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Written by Sebastian on Feb 03, 2021 in Dev

Phoenix UI testing with Cypress, Part 1

I still remember the days I tried to achieve UI testing with Selenium, PhantomJS and various other tools. It was a hassle. It didn’t run on CI because it needed some kind of window manager. It was unstable.

Introducing Cypress

Cypress has solved these issues, hiding the complexity of UI testing and leaving you with the task of writing the tests. It comes with it’s own UI where you can run tests and see what the test does in realtime. Cypress can do screenshots, videos of the tests and more.

In this tutorial, we’ll mostly use cypress run. This command acts like mix test does: Running your tests and displaying the result within your terminal window. I encourage you to check out the other features of Cypress on your own.

Cypress Setup

I assume you have an existing Elixir/Phoenix project you want to start using Cypress with. Your frontend files are located at assets/ while your elixir tests are stored within test/.

To install Cypress, we use npm: cd assets/ && npm install cypress --save-dev. Cypress installs itself and is afterwards available as a command line tool at assets/node_modules/cypress/bin/cypress.

Cypress by default expects all test files and support files to be located at cypress/. I’d argue a much better place for these files is in test/cypress, so this is where we’re going to place them. If you like your Cypress tests to live someplace else, you’ll find this guide helpful to figure out which adjustments you need to make.

Config and support files

Let’s start with the config file cypress.json in your root directory:

  "componentFolder": false,
  "downloadsFolder": "tmp/cypress/downloads",
  "fixturesFolder": "test/cypress/fixtures",
  "integrationFolder": "test/cypress/integration",
  "pluginsFile": false,
  "screenshotOnRunFailure": false,
  "screenshotsFolder": "tmp/cypress/screenshots",
  "supportFile": false,
  "testFiles": "**/*.*",
  "video": false,
  "videosFolder": "tmp/cypress/videos"

As you can see, we overwrite all folders, either pointing to test/cypress or tmp/cypress (for files to be ignored). We also don’t use support files or plugins and deactivate screenshots and videos.

The first test

Now it’s time to write the first test, a simple request to our homepage. Tests for Cypress are placed in the integrations folder which means creating the file test/cypress/integration/index_spec.js:

describe('Homepage', () => {
  it('Visit homepage without interaction', () => {

You can run this test using the command ./assets/node_modules/cypress/bin/cypress run but it will fail if your Phoenix server does not run. Try it again after starting the server with mix phx.serer in another terminal window.

The All-in-one shell file

We want to run the tests with one command, both locally and on a CI server. I used the shell script suggested by and modified them a bit. Create a file, make it executable (chmod +x and put the following code into it:


MIX_ENV=cypress mix ecto.reset
echo "===RUN npm install cypress --save-dev ==="
echo "===IN THE assets/ FOLDER==="
MIX_ENV=cypress mix phx.server &
pid=$! # Store server pid
until $(curl --output /dev/null --silent --head --fail http://localhost:4002); do
    printf '.'
    sleep 5
echo ""
./assets/node_modules/.bin/cypress run
kill -9 $pid # kill server
exit $result

As you might have noticed, the MIX_ENV is set to cypress. To create this env, we need the new configuration file config/cypress.exs:

use Mix.Config

# Configure your database
config :phonix, Phonix.Repo,
  username: "postgres",
  password: "postgres",
  database: "phonix_cypress",
  hostname: "localhost",
  pool_size: 10

config :phonix, PhonixWeb.Endpoint,
  http: [port: 4002],
  server: true

# Print only warnings and errors during test
config :logger, level: :warn

This approach is also copied from It’s a great idea to separate the test environment from the ui test environment. As Alan suggests, you could use a tool like ex_check which can run your normal tests and your ui tests in parallel, which is only possible if you use different databases and thus different environments.

We are using a different port in the cypress env (4002), so make sure to adjust your tests accordingly.

Now you can run your UI tests by executing ./ This script should run on your CI environment as well as locally. Just make sure to run npm install in your CI run!

What else?

I intend to write a second part, figuring out how to use fixtures or reset the database between tests. I saw a few blog posts on how to do this, utilizing sockets in phoenix to take commands. I don’t really like the approach and might come up with a way to work with the database directly. We’ll see!