If JavaScript appears not to run in a Capybara test using Poltergeist, first verify that Capybara is actually using Poltergeist and that it can find a compatible PhantomJS executable. Then enable JavaScript error reporting and inspect the failing page. If the application uses unsupported ES6 syntax such as let or const, waiting longer will not fix it: transpile the code, use a suitable polyfill for a missing API, or move the test to a modern browser driver. Poltergeist has been archived since November 27, 2020, so recurring engine problems are a reason to plan a migration.
Confirm that Capybara is running Poltergeist
A test can be labeled as a JavaScript test without actually using the intended driver. Check the test setup for all three pieces: the Poltergeist gem, its Capybara integration, and the JavaScript-driver assignment. Also verify that the PhantomJS executable is available to the process running the tests.
Check the gem, require, and driver setting
For an older Ruby application that still uses this stack, the basic setup is:
# Gemfile
gem 'capybara'
gem 'poltergeist'
# spec/support/capybara.rb, or the equivalent test setup file
require 'capybara/poltergeist'
Capybara.javascript_driver = :poltergeist
Keep the Poltergeist require in the test process’s setup path, not only in application code that may not be loaded by the test runner. If your project already has a driver registration or custom configuration, inspect that registration too; a later assignment or per-test driver choice may override the global setting.
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Check the PhantomJS executable
Poltergeist controls PhantomJS as a separate executable. Run which phantomjs in the same environment where the tests run, then run phantomjs --version. If the command is absent, unavailable to the test process, or points to a different binary than expected, fix the executable path or installation before debugging page JavaScript.
On Linux, do not assume the distribution package is suitable: Poltergeist’s README specifically warns against the PhantomJS package from the official Ubuntu repositories because it does not work well with Poltergeist. Use a binary compatible with the Poltergeist version and environment instead. No single current PhantomJS version is established as compatible with every legacy project, so record the actual versions rather than copying an unverified version pin.
Make JavaScript errors visible
A page exception can otherwise look like an element that never appeared, an assertion that timed out, or a click that did nothing. Enable Poltergeist’s JavaScript error reporting so errors in the page are raised in the test output. For example, register a debugging driver explicitly:
Capybara.register_driver :poltergeist_debug do |app|
Capybara::Poltergeist::Driver.new(app, js_errors: true, debug: true)
end
Capybara.javascript_driver = :poltergeist_debug
If the project already registers Poltergeist, add js_errors: true and debug: true to that registration rather than creating a second driver unnecessarily. Poltergeist options may be written with symbol keys or older hash-rocket syntax depending on the Ruby version; use the syntax accepted by the project’s Ruby. If turning on both settings changes the reported failure, separate them and retry with one setting at a time.
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save_screenshot('tmp/capybara-failure.png')Keep the screenshot with the test log. It can reveal a consent overlay, a page that never loaded, a wrong viewport, a font or layout issue, or a control positioned differently than the test assumes. Poltergeist debug output can also report click coordinates, which helps distinguish a JavaScript failure from a target being covered or out of place.
For a reproducible failure, collect the Poltergeist and PhantomJS versions, operating system, complete stack trace, relevant browser-console output, the screenshot, and the smallest set of steps or test code that triggers the issue. This is especially useful for failures that occur only in CI or only intermittently.
Check whether PhantomJS can parse the application’s JavaScript
PhantomJS uses an old WebKit-based browser engine, not a current Chrome or Firefox engine. Poltergeist’s README says PhantomJS does not support ES6 features and specifically identifies let and const as possible silent failure points. A bundle that works in a current browser can therefore fail before Capybara has a chance to find the elements it expects.
Identify syntax and API compatibility separately
- Syntax failure: If PhantomJS cannot parse the script, the application may stop initializing. Transpile the test bundle to syntax the engine understands, then rerun the failing case with JavaScript errors enabled.
- Missing browser API: If the syntax parses but the code calls an API PhantomJS lacks, add a compatible polyfill through Poltergeist’s
extensionsoption where that is practical. - Engine-dependent behavior: If the application depends on browser behavior that cannot be reproduced reliably in this engine, use a modern browser driver instead of accumulating patches.
Do not treat a longer Capybara wait as a remedy for a parse error or absent browser feature. A wait can help only when the application is still functioning and the expected state arrives later.
Separate asynchronous timing from failed JavaScript
Capybara retries many failed asynchronous lookups while waiting for the page to reach the requested state. Its documented default_max_wait_time is two seconds. When a legitimate client-side operation takes longer, raise the setting deliberately, preferably for the relevant test or suite, and assert the state the user would observe.
# In the test configuration, if this application legitimately needs more time:
Capybara.default_max_wait_time = 5
# Prefer an eventual-state assertion over a fixed sleep:
expect(page).to have_css('.results-loaded')
The five-second value above is an example project choice, not a universal requirement. Start with the observed delay and keep the timeout bounded; an inflated timeout can make real failures slow and obscure whether the page is progressing.
Avoid inserting arbitrary sleep calls as the default fix. Fixed sleeps waste time when the page is fast and still fail when it is slower than the chosen delay. First confirm that the script ran without errors; then wait for an observable state, such as a result element or updated text.
Choose the right script-evaluation method
Use execute_script for JavaScript whose purpose is a side effect and whose return value is not needed. Use evaluate_script when the test needs a returned value, but keep the returned data simple: the representation of complex objects can vary by driver.
# Side effect only
page.execute_script("document.body.setAttribute('data-test-ready', 'true')")
# Simple returned value
ready = page.evaluate_script("document.body.getAttribute('data-test-ready')")
If a script call appears to fail, reduce it to a small expression and inspect the browser-side error output. Do not infer that the application bundle executed successfully merely because Capybara can evaluate a short, independent expression.
Diagnose clicks that appear to do nothing
Poltergeist performs coordinate-based, user-like clicks. A click can miss when an overlay covers the target, the page has not finished laying out, or the element is outside the expected position. Inspect a screenshot and the debug output before changing the test to force a click.
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- If a consent banner or modal covers the control, dismiss it or set up the page state the test actually intends to exercise.
- If the element is not visible or is displaced, fix the page timing, viewport, or layout condition before clicking.
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find_button('Save').trigger('click')only when dispatching a DOM event is the deliberate test intent. It does not verify that a user could physically reach and click the control.
Handle timeouts, DeadClient errors, and crashes
A timeout may mean the page is genuinely slow, the application script stopped on unsupported syntax, the selected driver is wrong, or PhantomJS itself has failed. Use the preceding checks to narrow the cause before changing timeouts. For a DeadClient error or other process crash, determine whether the failure reproduces consistently, preserve the stack trace, and record the OS and both driver and browser versions. A sporadic crash may be related to the old WebKit embedded in PhantomJS; repeated blind retries do not address that underlying risk.
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session.driver.quitWhen the problem is isolated and reproducible, a focused issue report with a minimal example and the captured environment details is more useful than a report that only says JavaScript failed.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Choose between a patch and migration
Poltergeist’s repository has been archived and made read-only since November 27, 2020. Current Capybara documentation says JavaScript tests need a different driver and documents Selenium-based options. The practical choice depends on whether this is a short-lived legacy suite or a test suite that must keep tracking a modern application.
| Option | Addresses | Trade-off |
|---|---|---|
| Enable errors, debug logs, and screenshots | Hidden page exceptions, uncertain page state, and hard-to-reproduce failures | Improves diagnosis but does not update PhantomJS’s engine. |
| Transpile syntax or add a polyfill | A known syntax or missing-API incompatibility | Can preserve a legacy stack, but retains old engine limits and adds compatibility maintenance. |
| Increase Capybara’s wait time | A legitimately slow asynchronous update | Helps synchronization only; it cannot fix an exception, parse failure, or crashed browser. |
| Move to a maintained Selenium-compatible driver | Recurring incompatibility with the legacy browser engine | Requires browser and driver setup, but provides a durable path for modern browser behavior. |
For a short-term stabilization, first expose errors and establish whether the problem is syntax, an API, timing, or a click target. For an actively maintained application, recurring PhantomJS-only failures are a strong migration signal. Plan the new driver and browser in local development and CI, then move tests in a controlled sequence rather than assuming that a larger timeout will make the old engine current.
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Or skip the browser setup
If the task is to capture a page image or PDF rather than exercise Capybara interactions, ScreenshotNeo is an alternative to try first: it returns a screenshot or PDF from a single API request. It is not a substitute for an end-to-end test that clicks controls and asserts application behavior.
For example, capture a page as WebP with cURL (see the ScreenshotNeo API documentation):
curl -G "https://api.screenshotneo.com/v1/shot" -d access_key=YOUR_API_KEY --data-urlencode url=https://stripe.com -o shot.webpThe same one-request pattern in Python is:
import requests
r = requests.get("https://api.screenshotneo.com/v1/shot", params={"access_key": "YOUR_API_KEY", "url": "https://stripe.com"}, timeout=90)
open("shot.webp", "wb").write(r.content)In Node.js:
const q = new URLSearchParams({ access_key: 'YOUR_API_KEY', url: 'https://stripe.com' });
const res = await fetch(`https://api.screenshotneo.com/v1/shot?${q}`);ScreenshotNeo accepts consent banners before capture and removes more than 60 known consent platforms, newsletter popups, and chat widgets; each cleanup step can be turned off. Bot checks, blank pages, failed loads, timeouts, and cache hits are not billed, and response headers report the page verdict and billing status. It also offers an MCP server with take_screenshot, get_page_info, and capture_pdf tools for AI agents. The Free plan includes 1,000 shots per month without a card; paid plans start at $5 for 3,000 shots. Every feature is on every plan. See ScreenshotNeo, or sign up free for 1,000 screenshots a month with no card.
Frequently Asked Questions
Can I use Poltergeist to test JavaScript in Capybara today?
You can retain it in a legacy project if its dependencies and PhantomJS executable still work in your environment, but the archived project and old browser engine make it a poor default for new or modern-browser coverage.
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Does ScreenshotNeo run my Capybara test suite?
No. It captures a webpage through an API or MCP tool; it does not replace a Capybara driver for clicking through an application and asserting interactive behavior.
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