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Elixir & Phoenix: Scalable Backend Development · Lesson

Unit Testing with ExUnit

Write fast and reliable unit tests for your Elixir modules and functions using the built-in ExUnit framework.

Unit Testing with ExUnit is a free Elixir & Phoenix: Scalable Backend Development lesson on CoddyKit — lesson 1 of 4. You can read the complete lesson below for free — then practise it hands-on in the browser with a built-in code editor and a 24/7 AI tutor. It is part of the Elixir & Phoenix: Scalable Backend Development learning path, one of 4 lessons in the course, and your progress syncs across the web and the CoddyKit app.

Intro to ExUnit

Welcome to unit testing with ExUnit! ExUnit is Elixir's built-in testing framework, designed to help you write reliable and maintainable code.

Unit tests focus on small, isolated parts of your code, like individual functions or modules. They ensure each component works as expected, giving you confidence as your application grows.

Your First ExUnit Test

Elixir test files typically end with _test.exs and are placed in a test/ directory. Inside, you use ExUnit.Case to bring in testing functionalities.

A test block starts with test "description" do ... end. Here's a simple example:

defmodule MyMath do
  def add(a, b), do: a + b
end

defmodule MyMathTest do
  use ExUnit.Case, async: true

  test "adds two numbers correctly" do
    assert MyMath.add(1, 2) == 3
  end
end

Asserting with `assert`

The most common assertion in ExUnit is assert. It checks if an expression evaluates to a truthy value (anything other than false or nil).

You often use it to compare expected outcomes with actual results.

defmodule Calculator do
  def multiply(a, b), do: a * b
end

defmodule CalculatorTest do
  use ExUnit.Case, async: true

  test "multiplies positive numbers" do
    assert Calculator.multiply(2, 3) == 6
  end

  test "multiplies by zero" do
    assert Calculator.multiply(5, 0) == 0
  end
end

Refuting with `refute`

Sometimes, you want to ensure something is not true. That's where refute comes in. It's the opposite of assert.

refute passes if its argument evaluates to a falsy value (false or nil).

defmodule Validator do
  def is_negative(num), do: num < 0
end

defmodule ValidatorTest do
  use ExUnit.Case, async: true

  test "refutes positive numbers as negative" do
    refute Validator.is_negative(10)
    refute Validator.is_negative(0)
  end

  test "asserts negative numbers" do
    assert Validator.is_negative(-5)
  end
end

Testing for Exceptions

What if your function is supposed to raise an error under specific conditions? assert_raise lets you test for that!

You provide the expected exception type and an optional message, along with a function that should trigger the error.

defmodule Divider do
  def divide(a, b) do
    if b == 0, do: raise(ArgumentError, "Cannot divide by zero"), else: a / b
  end
end

defmodule DividerTest do
  use ExUnit.Case, async: true

  test "raises ArgumentError for division by zero" do
    assert_raise ArgumentError, "Cannot divide by zero", fn ->
      Divider.divide(10, 0)
    end
  end
end

Setting Up Test Context

Many tests need common setup, like creating a temporary resource or a mock user. ExUnit's setup callback runs before each test in a module.

It should return {:ok, assigns} where assigns is a keyword list or map that will be available in your test function's arguments.

defmodule UserProfileTest do
  use ExUnit.Case, async: true

  setup do
    # This runs before each test
    user = %{id: 101, name: "Jane Doe", email: "jane@example.com"}
    {:ok, user: user, admin: true}
  end

  test "user has a name", %{user: user} do
    assert user.name == "Jane Doe"
  end

  test "context includes admin flag", %{admin: admin_status} do
    assert admin_status == true
  end
end

Grouping Tests with `describe`

As your test files grow, you might want to logically group related tests. The describe macro helps organize tests within a module, making your test suite more readable.

You can even nest describe blocks!

defmodule StringUtils do
  def capitalize(str), do: String.capitalize(str)
  def reverse(str), do: String.reverse(str)
end

defmodule StringUtilsTest do
  use ExUnit.Case, async: true

  describe "capitalize/1" do
    test "capitalizes first letter" do
      assert StringUtils.capitalize("hello") == "Hello"
    end

    test "handles empty string" do
      assert StringUtils.capitalize("") == ""
    end
  end

  describe "reverse/1" do
    test "reverses a string" do
      assert StringUtils.reverse("world") == "dlrow"
    end
  end
end

Executing Your Tests

Once you've written your tests, running them is simple using Elixir's build tool, Mix.

  • To run all tests in your project: mix test
  • To run tests in a specific file: mix test test/my_module_test.exs
  • To run a specific test by line number: mix test test/my_module_test.exs:10
  • To run tests matching a description: mix test --only "adds two numbers"

ExUnit Challenge

Consider the following Elixir module designed to check if a number is odd. Which ExUnit assertion correctly tests if NumberChecker.is_odd(5) returns true?

defmodule NumberChecker do
  def is_odd(n) when is_integer(n) do
    rem(n, 2) != 0
  end
end

Unit Testing Summary

Great job! You've covered the fundamentals of unit testing with ExUnit.

  • ExUnit is Elixir's built-in testing framework.
  • Tests are defined in _test.exs files using use ExUnit.Case.
  • assert checks for truthy values, refute for falsy values.
  • assert_raise verifies that specific exceptions are thrown.
  • setup callbacks prepare context for your tests.
  • describe blocks help organize related tests.
  • You run tests using mix test.

These skills are crucial for building robust and reliable Elixir applications!

Frequently asked questions

Is the “Unit Testing with ExUnit” lesson free?

Yes — the full text of “Unit Testing with ExUnit” is free to read here on the web, and the Elixir & Phoenix: Scalable Backend Development course includes 4 lessons in total. To practise it interactively (a built-in code editor and a 24/7 AI tutor) and unlock the rest of the Elixir & Phoenix: Scalable Backend Development course, upgrade to CoddyKit PRO.

What will I learn in “Unit Testing with ExUnit”?

Write fast and reliable unit tests for your Elixir modules and functions using the built-in ExUnit framework. You practise Elixir & Phoenix: Scalable Backend Development with hands-on code you run directly in the browser, and a 24/7 AI tutor answers your questions as you work through the lesson.

Do I need any experience to start Elixir & Phoenix: Scalable Backend Development?

No prior experience is required. Elixir & Phoenix: Scalable Backend Development on CoddyKit is structured for beginners through advanced learners; this is — lesson 1 of 4, so you can start here or from the beginning and move at your own pace.

How long does the “Unit Testing with ExUnit” lesson take?

Most CoddyKit lessons take about 5–10 minutes. Each one is bite-sized and interactive, so you make steady progress and pick up exactly where you left off across the web and the app.

Can I write and run code in this Elixir & Phoenix: Scalable Backend Development lesson?

Yes. Every Elixir & Phoenix: Scalable Backend Development lesson includes a built-in code editor, so you write and run real code right in your browser and get instant AI feedback — no local setup required.

All lessons in this course

  1. Unit Testing with ExUnit
  2. Integration Testing Phoenix Applications
  3. Mocking, Stubs, and Test Data
  4. Property-Based Testing with StreamData
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