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Pascual Vila

Frontend Instructor // Code Syllabus

Common Pitfalls & Errors

The Error //

Refactoring code with no existing test coverage and no additional verification step

// Risky: no way to verify behavior was actually preserved
// Refactor happens directly, with only a manual code read-through

// Safer: characterization tests added FIRST, specifically as a safety net
// 1. Generate tests capturing current behavior
// 2. Refactor
// 3. Confirm the same tests still pass

The Solution //

Without a test suite, there's no automated way to confirm an AI-assisted refactor genuinely preserved behavior — a subtle, unintended change can slip through unnoticed in a code read-through alone. Generate characterization tests capturing the code's current, actual behavior before refactoring untested code.

The Error //

Requesting a large, sweeping refactor across an entire file in a single pass

// Risky: large diff, hard to review carefully for subtle changes
"Modernize this entire 400-line file"

// Safer: small, reviewable, one change at a time
"Extract the duplicated validation logic into a shared function"

The Solution //

A large, all-at-once refactor produces a diff too large to carefully review for accidental behavior changes — increasing the chance a subtle, unintended change goes unnoticed simply due to review fatigue and diff size. Scope refactoring requests to one specific, named improvement at a time.