Testing Phoenix Applications Elixir

What You’ll Learn 📚

In this tutorial, we’ll cover:

• Core concepts of testing in Phoenix
• Key terminology and definitions
• Step-by-step examples from simple to complex
• Common questions and troubleshooting tips

Introduction to Testing in Phoenix

Testing is like a safety net for your code. It ensures that your application behaves as expected and helps catch bugs early. In Phoenix, testing is built-in and leverages the power of Elixir’s testing framework, ExUnit.

Core Concepts

• ExUnit: The testing framework that comes with Elixir, used for writing and running tests.
• Test Case: A single unit of testing, usually a function that checks a specific piece of functionality.
• Assertions: Statements that check if a condition is true. If not, the test fails.

Key Terminology

• Fixtures: Predefined data used to set up the test environment.
• Mocks: Fake implementations of modules or functions used to isolate tests.
• Test Coverage: A measure of how much of your code is tested by your test suite.

Starting with the Simplest Example

defmodule MyApp.HelloTest do
  use ExUnit.Case

  test "greets the world" do
    assert MyApp.hello() == :world
  end
end

Progressively Complex Examples

Example 1: Testing a Controller

defmodule MyAppWeb.PageControllerTest do
  use MyAppWeb.ConnCase

  test "GET /", %{conn: conn} do
    conn = get(conn, "/")
    assert html_response(conn, 200) =~ "Welcome to Phoenix!"
  end
end

Example 2: Testing a Model

defmodule MyApp.Accounts.UserTest do
  use MyApp.DataCase

  alias MyApp.Accounts.User

  test "valid changeset" do
    changeset = User.changeset(%User{}, %{name: "John", email: "john@example.com"})
    assert changeset.valid?
  end
end

Example 3: Testing with Mocks

defmodule MyApp.SomeServiceTest do
  use ExUnit.Case
  import Mox

  setup :verify_on_exit!

  test "calls external service" do
    MyApp.ExternalServiceMock
    |> expect(:call, fn _ -> :ok end)

    assert MyApp.SomeService.call_external() == :ok
  end
end

Common Questions and Answers

1. Why do we need tests?

   Tests help ensure your code works as expected and prevent future changes from introducing bugs.

2. What is the difference between unit and integration tests?

   Unit tests focus on small parts of your code in isolation, while integration tests check how different parts work together.

3. How do I run tests in Phoenix?

   Use the command mix test in your terminal to run all tests.

4. What if a test fails?

   Don’t panic! Check the error message to understand what went wrong and fix the issue in your code.

5. How can I improve test coverage?

   Write tests for all critical parts of your application, including edge cases and error handling.

Troubleshooting Common Issues

If your tests aren’t running, ensure you’ve set up your test environment correctly and that your test files are named with _test.exs.

Remember, practice makes perfect! The more you write tests, the more comfortable you’ll become. Keep experimenting and learning. 🚀

Practice Exercises

• Create a new Phoenix project and write a test for a simple function.
• Try writing a test for a controller action that returns JSON.
• Experiment with mocking a service in your tests using Mox.

For more information, check out the Phoenix Testing Guide and the ExUnit Documentation.