Use BackstopJS’s Puppeteer engine and its onBeforeScript hook to authenticate before a scenario opens a page protected by HTTP Basic authentication. In the hook, call Puppeteer’s page.authenticate({ username, password }) with credentials supplied through environment variables. The example below combines the documented APIs; it is a setup example, not tested code.
Configure BackstopJS for HTTP Basic authentication
BackstopJS makes the browser page available to custom scripts through its onBefore hook, which runs before each scenario. Puppeteer’s Page.authenticate() supplies credentials for HTTP authentication. Together, these let the browser authenticate as it requests the protected page. BackstopJS identifies Puppeteer as its default engine in its current README; check your installed version and configuration if your project differs.
1. Add the hook and protected scenario
In your backstop.json, set the engine, point onBeforeScript to the script filename, and use the protected URL in a scenario. This fragment assumes the default location for engine scripts; set paths.engine_scripts if your scripts are stored elsewhere.
{
"engine": "puppeteer",
"onBeforeScript": "auth.js",
"scenarios": [
{
"label": "Protected page",
"url": "https://staging.example.test/protected",
"readySelector": "main"
}
]
}2. Authenticate in the hook
Create backstop_data/engine_scripts/auth.js (or put the script in your configured engine-scripts directory):
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module.exports = async (page) => {
const username = process.env.BASIC_AUTH_USER;
const password = process.env.BASIC_AUTH_PASSWORD;
if (!username || !password) {
throw new Error('Set BASIC_AUTH_USER and BASIC_AUTH_PASSWORD');
}
await page.authenticate({ username, password });
};BackstopJS documents the hook signature as onBefore(page, scenario, viewport, isReference, Engine, config); this example uses only the supplied page. Set BASIC_AUTH_USER and BASIC_AUTH_PASSWORD in your local shell or CI secret store. Do not put real credentials in backstop.json, the script, or source control. If a scenario defines its own hook, BackstopJS allows a scenario-level override, so check the effective configuration for the scenario you are running.
Run and verify the visual test
Run the BackstopJS command your project uses for reference capture or test execution. The exact command depends on how BackstopJS is installed and invoked in your project; the authentication configuration itself does not require credentials in the scenario URL.
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- Confirm the browser reaches the intended protected URL and displays authenticated content—not an authentication prompt, a 401 response, or a redirect to a separate sign-in form.
- Use a
readySelectorsuch asmainwhen the authenticated page renders asynchronously. BackstopJS also supports readiness events and delays; prefer an observable condition when one is available. - Choose the capture region to match what the visual test protects: the full document, viewport, or explicit CSS selectors.
- Review the visual report before approving changed references. Approving a new reference changes the baseline used in later comparisons.
Choose the right authentication mechanism
HTTP Basic authentication
Use page.authenticate() when the browser receives an HTTP Basic-auth challenge. It supplies credentials at the HTTP-auth layer; it does not fill out a website’s login form.
Form-based login or an existing browser session
If the site has a form-based login, use a deliberate login interaction or restore appropriate session state instead of calling page.authenticate(). BackstopJS documents cookies and custom scripts for browser setup. Its Playwright integration documents storageState for loading cookies and localStorage before tests; that is session-state setup, not a documented way to supply HTTP Basic credentials.
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Puppeteer or Playwright engine
Puppeteer is the default engine in BackstopJS’s current README. For a Playwright-based project, switch to the documented Playwright engine and its scripts rather than assuming a Puppeteer hook works unchanged. The BackstopJS material cited here documents Playwright’s storageState for cookies and localStorage, but does not establish equivalent HTTP Basic-auth behavior; verify the current Playwright API and your installed integration before choosing that route.
Troubleshoot authentication and capture issues
- The page remains unauthorized: Check that both environment variables are present in the process running BackstopJS and that the credentials belong to the protected host. Confirm the URL is actually protected by HTTP Basic authentication.
- The browser shows a login form: This is likely form-based authentication, not an HTTP Basic challenge. Use a browser login flow or session state instead.
- The capture is blank or incomplete: Check that the scenario reaches the authenticated content and revise its
readySelector, readiness event, or delay to match the page’s rendering behavior. - The wrong part of the page is compared: Adjust the scenario’s capture selector or choose the document or viewport region appropriate to the visual change you need to detect.
- Authentication slows the run: Puppeteer documents that authentication enables request interception behind the scenes, which might affect performance. Account for that when interpreting run time.
- The hook is not found or does not run: Check
onBeforeScript, the configuredpaths.engine_scriptsdirectory, and whether a scenario-level hook overrides the root setting. Older BackstopJS releases or custom engines may differ, so check the configuration against your installed version.
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Frequently Asked Questions
Does the BackstopJS authentication example work with a form login?
No. The example handles HTTP Basic authentication. A form login needs a browser interaction or restored session state.
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