Common Bug Patterns

Checklist of frequent bug patterns to scan before forming hypotheses. Ordered by frequency. Check these FIRST — they cover ~80% of bugs across all technology stacks.

<patterns>

Null / Undefined Access

• Null property access — accessing property on null or undefined, missing null check or optional chaining
• Missing return value — function returns undefined instead of expected value, missing return statement or wrong branch
• Destructuring null — array/object destructuring on null/undefined, API returned error shape instead of data
• Undefaulted optional — optional parameter used without default, caller omitted argument

Off-by-One / Boundary

• Wrong loop bound — loop starts at 1 instead of 0, or ends at length instead of length - 1
• Fence-post error — "N items need N-1 separators" miscounted
• Inclusive vs exclusive — range boundary < vs <=, slice/substring end index
• Empty collection — .length === 0 falls through to logic assuming items exist

Async / Timing

• Missing await — async function called without await, gets Promise object instead of resolved value
• Race condition — two async operations read/write same state without coordination
• Stale closure — callback captures old variable value, not current one
• Initialization order — event handler fires before setup complete
• Leaked timer — timeout/interval not cleaned up, fires after component/context destroyed

State Management

• Shared mutation — object/array modified in place affects other consumers
• Stale render — state updated but UI not re-rendered, missing reactive trigger or wrong reference
• Stale handler state — closure captures state at bind time, not current value
• Dual source of truth — same data stored in two places, one gets out of sync
• Invalid transition — state machine allows transition missing guard condition

Import / Module

• Circular dependency — module A imports B, B imports A, one gets undefined
• Export mismatch — default vs named export, import X vs import { X }
• Wrong extension — .js vs .cjs vs .mjs, .ts vs .tsx
• Path case sensitivity — works on Windows/macOS, fails on Linux
• Missing extension — ESM requires explicit file extensions in imports

Type / Coercion

• String vs number compare — "5" > "10" is true (lexicographic), 5 > 10 is false
• Implicit coercion — == instead of ===, truthy/falsy surprises (0, "", [])
• Numeric precision — 0.1 + 0.2 !== 0.3, large integers lose precision
• Falsy valid value — value is 0 or "" which is valid but falsy

Environment / Config

• Missing env var — environment variable missing or wrong value in dev vs prod vs CI
• Hardcoded path — works on one machine, fails on another
• Port conflict — port already in use, previous process still running
• Permission denied — different user/group in deployment
• Missing dependency — not in package.json or not installed

Data Shape / API Contract

• Changed response shape — backend updated, frontend expects old format
• Wrong container type — array where object expected or vice versa, data vs data.results vs data[0]
• Missing required field — required field omitted in payload, backend returns validation error
• Date format mismatch — ISO string vs timestamp vs locale string
• Encoding mismatch — UTF-8 vs Latin-1, URL encoding, HTML entities

Regex / String

• Sticky lastIndex — regex g flag with .test() then .exec(), lastIndex not reset between calls
• Missing escape — . matches any char, $ is special, backslash needs doubling
• Greedy overmatch — .* eats through delimiters, need .*?
• Wrong quote type — string interpolation needs backticks for template literals

Error Handling

• Swallowed error — empty catch {} or logs but doesn't rethrow/handle
• Wrong error type — catches base Error when specific type needed
• Error in handler — cleanup code throws, masking original error
• Unhandled rejection — missing .catch() or try/catch around await

Scope / Closure

• Variable shadowing — inner scope declares same name, hides outer variable
• Loop variable capture — all closures share same var i, use let or bind
• Lost this binding — callback loses context, need .bind() or arrow function
• Scope confusion — var hoisted to function, let/const block-scoped

</patterns> <usage>

How to Use This Checklist

1. Before forming any hypothesis, scan the relevant categories based on the symptom
2. Match symptom to pattern — if the bug involves "undefined is not an object", check Null/Undefined first
3. Each checked pattern is a hypothesis candidate — verify or eliminate with evidence
4. If no pattern matches, proceed to open-ended investigation

Symptom-to-Category Quick Map

Symptom	Check First
"Cannot read property of undefined/null"	Null/Undefined Access
"X is not a function"	Import/Module, Type/Coercion
Works sometimes, fails sometimes	Async/Timing, State Management
Works locally, fails in CI/prod	Environment/Config
Wrong data displayed	Data Shape, State Management
Off by one item / missing last item	Off-by-One/Boundary
"Unexpected token" / parse error	Data Shape, Type/Coercion
Memory leak / growing resource usage	Async/Timing (cleanup), Scope/Closure
Infinite loop / max call stack	State Management, Async/Timing

</usage>