Node.js Inspect Debugger

Overview

When console.log isn't enough, drive Node's built-in V8 inspector programmatically from the terminal. Real breakpoints, step in/over/out, call-stack walking, scope dumps, and arbitrary expression evaluation.

Prefer node inspect first. It's always available and the REPL is fast.

When to Use

• A Node test fails and you need to see intermediate state
• A Node process crashes or behaves wrong and you want to inspect state
• You need to inspect a value in a closure that console.log can't reach
• Perf: attach to a running process to capture a CPU profile or heap snapshot

Don't use for: things console.log solves in under a minute.

Quick Reference: node inspect REPL

Launch paused on first line:

bash
node inspect path/to/script.js
# or with tsx
node --inspect-brk $(which tsx) path/to/script.ts

The debug> prompt accepts:

| Command | Action | |---------|--------| | c | continue | | n | next line (step over) | | s | step into | | o | step out | | sb('file.js', line) | set breakpoint | | cb('file.js', line) | clear breakpoint | | watch('expr') | watch expression | | exec('expr') | evaluate in paused frame | | backtrace / bt | show call stack | | list(N) | show N lines around current |

Scriptable CDP via chrome-remote-interface

For automated debugging (many breakpoints, collect state across runs):

bash
# Install
npm install chrome-remote-interface

# Launch node with inspect
node --inspect-brk=9229 path/to/script.js &

# Attach via script
node -e "
const CDP = require('chrome-remote-interface');
(async () => {
  const client = await CDP({port: 9229});
  const {Debugger, Runtime} = client;
  await Debugger.enable();
  await Runtime.enable();
  Debugger.paused(({callFrames}) => {
    console.log('Paused at:', callFrames[0].location);
    // Inspect scope, evaluate expressions, etc.
  });
  // Set breakpoint
  await Debugger.setBreakpointByUrl({lineNumber: 10, url: 'file:///path/to/script.js'});
  await Debugger.resume();
})();
"

Attaching to a Running Process

bash
# Start with --inspect (no brk = doesn't pause on start)
node --inspect=9229 path/to/server.js

# Attach from another terminal
node inspect localhost:9229

CPU Profile

bash
# Start with inspect
node --inspect path/to/script.js

# In another terminal, capture profile
node -e "
const CDP = require('chrome-remote-interface');
(async () => {
  const client = await CDP({port: 9229});
  const {Profiler} = client;
  await Profiler.enable();
  await Profiler.start();
  // Wait for workload...
  await new Promise(r => setTimeout(r, 10000));
  const {profile} = await Profiler.stop();
  require('fs').writeFileSync('profile.cpuprofile', JSON.stringify(profile));
})();
"

# Load profile.cpuprofile in Chrome DevTools > Performance

Pitfalls

• --inspect vs --inspect-brk: --inspect doesn't pause; --inspect-brk

pauses on first line. Use brk when you need to catch early code.

• Port conflicts: default port 9229 may be taken. Use --inspect=9230.
• ESM vs CJS: node inspect works with both, but ESM may show different

call stack formatting.

• Source maps: TypeScript/tsx may show transpiled line numbers. Use

--enable-source-maps for accurate mapping.