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:

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):

# 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

# 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

# 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.