Code Quality Checklist

Contents

• Error Handling (anti-patterns, best practices, questions)
• Performance & Caching (CPU, database/IO, caching, memory)
• Boundary Conditions (null/undefined, empty collections, numeric, string)

Error Handling

Anti-patterns to Flag

• Swallowed exceptions: Empty catch blocks or catch with only logging. Example:

  try { ... } catch (e) { }  // Silent failure
  try { ... } catch (e) { console.log(e) }  // Log and forget
  

• Overly broad catch: Catching Exception/Error base class instead of specific types
• Error information leakage: Stack traces or internal details exposed to users
• Missing error handling: No try-catch around fallible operations (I/O, network, parsing)
• Async error handling: Unhandled promise rejections, missing .catch(), no error boundary

Best Practices to Check

• Errors are caught at appropriate boundaries
• Error messages are user-friendly (no internal details exposed)
• Errors are logged with sufficient context for debugging
• Async errors are properly propagated or handled
• Fallback behavior is defined for recoverable errors
• Critical errors trigger alerts/monitoring

Questions to Ask

• "What happens when this operation fails?"
• "Will the caller know something went wrong?"
• "Is there enough context to debug this error?"

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Performance & Caching

CPU-Intensive Operations

• Expensive operations in hot paths: Regex compilation, JSON parsing, crypto in loops
• Blocking main thread: Sync I/O, heavy computation without worker/async
• Unnecessary recomputation: Same calculation done multiple times
• Missing memoization: Pure functions called repeatedly with same inputs

Database & I/O

• N+1 queries: Loop that makes a query per item instead of batch

  // Bad: N+1
  for (const id of ids) {
    const user = await db.query(`SELECT * FROM users WHERE id = ?`, id)
  }
  // Good: Batch
  const users = await db.query(`SELECT * FROM users WHERE id IN (?)`, ids)
  

• Missing indexes: Queries on unindexed columns
• Over-fetching: SELECT * when only few columns needed
• No pagination: Loading entire dataset into memory

Caching Issues

• Missing cache for expensive operations: Repeated API calls, DB queries, computations
• Cache without TTL: Stale data served indefinitely
• Cache without invalidation strategy: Data updated but cache not cleared
• Cache key collisions: Insufficient key uniqueness
• Caching user-specific data globally: Security/privacy issue

Memory

• Unbounded collections: Arrays/maps that grow without limit
• Large object retention: Holding references preventing GC
• String concatenation in loops: Use StringBuilder/join instead
• Loading large files entirely: Use streaming instead

Questions to Ask

• "What's the time complexity of this operation?"
• "How does this behave with 10x/100x data?"
• "Is this result cacheable? Should it be?"
• "Can this be batched instead of one-by-one?"

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Boundary Conditions

Null/Undefined Handling

• Missing null checks: Accessing properties on potentially null objects
• Truthy/falsy confusion: if (value) when 0 or "" are valid
• Optional chaining overuse: a?.b?.c?.d hiding structural issues
• Null vs undefined inconsistency: Mixed usage without clear convention

Empty Collections

• Empty array not handled: Code assumes array has items
• Empty object edge case: for...in or Object.keys on empty object
• First/last element access: arr[0] or arr[arr.length-1] without length check

Numeric Boundaries

• Division by zero: Missing check before division
• Integer overflow: Large numbers exceeding safe integer range
• Floating point comparison: Using === instead of epsilon comparison
• Negative values: Index or count that shouldn't be negative
• Off-by-one errors: Loop bounds, array slicing, pagination

String Boundaries

• Empty string: Not handled as edge case
• Whitespace-only string: Passes truthy check but is effectively empty
• Very long strings: No length limits causing memory/display issues
• Unicode edge cases: Emoji, RTL text, combining characters

Common Patterns to Flag

// Dangerous: no null check
const name = user.profile.name

// Dangerous: array access without check
const first = items[0]

// Dangerous: division without check
const avg = total / count

// Dangerous: truthy check excludes valid values
if (value) { ... }  // fails for 0, "", false

Questions to Ask

• "What if this is null/undefined?"
• "What if this collection is empty?"
• "What's the valid range for this number?"
• "What happens at the boundaries (0, -1, MAX_INT)?"