testing

Testing

What to Test

Test behavior, not implementation. A test should verify what a function does, not how it does it. If you refactor the internals and the test breaks, the test was testing the wrong thing.

Good test: "given a valid email, returns true" Bad test: "calls regex.match with pattern /^[a-z].../"

Meaningful Coverage vs Line Coverage

100% line coverage is a vanity metric. You can have 100% coverage and still ship bugs if your tests don't exercise meaningful paths.

Focus coverage on:

• Business logic (the rules that make your app unique)
• Error handling paths (what happens when things go wrong)
• Boundary conditions (empty, null, max values, off-by-one)
• Integration points (where your code meets external systems)

Skip coverage on:

• Simple getters/setters
• Framework boilerplate
• Generated code
• Pure delegation (functions that just call another function)

Edge Cases Worth Testing

Every function has these potential edge cases. Consider which apply:

• Null / undefined / empty string
• Empty array / empty object
• Single element
• Very large input
• Negative numbers / zero
• Unicode and special characters
• Concurrent access
• Network timeout / failure

You don't need to test ALL of these for every function. Think about which ones are realistic for your specific case.

Test-First Workflow

Writing tests first helps when:

• The behavior is well-defined but the implementation isn't clear
• You're fixing a bug (write the failing test first, then fix)
• You're implementing a spec (tests become the spec's executable form)

Test-first hurts when:

• You're exploring and don't know what the API should look like
• You're prototyping and will throw the code away
• The test would be trivial (testing that a constant equals itself)

When doing test-first: write the test, watch it fail, implement the minimum to pass, then refactor. Don't write all the tests up front — go one at a time.

Test Structure

Follow Arrange-Act-Assert (AAA):

// Arrange: set up the test conditions
const input = createValidInput();

// Act: call the thing being tested
const result = processInput(input);

// Assert: verify the outcome
expect(result.status).toBe('success');