React Performance: JS Perf — Length Check First. Keywords: length check array compare. Platform: React/Next.js. Description: Check array lengths before expensive comparisons. Do: Early return if lengths differ. Don't: Always run expensive comparison. Good Example: if (a.length !== b.length) return true; // then compare. Bad Example: a.sort().join() !== b.sort().join() // even when lengths differ. Severity: Medium-High.