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BackstopJS uses Puppeteer for headless Chrome by default. The simplest containerized workflow is to configure "engine": "puppeteer" in backstop.json, then run backstop test --docker. BackstopJS selects an image matching the version in your project’s package metadata and mounts the working directory at /src. For CI, pay particular attention to terminal allocation and write permissions on generated files.
Configure BackstopJS to use headless Chrome
BackstopJS documents Puppeteer as its default engine and uses Chrome in headless mode. In backstop.json, make the engine explicit so the configuration is clear:
{
"engine": "puppeteer"
}BackstopJS merges engineOptions with its defaults. The documented defaults include ignoreHTTPSErrors: true and a headless setting based on debugWindow. You can pass browser flags or navigation settings there, for example:
{
"engine": "puppeteer",
"engineOptions": {
"args": ["--some-browser-flag"],
"headless": true,
"gotoParameters": {
"waitUntil": "networkidle0"
}
}
}Treat these as configuration examples, not universal recommendations. In particular, review certificate handling, browser flags, and navigation behavior against your site and security requirements rather than copying options without understanding their effect.
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BackstopJS notes that different environments can render the same webpage differently, especially text. Docker can help control environment differences, but it does not guarantee pixel-identical output across every machine or run.
Run BackstopJS in Docker
Use BackstopJS-managed Docker
- Install Docker and make sure its daemon is running.
- From the project directory, run
backstop test --docker. - Review the generated reference and test output in the project’s configured BackstopJS paths.
BackstopJS documents this command as using an image version that matches the BackstopJS version in the project’s package metadata. Its template mounts the current working directory at /src. On Linux, the README gives this template for avoiding root-owned generated files:
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docker run --rm -it --user $(id -u):$(id -g) --mount type=bind,source="{cwd}",target=/src backstopjs/backstopjs:{version} {backstopCommand} {args}The placeholders represent the current working directory, image version, BackstopJS command, and arguments. The BackstopJS command can generate Docker invocations itself; use the documented backstop test --docker path unless you specifically need to manage the container invocation.
Invoke an image directly
If you need to run the image manually, mount the project directory at /src. Docker Hub’s overview demonstrates this basic sequence:
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docker run --rm -it --mount type=bind,source="$PWD",target=/src backstopjs/backstopjs init
docker run --rm -it --mount type=bind,source="$PWD",target=/src backstopjs/backstopjs reference
docker run --rm -it --mount type=bind,source="$PWD",target=/src backstopjs/backstopjs testThe overview describes the image as BackstopJS 3.x and says openReport is not supported in that documented image. It also said it had been updated almost two years earlier when accessed. Do not assume its command support or version description applies to every newer image tag: inspect the exact tag and workflow you plan to use.
Choose the image and browser version deliberately
For the managed --docker workflow, the project’s BackstopJS package version determines the matching container version by default. If you invoke an image manually or build your own, check the actual tag rather than assuming the general Docker Hub overview describes it.
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- Please Note: To reach 10Gbps speed, keep the cable under 3.3 ft. For USB A Male to USB C adapters, try flipping the USB C connector. USB C Male to USB A adapters support bidirectional 10Gbps transfer within 3.3 ft
For example, Docker Hub’s layer metadata for backstopjs/backstopjs:6.3.25 identifies /usr/bin/chromium as the Puppeteer executable path, Node 20.17.0, and linux/amd64. That page also reports a compressed image size of 1.57 GB. These are facts about that specific tag and its listed layer, not universal defaults for BackstopJS containers; confirm the metadata for the tag and architecture you use before hard-coding a browser path.
If you build a custom image, Puppeteer’s Docker guidance emphasizes installing the browser’s required system libraries, using compatible browser and Puppeteer versions, and making profile, configuration, and cache locations writable. A browser binary alone is not enough to ensure Chrome can start.
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Keep output writable and CI output usable
BackstopJS writes screenshots and report resources beneath configurable paths that default to backstop_data relative to the current working directory. Because the project directory is mounted at /src, the container user must be able to write to the relevant directories.
- Linux file ownership: Match the container process to the host UID and GID with the README’s
--user $(id -u):$(id -g)template. This helps avoid files that your normal host user cannot edit or remove. - CI and piped logs: Remove Docker’s
-toption when output is piped in CI or an IDE. A pseudo-terminal is not appropriate for every non-interactive output stream. - Custom container paths: Ensure Chrome can write its profile and cache in the container’s actual runtime environment, especially when the filesystem is read-only or only selected directories are mounted.
Troubleshoot Chrome launch and Docker errors
| Symptom | Likely cause | What to check or change |
|---|---|---|
Chrome says it cannot run as root without --no-sandbox. |
An older generated BackstopJS configuration may be involved; the README calls out configurations created before BackstopJS 3.5. | Prefer running the container as the host user using the UID/GID option. Do not disable Chrome’s sandbox indiscriminately: doing so reduces an important browser security boundary. If a sandbox change is unavoidable in your environment, assess and document that risk. |
| Chrome executable cannot be found. | The configured browser path does not match the image tag, or the browser is absent. | Inspect the exact image’s metadata and installed browser. The /usr/bin/chromium path is documented for the 6.3.25 layer, not all tags. Update the path or use the matching image. |
| Chrome fails during startup in a custom image. | Missing shared libraries, incompatible browser and Puppeteer versions, or inaccessible profile/cache directories. | Follow Puppeteer’s Docker troubleshooting guidance; install required browser dependencies, align versions, and make runtime profile and cache paths writable. |
| Generated screenshots or reports are owned by root or cannot be written. | The container user differs from the host user, or the mounted output directory is not writable. | Use the matching host UID/GID on Linux and check permissions on the configured BackstopJS output paths under the mounted project directory. |
| CI logs fail or behave oddly with piped output. | The Docker command allocates a pseudo-terminal using -t. |
Remove -t for piped output or non-interactive CI execution. |
openReport is unavailable. |
The Docker Hub overview that documents the v3.x image says that image does not support it. | Verify the exact tag and supported commands for your workflow; do not generalize the overview’s limitation to every version. |
Or skip the browser setup
If you need screenshots through an API instead of maintaining a BackstopJS container, ScreenshotNeo offers a single-request screenshot API and an MCP server. Its clean-shot workflow accepts cookie or consent banners like a visitor and removes more than 60 known consent platforms, newsletter popups, and chat widgets before capture; those steps can be turned off. Bot checks or CAPTCHAs, blank pages, timeouts, failed loads, and cache hits are not billed, and responses identify the page verdict and billing status in headers. AI agents can use its MCP tools for screenshots, page information, and PDF capture.
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curl -G "https://api.screenshotneo.com/v1/shot" -d access_key=YOUR_API_KEY --data-urlencode url=https://stripe.com -o shot.webpScreenshotNeo’s free plan includes 1,000 screenshots a month with no card, and paid plans start at $5 for 3,000. Sign up for ScreenshotNeo’s free plan.
Frequently Asked Questions
Can I use a custom Chrome executable with BackstopJS?
Yes, when your selected engine and container configuration support the executable path you specify. Confirm that path against the exact image tag; the documented /usr/bin/chromium path applies to the 6.3.25 layer only.
Does Docker make BackstopJS screenshots identical across environments?
No. BackstopJS says environments can render pages differently, including text. A container can help control environment differences, but it is not a guarantee of identical rendering.
Quick Recap
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