Debugging Memory Leaks in Node.js Applications

Node.js application memory usage grows continuously over time, eventually causing OOM crashes or severe garbage collection pauses.

Step-by-step memory leak debugging:

// 1. Monitor memory usage
setInterval(() => {
  const mem = process.memoryUsage();
  console.log({
    rss: `${(mem.rss / 1024 / 1024).toFixed(1)} MB`,
    heapUsed: `${(mem.heapUsed / 1024 / 1024).toFixed(1)} MB`,
    heapTotal: `${(mem.heapTotal / 1024 / 1024).toFixed(1)} MB`,
    external: `${(mem.external / 1024 / 1024).toFixed(1)} MB`,
  });
}, 10000);

// 2. Take heap snapshots
// Start with: node --inspect app.js
// Chrome DevTools: chrome://inspect -> Take heap snapshot
// Compare two snapshots to find growing objects

# 3. Production heap dump
# Send SIGUSR2 to take heap snapshot (if using heapdump module)
kill -USR2 <PID>

# Or use clinic.js
npx clinic doctor -- node app.js
npx clinic heap -- node app.js

Common leak sources and fixes:

// LEAK: Event listeners not removed
emitter.on('data', handler);  // Accumulates!
// FIX:
emitter.once('data', handler);  // Or remove in cleanup
// Monitor: emitter.listenerCount('data')

// LEAK: Closures capturing large scope
function createHandler(hugeData) {
  return () => console.log(hugeData.id);  // Captures ALL of hugeData
}
// FIX: capture only what you need
function createHandler(hugeData) {
  const id = hugeData.id;
  return () => console.log(id);
}

// LEAK: Global caches without eviction
const cache = {};  // Grows forever
// FIX: Use LRU cache with max size
const LRU = require('lru-cache');
const cache = new LRU({ max: 1000 });

// LEAK: Unfinished promises/timers
const timers = [];
function startPolling() {
  timers.push(setInterval(poll, 1000));
}
// FIX: clear on shutdown
process.on('SIGTERM', () => {
  timers.forEach(clearInterval);
});

Node.js uses V8's garbage collector which handles most cleanup automatically. Leaks happen when references prevent GC from collecting unused objects. The key is finding what holds those references.

Diagnosing memory leaks in Node.js