missing reference: With
--requireReference, a missing baseline is meant to fail the run. Generate and review the reference in the baseline workflow, commit it, then rerun CI. - Many diffs appear after an environment change: Verify that the browser target, dimensions, Storybook build, and capture environment match the baseline workflow before approving any changes. The documented workflow requires comparing screenshots; it does not establish that arbitrary environment changes are harmless.
- The configured browser or Node version fails to launch: The setup guide’s version statements date from 2024, so check the requirements for your installed Loki version and CI image rather than assuming those older figures guarantee present compatibility.
- The
gmdiff engine is unavailable: The getting-started guide lists GraphicsMagick as optional for this engine. Install and configure it if you choosegm, or select one of Loki’s other documented engines. - A change is expected but the test still fails: Inspect the diff, then approve the intended change with
yarn loki approveor Loki’s suggested failed-test update command. Avoid updating all references without reviewing them.
What Loki does not replace
Loki compares rendered screenshots of Storybook stories. The cited setup material describes screenshot comparison, not a replacement for unit tests, interaction or behavior tests, or accessibility checks. Keep those tests for the questions a screenshot cannot answer.
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Frequently Asked Questions
Does Loki work without Storybook?
The documented Loki workflow captures Storybook stories, so this guide assumes a React project with Storybook configured.
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Can Loki automatically accept visual changes in CI?
The documented CI setup uses --requireReference so missing baselines fail rather than being created automatically. Review and approve intended changes through your baseline workflow.
Does Loki only test React components?
Loki captures Storybook stories; React is the project context here, while the relevant coverage depends on which UI states your stories represent.
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