The Tool Desk
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Confirm Capybara is using Poltergeist
Poltergeist is a Capybara driver that controls PhantomJS. A test can appear to use it while the JavaScript driver is unset, misspelled, or not loaded. Check the gem, require, and driver assignment together.
# Gemfile
gem 'poltergeist'
# In your test setup, after the gems are available
require 'capybara/poltergeist'
Capybara.javascript_driver = :poltergeistIf you use RSpec, keep this configuration in the test setup file that your suite actually loads. Merely adding the gem does not select Poltergeist for JavaScript-marked tests. If your project registers drivers explicitly, make sure the registration name and Capybara.javascript_driver value match.
Check the PhantomJS executable
Poltergeist also needs a compatible PhantomJS binary available on the test process’s PATH. Check from the same shell or CI job that runs the tests, rather than from a different interactive session. If the executable is missing or cannot be run, fix that environment before investigating page JavaScript.
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On Linux, the Poltergeist README specifically warns against using the phantomjs package from the official Ubuntu repositories because it does not work well with Poltergeist. Use a compatible binary instead. Record the PhantomJS and Poltergeist versions when reporting or comparing failures; the available project guidance does not establish one current, universally compatible version pair.
Make page JavaScript errors visible
A silent JavaScript exception can make a page look as if its scripts never ran. Configure Poltergeist to raise page JavaScript errors and enable debug output while reproducing the problem.
require 'capybara/poltergeist'
Capybara.register_driver :poltergeist do |app|
Capybara::Poltergeist::Driver.new(app, :js_errors => true, :debug => true)
end
Capybara.javascript_driver = :poltergeistUse this in place of a conflicting driver registration, not as a second competing registration under the same name. The :js_errors option surfaces exceptions from page JavaScript as test failures; :debug provides additional driver output. Once the issue is understood, decide whether verbose debug logging should remain enabled in routine test runs.
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At the failing step, save a screenshot and preserve the test output. A screenshot can show whether the page loaded, whether a modal or overlay is present, and whether the expected control is actually visible. In the failure report, include the operating system, Poltergeist version, PhantomJS version, stack trace, and the shortest reproducible test. These details help distinguish an application error from a driver, binary, or environment problem.
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Check for PhantomJS syntax and engine limitations
PhantomJS uses an old JavaScript engine. The Poltergeist README warns that PhantomJS does not support ES6 features and calls out let and const as syntax that can fail silently. If the application bundle or code executed by the page uses syntax the engine cannot parse, the browser may stop executing that script before Capybara can find the resulting UI.
Choose a fix that matches the failure
- Unsupported syntax in application code: transpile the test build to syntax PhantomJS understands. Verify the actual bundle served to the test browser, not just the source files.
- A missing browser API: provide an appropriate polyfill through Poltergeist’s
:extensionsoption if the API can safely be polyfilled and the test still represents the behavior you intend to verify. - Dependence on modern engine behavior: move the test to a modern browser driver. A polyfill cannot make an obsolete engine behave like a current browser in every respect.
Do not treat “the test passes after transpiling” as proof that the application works identically in a current browser. Transpilation can help keep a legacy test suite running, but it preserves the old engine’s limits and can conceal differences between the test browser and production browsers.
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Separate script evaluation from asynchronous waiting
Not every apparent JavaScript failure is a parser or engine failure. Sometimes the test asks for a result in a way the driver handles differently, or checks for UI state before asynchronous work has finished. Diagnose those as separate cases.
Use the right script helper
Capybara’s evaluate_script returns a value, but returning complex JavaScript objects can be driver-specific. Use it when the test needs a simple value. If the script is intended only to cause a side effect and no value is needed, use execute_script.
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page.evaluate_script('document.title')
# Cause a side effect; no return value is needed
page.execute_script("document.body.setAttribute('data-test-state', 'ready')")Keep assertions focused on observable behavior when possible—for example, waiting for the element or text that should appear—rather than relying on a complex object returned across the browser-driver boundary.
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Let Capybara retry asynchronous lookups
Capybara automatically retries failed asynchronous lookups. Its documented default_max_wait_time is two seconds. For work that legitimately takes longer, adjust the wait setting deliberately and assert the eventual state instead of adding arbitrary sleeps.
# Set in test configuration only when the application needs a longer wait
Capybara.default_max_wait_time = 5
# Prefer an assertion that waits for the expected state
expect(page).to have_text('Results loaded')The five-second value above is an example configuration, not a universal recommendation. Use a timeout that reflects the expected behavior in your environment. A longer wait can help with slow AJAX or client rendering; it cannot make PhantomJS parse unsupported syntax or repair a JavaScript exception.
Diagnose clicks that do not trigger the expected behavior
Poltergeist performs coordinate-based, user-like clicks. A click can miss when the target is covered by an overlay, sits in an unexpected position, or is affected by layout, font, or viewport differences. Inspect the debug output and screenshot at the moment of failure, and fix the page state or test setup if the target is obstructed.
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Use a DOM event trigger only when that is deliberately what the test means to exercise. For example, find_button('Save').trigger('click') dispatches a click event directly; it does not verify that a user could physically reach and click the button. Prefer a normal user-like click when testing the interaction a visitor would perform.
Troubleshoot by symptom
| Symptom | Likely cause | Next step |
|---|---|---|
| JavaScript-marked test does not use Poltergeist | The driver is not required or selected, or a custom registration does not match the configured name. | Check the gem, require 'capybara/poltergeist', driver registration, and Capybara.javascript_driver. |
| Page appears to stop running a script without a clear error | PhantomJS may be encountering unsupported ES6 syntax, including let or const. |
Enable :js_errors, inspect the served bundle, and transpile it or move the test to a modern driver. |
| Element lookup fails just after an AJAX update | The assertion may run before the expected state appears, or the operation may exceed the configured wait. | Assert the eventual UI state and adjust default_max_wait_time only if the work legitimately needs longer. |
| Click does not produce the expected result | An overlay, viewport, layout, or coordinate issue may prevent the user-like click from reaching the target. | Review a screenshot and debug output; correct the page state or, when event dispatch is the intended test, use a deliberate DOM trigger. |
DeadClient or another PhantomJS crash |
The old embedded WebKit or another driver/environment issue may be involved; the symptom alone does not identify a cause. | Check reproducibility, retain the stack trace and versions, and report a focused issue when useful. Quit manually created sessions with session.driver.quit to avoid memory leaks. |
| PhantomJS starts inconsistently on Linux | The installed binary may be unsuitable; Poltergeist warns against the official Ubuntu repository package. | Verify the executable available on the test process’s PATH and use a compatible PhantomJS binary. |
Decide whether to patch Poltergeist or migrate
Poltergeist’s GitHub repository is archived and read-only as of November 27, 2020. That status matters when a test suite depends on an engine that cannot keep pace with modern JavaScript. Current Capybara documentation says JavaScript tests need a different driver and documents Selenium-based drivers.
| Option | What it addresses | Trade-off |
|---|---|---|
| Transpile syntax or add a polyfill | A narrow compatibility issue in the code PhantomJS receives. | Can preserve an old suite, but retains PhantomJS’s engine limits and the maintenance risk of an archived driver. |
| Increase Capybara’s wait time | Slow asynchronous work that eventually reaches the expected state. | Does not repair syntax errors, unsupported APIs, or browser crashes. |
| Move JavaScript tests to a maintained Selenium-compatible driver | Recurring incompatibility caused by the obsolete browser engine. | Requires browser and driver setup, but removes the dependency on PhantomJS for those tests. |
For a temporary, isolated failure, a transpilation or wait adjustment may be proportionate. For recurring failures tied to modern syntax, engine behavior, or crashes, plan migration. In CI, verify that the browser and driver setup is reproducible in the same environment as the test job; the available project guidance does not specify one universal Selenium setup or version combination.
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