---
name: test-writer
description: Writes tests following Elixir/Phoenix best practices. Ensures DRY tests with proper helper functions, no duplicated setup code, and correct parameter defaults. Use when writing or modifying tests.
---

# Test Writer Skill

## Overview

This skill provides guidelines for writing clean, maintainable Elixir/Phoenix tests following best practices.

## Core Principles

### 1. DRY Tests

Avoid duplication by creating focused helper functions:

**Bad:**
```elixir
test "GET /users returns index", %{conn: conn} do
  conn = get(conn, "/users")
  body = html_response(conn, 200)
  assert body =~ "Users"
end

test "GET /users/:id returns show", %{conn: conn} do
  conn = get(conn, "/users/1")
  body = html_response(conn, 200)
  assert body =~ "User"
end
```

**Good:**
```elixir
defp goto_users_page(conn, suffix \\ ""), do: get(conn, "/users" <> suffix)

test "GET /users returns index", %{conn: conn} do
  conn = goto_users_page(conn)
  assert html_response(conn, 200) =~ "Users"
end

test "GET /users/:id returns show", %{conn: conn} do
  conn = goto_users_page(conn, "/1")
  assert html_response(conn, 200) =~ "User"
end
```


### 2. Separate Helpers for Different Assertion Patterns

Don't use conditionals in helpers to handle different cases:

**Bad:**
```elixir
defp goto_users_page(conn, suffix \\ "", check_title \\ true) do
  path = "/users" <> suffix
  conn = get(conn, path)
  body = html_response(conn, 200)
  if check_title, do: assert body =~ "Users"
  assert body =~ "AppLayout"
  body
end
```

**Good:**
```elixir
defp goto_users_page(conn, suffix \\ "") do
  path = "/users" <> suffix
  conn = get(conn, path)
  body = html_response(conn, 200)
  assert body =~ "Users"
  assert body =~ "AppLayout"
  body
end

defp goto_user_page(conn, suffix) do
  path = "/users" <> suffix
  conn = get(conn, path)
  body = html_response(conn, 200)
  assert body =~ "AppLayout"
  body
end
```

## Test Structure

### Context Block

```elixir
describe "resource name" do
  # Shared setup in context if needed
  # test "scenario" do ...
end
```

### Value Aliasing

Never reuse value names. Elixir is immutable, but value aliasing is confusing. Use unique, meaningful names for the left hand side of assignments. Or use pipes `|>` to eliminate the need for naming.

```elixir
test "GET /users returns index", %{conn: conn} do
  # don't reassign
  response = get(conn, "/users")
  # ...
end
```

## Common Patterns

### HTML Pages with Layout

```elixir
defp goto_resource_page(conn, suffix \\ ""), do: ...
# Asserts common layout elements and page-specific content
```

### API Endpoints

```elixir
test "GET /api/resource returns JSON", %{conn: conn} do
  conn = conn |> put_req_header("accept", "application/json")
  conn = get(conn, "/api/resource")
  response = json_response(conn, 200)
  # Assert structure
end
```

### Error Handling

```elixir
test "returns 404 for nonexistent", %{conn: conn} do
  assert html_response(get(conn, "/nonexistent"), 404)
end
```

## Running Tests

### One focused test file

```bash
cd /path/to/app
mix test test/path/to_test.exs
```

### all tests
```bash
make test
```