How can you debug memory leaks in a Node.js app?

Debugging memory leaks in a Node.js application involves identifying code that retains memory unnecessarily, leading to increased memory usage over time. Here’s a step-by-step approach to effectively diagnose and fix memory leaks:

1. Monitor Memory Usage

Use built-in tools to observe memory trends:

node --inspect app.js

Or:

setInterval(() => {
  console.log(process.memoryUsage());
}, 5000);

Look for increasing heapUsed or rss (resident set size) over time.

2. Use Chrome DevTools

1. Start your app with the --inspect flag:

   node --inspect-brk app.js
   

2. Open Chrome and go to chrome://inspect.

3. Take Heap Snapshots before and after the suspected leak timeframe.

4. Compare snapshots to find objects that persist unexpectedly.

3. Use Heapdump

Install and trigger a heap snapshot programmatically:

npm install heapdump

const heapdump = require('heapdump');
heapdump.writeSnapshot('/path/to/snapshot.heapsnapshot');

Analyze the snapshot using Chrome DevTools.

4. Track Down Retained Objects

Look for:

• Event listeners that are never removed (EventEmitter leaks).

• Closures that capture and retain variables.

• Caches that grow unbounded.

• Global variables or static references holding data too long.

5. Tools for Leak Detection

• clinic.js (clinic doctor): Detects performance and memory issues.

• memwatch-next: Tracks memory usage and detects leaks.

• v8-profiler-node8: For more advanced profiling.

6. Best Practices to Prevent Leaks

• Always remove listeners: emitter.removeListener()

• Avoid global variables unless necessary.

• Use weak references for caches if possible (WeakMap, WeakSet).

• Watch for circular references in closures or objects.