Writing Tests (Opus 级)

Tests verify behavior, not implementation. A test that breaks when you rename a variable is a bad test.

Step 1: Understand what to test

Before writing any test:

• Read the function or module you are testing in full
• Identify its contract: given input X, what output or side effect is promised?
• Find existing tests to understand the pattern in this codebase

Step 2: Identify the cases

For every function or behavior, enumerate:

Happy path

The normal input that produces the expected output.

Boundary cases

• Empty input (empty string, empty array, zero, null)
• Single element vs. many elements
• Min and max values
• Exact boundary values (e.g. >= vs >)

Error cases

• Invalid input: what should happen?
• Missing required fields
• Network failure, file not found, permission denied

Edge cases specific to the logic

Think about what could go wrong given how the code works.

Step 3: Write the tests

Follow the existing test framework and style in the codebase. Do not introduce a new testing library.

Structure each test as:

1. Arrange: set up inputs and any mocks or state
2. Act: call the function
3. Assert: verify the outcome

it("returns null when the token is expired", () => {
  // Arrange
  const token = createToken({ expiresAt: new Date("2000-01-01") });
  // Act
  const result = validateToken(token);
  // Assert
  expect(result).toBeNull();
});