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To inject a local JavaScript file into the page that Capybara is currently displaying, use a JavaScript-capable driver and pass the file contents to page.execute_script:
script_path = Rails.root.join('spec', 'support', 'scripts', 'helper.js')
page.execute_script(File.read(script_path))File.read runs in the Ruby test process. Capybara then sends the resulting source string to Chrome for execution. This is the documented string-taking API; it is not a method that accepts a filesystem path directly. The default :rack_test driver cannot execute JavaScript, so the session must use Selenium Chrome or another JavaScript-capable driver.
What you need before injecting a file
There are two separate locations involved:
- The test runner’s filesystem: Ruby can read
spec/support/scripts/helper.jsfrom this machine. - The browser document: Chrome can execute JavaScript only after a document has been loaded in the current Capybara session.
A local path is meaningful to Ruby, not to the browser. If you want Chrome to fetch a file itself, the file must be exposed at a URL that the browser can reach.
Use a JavaScript-capable Capybara driver
Capybara’s default :rack_test driver does not run JavaScript. Capybara lists Selenium-backed Chrome drivers, including :selenium_chrome_headless, in its project documentation (Capybara README). Selenium’s WebDriver documentation also demonstrates configuring Chrome in headless mode and executing browser JavaScript (Selenium WebDriver documentation).
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A typical setup in a Rails test suite looks like this:
# Gemfile
group :test do
gem 'capybara'
gem 'selenium-webdriver'
end
# spec/rails_helper.rb or test_helper.rb
require 'capybara/rspec'
Capybara.configure do |config|
config.default_driver = :selenium_chrome_headless
endThe exact browser, ChromeDriver and Selenium versions are project-dependent. Install a compatible Chrome/Chromium browser and driver, then confirm that an ordinary JavaScript feature spec passes before troubleshooting injection.
Method 1: read the local file and execute its source
This is the simplest approach when the script is a test helper, fixture or generated file that exists on the machine running Ruby.
- Navigate to the page that should receive the script.
- Build an absolute path in Ruby.
- Read the file as a string.
- Pass that string to the session-level
execute_scriptmethod.
visit '/dashboard'
script_path = Rails.root.join('spec', 'support', 'scripts', 'helper.js')
raise "Missing script: #{script_path}" unless File.file?(script_path)
page.execute_script(File.read(script_path))Capybara documents page.execute_script(source) for scripts where you do not need a return value (Capybara session API). The source is evaluated in the current document. A relative file path, a Pathname object or a URL will not be interpreted as JavaScript source unless you read or construct the source yourself.
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A complete RSpec example
require 'rails_helper'
RSpec.describe 'dashboard helper', type: :feature, js: true do
it 'injects the helper into the current page' do
visit '/dashboard'
script_path = Rails.root.join('spec', 'support', 'scripts', 'helper.js')
page.execute_script(File.read(script_path))
expect(page).to have_css('[data-helper-ready="true"]')
end
endThe js: true metadata is useful in RSpec Rails projects that switch JavaScript examples to a Selenium driver. If your suite uses a different driver-selection convention, set the driver explicitly for the example or configure it globally.
Make the injected script observable
Tests are easier to diagnose when the helper leaves a deliberate signal. For example, the file could set an attribute:
// spec/support/scripts/helper.js
document.documentElement.dataset.helperReady = 'true';Then assert the signal from Ruby:
expect(page).to have_css('html[data-helper-ready="true"]')Choose a signal that represents the behavior your test needs. Avoid asserting an implementation-only global unless the global itself is the contract.
Method 2: append a script element with a browser-reachable URL
Use a dynamically created <script src="..."> element when the browser should load the file as a resource. This is useful for an asset served by the application, a fixture route or a test server.
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visit '/dashboard'
page.execute_script(<<~JS)
const script = document.createElement('script');
script.src = '/assets/helper.js';
document.head.appendChild(script);
JSThis example assumes that /assets/helper.js resolves from the page's origin and that the application serves it. A file that exists only under the test runner's spec/ directory cannot be used as the browser URL. For a cross-origin URL, content-security policy, CORS behavior and other browser loading rules can affect the result.
Wait for the external file to finish loading
Appending a script starts a network request; execute_script itself does not make your test wait for that resource's application work. Add a load marker in the file and wait for it through Capybara's normal synchronization:
// served /assets/helper.js
document.documentElement.dataset.helperReady = 'true';page.execute_script(<<~JS)
const script = document.createElement('script');
script.src = '/assets/helper.js';
script.onload = () => { document.documentElement.dataset.helperLoaded = 'true'; };
script.onerror = () => { document.documentElement.dataset.helperLoadError = 'true'; };
document.head.appendChild(script);
JS
expect(page).to have_css('html[data-helperLoaded="true"]', wait: 10)Use the attribute name consistently; HTML attributes are case-insensitive, but a consistent lowercase convention avoids confusing selectors. If the resource performs asynchronous work after its load event, expose a separate application-ready marker and wait for that marker instead.
Choosing between Capybara's script APIs
| API or technique | Use it when | Important limitation |
|---|---|---|
page.execute_script(source) |
You need to run JavaScript in the current page and do not need a return value. | It runs when called against the current document; it is not a pre-navigation hook. |
page.evaluate_script(expression) |
You need the value produced by a JavaScript expression. | Returned-value behavior can vary by driver. |
Append <script src> |
The browser should load a URL-backed resource. | The URL must be reachable from the browser and subject to page loading policy. |
Element-level execute_script |
The code intentionally needs one element as its context. | The element is bound as this; it is not equivalent to page-level execution. |
Capybara's session implementation describes the string-based methods and their driver-dependent argument support (session.rb). Its element implementation documents the element-scoped context (element.rb).
Read a value with evaluate_script
page.execute_script("window.__helperVersion = '1.4.0'")
version = page.evaluate_script('window.__helperVersion')
expect(version).to eq('1.4.0')Use this only when the returned value is part of what you need to verify. Drivers differ in which JavaScript values they can marshal back to Ruby, so simple strings, numbers, booleans and plain data are safer than browser-native objects.
Understand element context
button = find('button.save')
button.execute_script("this.dataset.scriptRan = 'true'")Here, this refers to the matched button. A script that expects document, page-level globals or the whole document should be run with page.execute_script instead.
Passing values into a script
Some drivers support arguments to script execution, but Capybara leaves the supported argument types to the selected driver. Keep the values simple and verify the behavior with your exact Selenium/browser combination. If portability matters, serialize a small JSON value into the source rather than relying on complex browser-object marshalling.
Timing: inject into the current document, not an imagined future page
execute_script acts on the document that exists when the call is made. Navigate first, then inject:
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expect(page).to have_css('#payment-form')
page.execute_script(File.read(Rails.root.join('spec', 'support', 'scripts', 'helper.js')))If the script reads application globals, wait until the application has created them. If it changes a particular node, wait until that node exists. Capybara's matchers synchronize with page changes, so a semantic assertion is usually more robust than a fixed sleep:
expect(page).to have_css('[data-app-ready="true"]', wait: 15)
page.execute_script(File.read(script_path))A fixed sleep can hide slow-page failures and still be too short on a busy CI runner. Use it only when the application has no observable readiness condition, and keep the duration deliberately bounded.
What if the script must run before page scripts?
Current-document injection is different from registering code that runs before the document's own scripts or before every navigation. The available Capybara API does not provide a portable, version-independent preload command for that requirement. Chrome DevTools Protocol solutions may exist for particular Selenium, ChromeDriver and browser versions, but the Ruby API and guarantees must be checked against those exact versions before adopting them.
If you control the application, a more stable solution is often to include the helper in the page's normal test asset bundle or expose a test-only initialization hook. That makes ordering explicit instead of depending on a post-navigation injection race.
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“JavaScript is not supported” or nothing happens
Cause: the session is using :rack_test or another non-JavaScript driver.
Fix: switch the example to :selenium_chrome_headless or another JavaScript-capable driver, install compatible browser and driver binaries, and confirm that a basic JavaScript assertion works.
The test passes a filename to execute_script
Cause: Capybara receives the path text instead of JavaScript source.
Fix: read the file in Ruby first:
page.execute_script(File.read(script_path))“File not found” in CI
Cause: the path is relative to a different working directory, the fixture was not checked in, or the CI checkout omits it.
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Fix: anchor the path to Rails.root (or another known project root), assert File.file?, and ensure the file is included in the test job's checkout.
The script runs but cannot find an element
Cause: injection occurred before the application rendered the target node, or the selector refers to a different page state.
Fix: navigate first and wait for the relevant selector or application-ready marker before calling execute_script.
The URL-backed script never loads
Cause: the URL is not served, resolves against the wrong origin, is blocked by content-security policy, or fails a cross-origin browser rule.
Fix: open the exact URL from the test browser's origin, inspect the browser console/network log, verify that the response is JavaScript, and use the script's onerror marker while diagnosing.
You expected a result from execute_script
Cause: execute_script is intended for side effects and does not provide the evaluated value.
Fix: use evaluate_script for a value, then keep the returned data to driver-supported primitive types.
An element-scoped script behaves differently
Cause: element-level execution binds this to that element.
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Fix: use the session-level method for document-wide code, or deliberately reference this when the element context is the behavior under test.
Keeping injected helpers reliable
- Use absolute, intention-revealing paths: keep helper files under a documented test-support directory and build paths from the project root.
- Make initialization idempotent: guard against duplicate listeners or duplicate DOM nodes if a test can inject more than once.
- Expose a narrow readiness signal: a data attribute or small global makes synchronization and failure diagnosis clearer.
- Keep production behavior isolated: load test-only helpers from test code or a test-only route rather than silently changing normal assets.
- Clean up state: each example should navigate or reset the session so globals, event listeners and DOM changes do not leak into another test.
- Log the resolved path on failure: the path shown in a CI failure should identify the exact file Ruby attempted to read.
The practical decision is straightforward: use File.read plus page.execute_script for a local fixture, append a script element for a browser-served URL, and treat pre-document execution as a separate, version-sensitive engineering problem.
Or skip the browser setup
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response = Net::HTTP.get_response(uri)
File.binwrite('shot.webp', response.body)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)const q = new URLSearchParams({ access_key: 'YOUR_API_KEY', url: 'https://stripe.com' });
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Frequently Asked Questions
Will the injected file run again after a redirect?
No. The code is evaluated in the document that exists at the moment of the call. A redirect or new navigation creates a new document, so inject again after that navigation if the new page needs the helper.
Can I use a file:// URL for a script in headless Chrome?
A browser-reachable URL is required, and file URLs introduce origin and security differences that do not match a normal application page. Serving the helper from the test application's origin is generally more predictable.
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Attach an onerror handler that writes a diagnostic data attribute, then assert that the success marker appears with a Capybara matcher and inspect the browser's network or console output when it does not.
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