Error Handling Test Skill

Discovery

Before writing any tests, scan the target code for:

• Explicit throws — throw new Error(...), custom error classes, re-throws
• Implicit throws — array/object destructuring on null, property access on undefined, calling non-functions
• Async failure surfaces — Promise.reject, async functions that throw, unhandled branches in .then() chains
• Fallbacks — default values, catch blocks that return something, optional chaining with fallback (?? default), try/catch that swallows errors silently
• Error propagation — does the function rethrow, wrap, or consume the error? Each behavior needs a different assertion

Non-Obvious Patterns to Cover

1. Assert the error type and message, not just that something throws

// WEAK — only verifies something threw
expect(() => fn()).toThrow();