Run Elixir Tests in Watch Mode

24 January 2024

Here's a way to run Elixir tests in watch mode. "Watch mode" is where your tests will re-run automatically when src changes.

Start New Mix Project

The simplest way to get an Elixir project set up for testing is with mix, which comes with Elixir distributions.

mix new my_project

The ExUnit library is built into Elixir for testing. But it doesn't have a built-in way to run tests in watch mode.

But there is a 3rd-party option, mix-test.watch.

When we created a new project, a mix.exs file was generated, with our dependencies list. To integrate with the library, we'll adjust it to read:

defp deps do
  [
    {:mix_test_watch, "~> 1.0", only: [:dev, :test], runtime: false}
  ]
end

Now to run the tests in watch mode, instead of mix test, we run:

mix test.watch

And as we change our code, our tests will re-run against it.

Now, do we really need 3rd-party libraries? When I mix help test, I get the suggestion that I should get watch mode with:

fswatch lib test | mix test --listen-on-stdin

But, at least in my case, the constantly restarts the test process (ie, not just on file save). Do you have any luck with this method?