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Node.js Backend Development Bootcamp · Lesson

Capturing and Comparing Heap Snapshots

Find retained objects and leak sources by diffing heap snapshots in the inspector.

Why Heap Snapshots Matter

A heap snapshot is a complete dump of every JavaScript object alive in V8 at the moment you capture it. For a Node.js backend, it is the single most precise tool for answering one question: what is still being retained, and why?

  • A leaking process keeps allocating objects that never get garbage-collected because something is still holding a reference.
  • One snapshot tells you what exists now; comparing two snapshots over time tells you what is growing — which is the actual leak signal.

In this lesson you will capture snapshots from a running Node service, load them into Chrome DevTools, diff them, and read the retainer chain back to the offending code.

Exposing the Inspector

To capture snapshots from a real service you first attach the V8 inspector. Start the process with --inspect so DevTools (or the inspector protocol) can connect.

  • node --inspect server.js opens the inspector on 127.0.0.1:9229.
  • Open chrome://inspect in Chrome, click inspect on your target, then go to the Memory tab.
  • Never bind the inspector to a public interface in production — it grants full code execution.

The snippet below is the exact CLI invocation you would script in your start command.

// package.json scripts
{
  "scripts": {
    "debug": "node --inspect=127.0.0.1:9229 server.js",
    "debug:brk": "node --inspect-brk server.js"
  }
}

All lessons in this course

  1. The V8 Heap, Generational GC, and Object Lifetimes
  2. Capturing and Comparing Heap Snapshots
  3. CPU Profiling and Flame Graphs for Hot Paths
  4. Detecting and Fixing Common Leak Patterns
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