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How BackstopJS tests an authenticated page
BackstopJS captures screenshots for scenarios and compares a new test capture with a reference image. The reference is the visual state your team has accepted; it is not simply whatever the page looked like on the last run.
- Configure a scenario for the URL and viewport you want to test, along with a way to establish authentication.
- Run
backstop test. BackstopJS captures the page and reports visual differences from the reference. - Inspect the report. If the change is intentional, run
backstop approveto update the reference; if it is not, fix the application or test setup.
The project documentation recommends integrating the CLI into a build process or running it before deployment. A failed layout test returns a nonzero status, so a CI job can treat visual regressions as a failing check.
Choose how to provide the login state
These are alternative setup patterns, not interchangeable guarantees. A cookie file is suitable when cookies alone represent the session. If authentication also depends on local storage, Playwright storage state may fit better. If state must be established dynamically, use a custom setup or an application-specific login flow.
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| Method | Use it when | Important detail |
|---|---|---|
cookiePath |
A valid session can be represented by a JSON cookie file. | BackstopJS’s default onBefore script imports the file. Its path is relative to the current working directory. |
onBeforeScript |
You need scenario-specific setup or another app-specific preparation step. | The script runs before each scenario and receives the page and scenario. Use APIs appropriate to the configured browser engine. |
Playwright storageState |
The session state includes cookies and local storage and you are using the Playwright engine. | Set engine to playwright and configure engineOptions.storageState with the state JSON file. |
BackstopJS documents Puppeteer as its default engine and Playwright as an engine option. Its documentation describes Playwright browser choices including Chromium, Firefox, and WebKit. Do not pass Playwright-specific storageState settings to the Puppeteer engine; the engines have distinct configuration options.
Import cookies with cookiePath
If you already have a suitable JSON cookie file, add cookiePath to the scenario. The configured path is interpreted relative to the directory where you run BackstopJS. A minimal scenario shape is:
{
"scenarios": [
{
"label": "Account overview",
"url": "https://example.com/account",
"cookiePath": "auth/account-cookies.json",
"readySelector": "[data-testid='account-overview']"
}
]
}This shows the relevant scenario fields, not a complete BackstopJS configuration file. The cookie file must contain a format the configured engine can import, and its session must still be valid when the test runs. A cookie file does not guarantee that every login flow can be reproduced: applications may require additional browser state, refreshed tokens, or an identity-provider step.
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Prepare state with an on-before script
Use a custom onBeforeScript when cookie import is insufficient or setup needs to vary by scenario. BackstopJS runs this hook before each scenario and makes the browser page and scenario available. Script paths can be placed under the configured paths.engine_scripts; the project recommends pointing that setting to a project directory.
The repository also documents a custom onBefore handler that receives page, scenario, viewport, isReference, Engine, and config. Select the hook style and browser APIs supported by your installed BackstopJS version and engine. For example, a Puppeteer-based setup can load cookies through Puppeteer’s page APIs before capture; that does not make Playwright configuration valid for Puppeteer. Puppeteer is a browser automation library with page interaction and screenshot capabilities, as described in Google Chrome for Developers’ overview.
Keep credentials and active session files out of public examples and source control. Load sensitive state from an appropriate private location in your local or CI environment rather than committing real tokens.
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Load Playwright storage state
For an application whose browser state includes local storage as well as cookies, configure the Playwright engine and point engineOptions.storageState at a Playwright state JSON file. BackstopJS documents this as a way to set cookies and local storage before an authenticated capture. The state file must be generated and maintained in a secure environment, and it can expire just like a session.
{
"engine": "playwright",
"engineOptions": {
"storageState": "auth/playwright-state.json"
},
"scenarios": [
{
"label": "Account overview",
"url": "https://example.com/account",
"readySelector": "[data-testid='account-overview']"
}
]
}Use this pattern only with the Playwright engine. Check the README for the BackstopJS version installed in your project if your configuration behaves differently from the current repository documentation.
Wait for the authenticated view, not just the login request
A valid session does not prove that the page is ready to capture. The application may still be redirecting, rendering client-side content, or loading data after the browser has accepted the session. Configure a readiness condition that corresponds to the actual view under test.
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readySelectorwaits for a chosen selector to exist, such as a stable account-page marker.readyEventwaits for the application to log a chosen string, when the app exposes a useful readiness message.delayadds a fixed pause; use it when needed, but a selector or explicit app event is generally more closely tied to the intended state.readyTimeoutsets the wait limit for readiness. If it expires, check whether the session redirected or the selector/event is wrong before simply extending the timeout.
BackstopJS also supports onReadyScript for actions after readiness, plus scenario interactions such as clicks, hovers, and key presses. Use them when the interaction is part of the view being tested—for example, opening a menu that should appear in the reference—not as a substitute for establishing authentication.
Choose capture scope intentionally. A scenario can capture the full page or selected CSS selectors. By default, BackstopJS captures the first match for a selector. Use selectorExpansion to capture all matches, and expect when you need to assert how many selected items should be present.
Make the result repeatable in CI
Visual comparisons are sensitive to differences in rendering environments and to changing page content. Run reference and test captures in a consistent environment, and inspect the report before approving a new baseline. BackstopJS documents Docker as one option for reducing environment variation; it is a reproducibility aid, not a guarantee that all differences disappear.
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- Use the same browser engine and comparable runtime environment for reference and test runs.
- Choose readiness conditions based on a real application marker rather than relying on a long arbitrary delay.
- Control application data and other dynamic content where your test environment permits it.
- Use CI reporting, including the documented JUnit output option, when your pipeline consumes test reports.
- Keep the authentication state refreshable and private. How it is renewed depends on your application and identity provider; BackstopJS does not define those policies.
Run the same command locally and in CI when diagnosing a mismatch. A nonzero backstop test result indicates the visual test failed; use the generated report to determine whether the page is genuinely different or the capture setup is unstable.
Troubleshoot common authentication and capture failures
| Symptom | Likely cause | What to check |
|---|---|---|
| The screenshot shows a login page or redirect. | The session is missing, expired, or not accepted for that URL. | Confirm the cookie file path is relative to the working directory, validate the session in the same environment, and check whether the app requires local storage or a scripted flow. |
| The scenario fails waiting for readiness. | The selector or event does not occur, or the page never reached the intended authenticated view. | Inspect the captured page and redirects; verify the selector/event spelling and that it is present after login. Adjust readyTimeout only if the page is legitimately slower. |
| Playwright storage settings have no effect. | The project is running a different engine or the option path does not match the installed version. | Confirm "engine": "playwright", the engineOptions.storageState path, and the README for your installed BackstopJS version. |
| Local and CI screenshots differ unexpectedly. | Browser/runtime rendering or page content differs between environments. | Use a consistent environment, consider Docker, and control changing content where practical. Compare the report before approving a new baseline. |
| A selector captures only one of several repeated elements. | BackstopJS selects the first match by default. | Use selectorExpansion to capture all matches and expect to check the expected count. |
Or skip the browser setup
If you need a screenshot of a page but do not need BackstopJS’s reference-image regression workflow, ScreenshotNeo can return a screenshot or PDF with one GET request. It accepts cookie and authorization options, but this is not a replacement for BackstopJS’s baseline comparison and test-report workflow.
See the ScreenshotNeo API documentation for request options. Example cURL request:
curl -G "https://api.screenshotneo.com/v1/shot" -d access_key=YOUR_API_KEY --data-urlencode url=https://example.com/account -o shot.webpPython:
import requests
r = requests.get(
"https://api.screenshotneo.com/v1/shot",
params={"access_key": "YOUR_API_KEY", "url": "https://example.com/account"},
timeout=90,
)
open("shot.webp", "wb").write(r.content)Node.js:
const q = new URLSearchParams({ access_key: 'YOUR_API_KEY', url: 'https://example.com/account' });
const res = await fetch(`https://api.screenshotneo.com/v1/shot?${q}`);Cookie and consent banners, newsletter popups, and chat widgets are removed before capture; bot checks, blank pages, failed loads, timeouts, and cache hits are not billed. An MCP server lets AI agents use screenshot tools, and the free plan includes 1,000 screenshots a month with no card; paid plans start at $5 for 3,000 screenshots. Sign up for free.
Frequently Asked Questions
Can BackstopJS use a saved login session?
Yes. A cookie file or Playwright storage-state file can carry browser session state, provided it is valid and includes what the application needs.
Does a successful login guarantee a stable visual test?
No. The scenario still needs a readiness condition tied to the authenticated view, and dynamic content can affect the image.
Which browsers can BackstopJS use for authenticated captures?
The repository documents Puppeteer as the default engine and Playwright options including Chromium, Firefox, and WebKit; confirm details against your installed version.
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