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README.md

cypress-rails

CircleCI

This is a simple gem to make it easier to start writing browser tests with Cypress for your Rails apps, regardless of whether your app is server-side rendered HTML, completely client-side JavaScript, or something in-between.

Why do this?

Rails ships with a perfectly competent browser-testing facility called system tests which depend on capybara to drive your tests, most often with Selenium. All of these tools work, are used by lots of people, and are a perfectly reasonable choice when writing full-stack tests of your Rails application.

So why would you go off the Rails to use Cypress and this gem, adding two additional layers to the Jenga tower of testing facilities that Rails ships with? Really, it comes down to the potential for an improved development experience. In particular:

  • Cypress's IDE-like open command provides a highly visual, interactive, inspectable test runner. Not only can you watch each test run and read the commands as they're executed, Cypress takes a DOM snapshot before and after each command, which makes rewinding and inspecting the state of the DOM trivially easy, something that I regularly find myself losing 20 minutes attempting to do with Capybara
  • cypress open enables an almost REPL-like feedback loop that is much faster and more information dense than using Capybara and Selenium. Rather than running a test from the command line, seeing it fail, then adding a debug breakpoint to a test to try to manipulate the browser or tweaking a call to a Capybara API method, failures to be rather obvious when using Cypress and fixing it is usually as easy as tweaking a command, hitting save, and watching it re-run
  • With very few exceptions, a Cypress test that works in a browser window will also pass when run headlessly in CI
  • Cypress selectors are just jQuery selectors, which makes them both more familiar and more powerful than the CSS and XPath selectors offered by Capybara. Additionally, Cypress makes it very easy to drop into a plain synchronous JavaScript function for making more complex assertions or composing repetitive tasks into custom commands
  • Cypress commands are, generally, much faster than analogous tasks in Selenium. Where certain clicks and form inputs will hang for 300-500ms for seemingly no reason when running against Selenium WebDriver, Cypress commands tend to run as fast as jQuery can select and fill an element (which is, of course, pretty fast)
  • By default, Cypress takes a video of every headless test run, taking a lot of the mystery (and subsequent analysis & debugging) out of test failures in CI

Nevertheless, there are trade-offs to attempting this (most notably around Cypress's limited browser support and the complications to test data management), and I wouldn't recommend adopting Cypress and writing a bunch of browser tests for every application. But, if the above points sound like solutions to problems you experience, you might consider trying it out.

Installation

tl;dr:

  1. Install the npm package cypress
  2. Install this gem cypress-rails
  3. Run rake cypress:init

Installing Cypress itself

The first step is making sure Cypress is installed (that's up to you, this library doesn't install Cypress, it just provides a little Rails-specific glue).

If you're on newer versions of Rails and using webpacker for your front-end assets, then you're likely already using yarn to manage your JavaScript dependencies. If that's the case, you can add Cypress with:

$ yarn add --dev cypress

If you're not using yarn in conjunction with your Rails app, check out the Cypress docs on getting it installed. At the end of the day, this gem just needs the cypress binary to exist either in ./node_modules/.bin/cypress or on your PATH.

Installing the cypress-rails gem

Now, to install the cypress-rails gem, you'll want to add it to your development & test gem groups of your Gemfile, so that you have easy access to its rake tasks:

group :development, :test do
  gem "cypress-rails"
end

Once installed, you'll want to run:

$ rake cypress:init

This will override a few configurations in your cypress.json configuration file.

Usage

Develop tests interactively with cypress open

When writing tests with Cypress, you'll find the most pleasant experience (by way of a faster feedback loop and an interactive, easy-to-inspect test runner) using the cypress open command.

When using Rails, however, you'll also want your Rails test server to be running so that there's something for Cypress to interact with. cypress-rails provides a wrapper for running cypress open with a dedicated Rails test server.

So, by running either:

$ cypress-rails open

Or, if you don't mind the extra cost of loading rake just so it can call system:

$ rake cypress:open

Add tests to cypress/integration. Simply click a test file in the Cypress application window to launch the test in a browser. Each time you save the test file, it will re-run itself.

Run tests headlessly with cypress run

To run your tests headlessly (e.g. when you're in CI), you'll want the run command

$ cypress-rails run

Or, with rake:

$ rake cypress:run

Write Ruby tests that wrap and invoke your cypress tests

You can also extend a provided CypressRails::TestCase, which itself inherits from Rails' built-in ActionDispatch::SystemTestCase.

That means you can add a test named test/system/cypress_test.rb:

require "test_helper"

class CypressTest < CypressRails::TestCase
  test_locator "cypress/integration/**/*.js"
end

And it will run alongside all the rest of your system tests. Because this would be invoked by your existing test scripts, you can benefit from whatever custom test helpers (e.g. database setup, test coverage, etc.) your other full-stack tests need, as well as specifying custom setup and teardown directives.

Each Cypress file matched by the test_locator is translated to a single test case, which—while slightly inefficient, as it spools Cypress up and down multiple times—also makes it easy to handle each Cypress file as you would any other Ruby test. It also allows for CLI usage like this (where the test name is an expansion of the file location with the path separators replaced with _):

$ rails test test/system --name test_cypress_integration_send_invoice_js

WARNING: keep in mind that any custom Ruby code you add before or after each Cypress test is run in the context of a CypressRails::TestCase will not be run when developing with cypress open! That means this is probably not the most rock-solid strategy for consistent test behavior when it comes to things like test data management.

Setting up continuous integration

Circle CI

Nowadays, Cypress and Circle get along pretty well without much customization. The only tricky bit is that Cypress will install its large-ish binary to ~/.cache/Cypress, so if you cache your dependencies, you'll want to include that path:

version: 2
jobs:
  build:
    docker:
      - image: circleci/ruby:2.6-node-browsers
      - image: circleci/postgres:9.4.12-alpine
        environment:
          POSTGRES_USER: circleci
    steps:
      - checkout

      # Bundle install dependencies
      - type: cache-restore
        key: v1-gems-{{ checksum "Gemfile.lock" }}

      - run: bundle install --path vendor/bundle

      - type: cache-save
        key: v1-gems-{{ checksum "Gemfile.lock" }}
        paths:
          - vendor/bundle

      # Yarn dependencies
      - restore_cache:
          keys:
            - v1-yarn-{{ checksum "package.json" }}
            # fallback to using the latest cache if no exact match is found
            - v1-yarn-

      - run: yarn install

      - save_cache:
          paths:
            - node_modules
            - ~/.cache
          key: v1-yarn-{{ checksum "package.json" }}

      # Run your cypress tests
      - run: bin/rake cypress:run

Configuration

You can change the behavior of this gem by setting these environment variables:

  • RAILS_CYPRESS_PORT: the port to run the Rails test server on (defaults to a random available port
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