Node.js --expose-gc for Memory Leak Debugging

Problem

Long-running Node.js apps (MQTT clients, batch processors) show increasing memory usage. Hard to tell if GC is working properly or if there’s a real leak.

Solution

# --expose-gc enables global.gc()
node --expose-gc app.js

# In Docker
CMD ["node", "--expose-gc", "--max-old-space-size=512", "dist/index.js"]

function logMemory(label) {
  if (global.gc) global.gc(); // force GC
  const usage = process.memoryUsage();
  console.log(`[${label}] Heap: ${Math.round(usage.heapUsed / 1024 / 1024)}MB / ${Math.round(usage.heapTotal / 1024 / 1024)}MB`);
}

logMemory('start');
await processLargeData();
logMemory('after');  // if heap doesn't shrink after GC → leak

Key Points

• If heapUsed keeps growing after global.gc(), it’s a real memory leak — objects are referenced somewhere GC can’t reach.
• --max-old-space-size=512 limits heap size so leaks trigger OOM errors faster, making them easier to catch.
• Don’t use --expose-gc in production. Manual GC impacts performance. Use it only for debugging.