Five production-breaking patterns where quadratic complexity hides behind readable, idiomatic JavaScript. Why .find() inside .map() passes code review and fails at scale; and the one-line fixes that drop O(n^2) to O(n).

• Pattern 1: The innocent .includes() inside .filter()
• Pattern 2: Deduplication by comparison
• Pattern 3: The cascading .map().filter().map()
• Pattern 4: The recursive tree flattener
• Pattern 5: The SQL query in a loop (the N+1 problem)