Node.js 'FATAL ERROR: Reached heap limit Allocation failed - JavaScript heap out of memory' (during webpack/Vite/Next.js/tsc builds, Jest, or in a server). How do I fix it?

Cause: V8 limits the size of the JavaScript heap (the old space). When a process needs more than that limit, Node crashes with this message. Common causes: large builds (webpack, Next.js, tsc on big monorepos, source maps), test runners holding many workers or modules, loading a huge file or query result into memory at once, a real memory leak in a long-running server, and containers or CI runners where the default limit doesn't match the memory actually available. How to check: - node -e "console.log(v8.getHeapStatistics().heap_size_limit/1024/1024)" prints the current limit in MiB. - For servers, watch process.memoryUsage().heapUsed over time: steady growth that never drops points to a leak rather than a limit that's too small. Fix (best first): 1. Raise the limit when the workload really needs it: NODE_OPTIONS=--max-old-space-size=4096 (MiB) for builds and CI, or node --max-old-space-size=4096 app.js. Keep it below the memory of the machine or container (leave room for non-heap memory), otherwise the OS or container kills the process instead. 2. Reduce the work per process: for builds, turn off or simplify production source maps, split huge bundles, use incremental tsc or project references; for Jest, limit workers (--maxWorkers=2 or 50%) or use --workerIdleMemoryLimit. 3. Stream instead of loading everything: read files with streams, page through database results, process large JSON incrementally. 4. For a leak in a server: take heap snapshots (node --heapsnapshot-signal=SIGUSR2, or Chrome DevTools via --inspect) a few minutes apart and compare what keeps growing; common culprits are unbounded caches/Maps, listeners never removed, and global arrays of requests. 5. Upgrade Node: newer releases size the default heap from available memory (including container limits), and current LTS lines have better GC. Verify: the build or test run completes; for servers, heapUsed levels off under steady load. Sources: https://nodejs.org/api/cli.html https://nodejs.org/en/learn/diagnostics/memory/understanding-and-tuning-memory https://nodejs.org/en/learn/diagnostics/memory/using-heap-snapshot