elixir-testing

name	elixir-testing
description	ExUnit testing patterns, Mox for mocking, StreamData for property-based testing, and Phoenix test cases for Elixir applications.

Elixir Testing

When to Activate

Use this skill when:

• Writing ExUnit tests for Elixir or Phoenix applications
• Setting up Mox for mock-based testing
• Implementing property-based tests with StreamData
• Writing Phoenix controller or LiveView tests
• Setting up test factories with ExMachina
• Debugging flaky concurrent tests in Elixir
• Setting up code coverage with excoveralls
• Testing GenServer behavior and OTP processes

ExUnit Basics

defmodule MyApp.CalculatorTest do
  use ExUnit.Case, async: true  # Always async: true unless DB or shared state

  describe "add/2" do
    test "adds two positive numbers" do
      assert Calculator.add(1, 2) == 3
    end

    test "handles negative numbers" do
      assert Calculator.add(-1, 1) == 0
    end
  end

  describe "divide/2" do
    test "returns {:error, :division_by_zero} when divisor is 0" do
      assert {:error, :division_by_zero} = Calculator.divide(10, 0)
    end
  end
end

Mox for Compile-Safe Mocks

# 1. Define behaviour
defmodule MyApp.Payments.Gateway do
  @callback charge(amount :: integer(), token :: String.t()) ::
    {:ok, map()} | {:error, String.t()}
end

# 2. Implementation
defmodule MyApp.Payments.StripeGateway do
  @behaviour MyApp.Payments.Gateway

  def charge(amount, token) do
    Stripe.Charge.create(%{amount: amount, source: token, currency: "usd"})
  end
end

# 3. Register mock in test/test_helper.exs
Mox.defmock(MyApp.MockGateway, for: MyApp.Payments.Gateway)

# 4. Configure application to use mock in test env
# config/test.exs
config :my_app, :payment_gateway, MyApp.MockGateway

# 5. Use in tests
defmodule MyApp.OrderServiceTest do
  use ExUnit.Case, async: true
  import Mox

  setup :verify_on_exit!

  test "processes payment on order creation" do
    expect(MyApp.MockGateway, :charge, fn 1000, "tok_test" -> {:ok, %{id: "ch_123"}} end)

    assert {:ok, order} = OrderService.create(%{amount: 1000, token: "tok_test"})
    assert order.payment_id == "ch_123"
  end
end