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Start your app’s development server, point a BackstopJS scenario at the exact local URL it serves, then capture a baseline with backstop reference. After changing the app, run backstop test to compare screenshots. BackstopJS visits URLs; it does not start your server for you. The steps below cover local setup, page readiness, Docker, and common failures.
Install and initialize BackstopJS
- From your project directory, install BackstopJS locally with
npm install backstopjs. A local install makes it available to project scripts and integrations. If BackstopJS is already installed globally, you can use that installation instead. BackstopJS npm documentation - Check your working tree and any existing BackstopJS configuration before initializing. Then, if the project does not already have a setup, run
npx backstop initfrom the project directory. Initialization can overwrite existing files, so do not run it blindly over a configuration you need to preserve. The documented command isbackstop init; use the invocation appropriate to your local or global installation.
You can expose the workflow through npm scripts. These names are examples; use names that fit your project:
{
"scripts": {
"visual:init": "backstop init",
"visual:reference": "backstop reference",
"visual:test": "backstop test",
"visual:approve": "backstop approve"
}
}Start the server and set the scenario URL
Start the development server with the command your application uses. BackstopJS does not prescribe a universal start command or port: use the URL actually shown by your app, including its scheme, hostname, and port. For example, set http://localhost:3000/ only if the app is listening there.
In backstop.json, give the scenario a descriptive label and set its url to the local page you want to capture. Use referenceUrl if the baseline should come from a different environment or address. A simple scenario has this shape:
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{
"scenarios": [
{
"label": "Local home page",
"url": "http://localhost:3000/"
}
]
}Replace the example address with your app’s real URL. If your reference is also local, point referenceUrl at the appropriate local or other reference page according to your test design. Scenario and configuration options are documented in the BackstopJS package documentation.
Capture, compare, and approve screenshots
- With the app reachable, run
npx backstop referenceto capture baseline screenshots. - Make the code or style change you want to evaluate, then run
npx backstop test. BackstopJS captures the configured scenarios and compares them with the stored references. - Review the report and decide whether each visual difference is expected. Run
npx backstop approveonly when you intend to replace the baseline with the latest test images.
Keep the browser choice, viewport, URL, page data, interactions, selectors, and mismatch settings consistent between runs. Otherwise, a screenshot difference may reflect changed capture conditions rather than a change you meant to test. BackstopJS README
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Make captures wait for the page to be ready
A server responding to a request does not necessarily mean the UI is ready for a useful screenshot. BackstopJS provides readiness controls including readySelector, readyEvent, readyTimeout, and delay. The documented default for readyTimeout is 30,000 ms. BackstopJS README
readySelector: wait for an element that appears only after the content you care about is ready.readyEvent: wait for an application event. The BackstopJS example uses the event namebackstopjs_ready; your app must emit it after its dependencies and relevant content are ready.delay: add a fixed wait after readiness conditions have been met. This can help with late visual updates, but a longer delay is not a substitute for a reliable readiness signal.
Where timestamps, random values, rotating promotions, or other changing data create noise, use known static content or representative stubs when appropriate. For state setup and interactions, onBeforeScript can prepare browser state such as cookies, while onReadyScript can perform UI interactions after readiness conditions are fulfilled. A scenario-level setting can override a global setting, so check scenario values if an inherited selector or other default seems to be ignored. BackstopJS README
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Use the right hostname with Docker
If you run BackstopJS with --docker, the browser runs inside a container. In that setup, localhost refers to the container rather than your host machine, so it will not reach a development server running on the host by using the ordinary host-local address. The BackstopJS documentation gives host.docker.internal as an example for Mac and Windows. Use a hostname supported by your Docker host environment, and change the URL only for the Docker case; ordinary non-Docker local runs should keep using the address your app serves. BackstopJS README
Troubleshoot common local-server failures
| Symptom | Likely cause | What to check or change |
|---|---|---|
| BackstopJS cannot load the scenario URL | The server is not running, or the scenario has the wrong scheme, host, port, or path. | Start the app separately and open the exact configured URL. Verify the actual port and route instead of assuming the example port. |
localhost works in a browser but not under --docker |
The capture browser is inside a container, where localhost points to that container. |
Use a host-access hostname supported by your Docker setup, such as the documented host.docker.internal example for Mac and Windows. |
| The screenshot is blank, incomplete, or inconsistent | The capture may happen before the relevant UI or dependencies are ready. | Set a meaningful readySelector or emit the configured readyEvent only after the page state is ready. Use delay for a known late update when needed, and check the readiness timeout. |
| Every run reports unrelated visual changes | Dynamic content or changed capture conditions may be affecting the comparison. | Stabilize changing data where practical, and keep viewport, browser, selectors, interactions, URL, and state consistent. |
| A readiness setting appears to have no effect | A scenario-level value may override the global configuration. | Inspect the scenario and global settings together; remove or correct a scenario override that clears the intended inherited value. |
| A new baseline was created unexpectedly | backstop approve updates the reference screenshots to the latest test images. |
Review differences before approving. Use the existing reference/test flow for comparison and approve only intentional changes. |
| Initialization replaced a file | backstop init may overwrite existing files. |
Check the working tree and existing configuration before initialization; restore or recover a needed file from version control if it was replaced. |
Or skip the browser setup
If you need a screenshot of a page rather than a repeatable local visual-regression baseline, ScreenshotNeo offers a one-request screenshot API. It is not a replacement for BackstopJS’s reference-and-compare workflow. The request below captures a public page as WebP; see the ScreenshotNeo API documentation for options.
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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 removes cookie banners, newsletter popups, and chat widgets before capture. Bot checks, blank pages, failed loads, timeouts, and cache hits are not billed. Its MCP server lets AI agents take screenshots, and the free plan includes 1,000 screenshots per month without a card; paid plans start at $5 for 3,000. Sign up for 1,000 free screenshots a month with no card.
Frequently Asked Questions
Does BackstopJS start my local development server?
No. Start the app yourself or use your own project orchestration; BackstopJS visits the scenario URL you configure.
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It replaces the reference screenshots with the latest test images, so use it only after reviewing and accepting the visual changes.
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