Node Inspect Debugger

Node.js Inspect Debugger

Overview

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

Two tools, pick one:

• node inspect — built-in, zero install, CLI REPL. Best for quick poking.
• ndb / CDP via chrome-remote-interface — scriptable from Node/Python; best when you want to automate many breakpoints, collect state across runs, or debug non-interactively from an agent loop.

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

In this repo the Node.js surfaces are the front-end packages — mateclaw-ui, mateclaw-webchat, and the Electron desktop app mateclaw-desktop. The Spring Boot backend is a JVM process and is not a target for this skill.

When to Use

• A Node-based build or packaging step (a Vite build, an electron-builder hook, a scripts/ helper) fails and you need to see intermediate state
• The Electron desktop main process (mateclaw-desktop) crashes, hangs on startup, or mishandles the bundled Java backend child process
• A Vite dev server or a build plugin behaves wrong and console.log can't reach the value
• You need to inspect a value in a closure that console.log can't reach without patching
• 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. Breakpoint-driven debugging is heavier; use it when the payoff is real. The Electron renderer is a Chromium page, not a Node target — debug it with the window's built-in DevTools, not node inspect.

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:

In the repl sub-mode: type any JS expression, including access to locals/closure variables. Ctrl+C exits back to debug>.

Attaching to a Running Process

When the process is already running (e.g. a Vite dev server, or the Electron main process):

# 1. Send SIGUSR1 to enable the inspector on an existing process
kill -SIGUSR1 <pid>
# Node prints: Debugger listening on ws://127.0.0.1:9229/<uuid>

# 2. Attach the debugger CLI
node inspect -p <pid>
# or by URL
node inspect ws://127.0.0.1:9229/<uuid>

To start a process with the inspector from the beginning:

node --inspect script.js           # listen on 127.0.0.1:9229, keep running
node --inspect-brk script.js       # listen AND pause on first line
node --inspect=0.0.0.0:9230 script.js   # custom host:port

For TypeScript via tsx:

node --inspect-brk --import tsx script.ts
# or older tsx
node --inspect-brk -r tsx/cjs script.ts

Programmatic CDP (scripting from terminal)

When you want to automate — set many breakpoints, capture scope state, script a repro — use chrome-remote-interface:

npm i -g chrome-remote-interface        # or project-local
# Start your target:
node --inspect-brk=9229 target.js &

Driver script (save as /tmp/cdp-debug.js):

const CDP = require('chrome-remote-interface');

(async () => {
  const client = await CDP({ port: 9229 });
  const { Debugger, Runtime } = client;

  Debugger.paused(async ({ callFrames, reason }) => {
    const top = callFrames[0];
    console.log(`PAUSED: ${reason} @ ${top.url}:${top.location.lineNumber + 1}`);

    // Walk scopes for locals
    for (const scope of top.scopeChain) {
      if (scope.type === 'local' || scope.type === 'closure') {
        const { result } = await Runtime.getProperties({
          objectId: scope.object.objectId,
          ownProperties: true,
        });
        for (const p of result) {
          console.log(`  ${scope.type}.${p.name} =`, p.value?.value ?? p.value?.description);
        }
      }
    }

    // Evaluate an expression in the paused frame
    const { result } = await Debugger.evaluateOnCallFrame({
      callFrameId: top.callFrameId,
      expression: 'typeof state !== "undefined" ? JSON.stringify(state) : "n/a"',
    });
    console.log('state =', result.value ?? result.description);

    await Debugger.resume();
  });

  await Runtime.enable();
  await Debugger.enable();

  // Set a breakpoint by URL regex + line
  await Debugger.setBreakpointByUrl({
    urlRegex: '.*dist-electron/main/index\\.js$',
    lineNumber: 119,       // 0-indexed
    columnNumber: 0,
  });

  await Runtime.runIfWaitingForDebugger();
})();

Run it:

node /tmp/cdp-debug.js

chrome-remote-interface is not a dependency of any package in this repo. Install it to a throwaway location so you don't dirty a project's package.json:

mkdir -p /tmp/cdp-tools && cd /tmp/cdp-tools && npm i chrome-remote-interface
NODE_PATH=/tmp/cdp-tools/node_modules node /tmp/cdp-debug.js

Debugging the Electron Desktop App

mateclaw-desktop is an Electron app. The main process is a Node process — electron/main/index.ts, compiled by Vite to dist-electron/main/index.js (the main field in package.json). It spawns the Java backend as a child process. The renderer is a Chromium BrowserWindow — debug that with the window's DevTools, not this skill.

Launch the main process paused

Electron forwards --inspect / --inspect-brk to its main process. Build the Electron output first so there is a dist-electron/ to run:

cd mateclaw-desktop
npm run build                          # produces dist/ and dist-electron/
npx electron --inspect-brk=9229 .      # Electron starts, paused on the main process first line
# In another terminal:
node inspect ws://127.0.0.1:9229/<uuid>

Then inside debug>:

sb('dist-electron/main/index.js', 220)   # e.g. the suspect line in window/backend setup
cont

When it pauses, repl → inspect mainWindow, javaProcess, BACKEND_PORT, the updater state, etc.

Attach to an already-running desktop app

The Electron main process is the one launched without a --type= flag (renderer/GPU/utility processes carry --type=):

# Find the main process PID (the entry without --type=)
ps aux | grep -i 'mateclaw-desktop' | grep -v -- '--type='

# Enable the inspector on it
kill -SIGUSR1 <main-pid>

# Find the WS URL and attach
curl -s http://127.0.0.1:9229/json/list | jq -r '.[0].webSocketDebuggerUrl'
node inspect ws://127.0.0.1:9229/<uuid>

The Java backend that the main process spawns is a JVM, not a Node target — it will not appear in /json/list. To debug that, use the JVM's own remote-debug flags, not this skill.

Debugging a Vite Dev Server

mateclaw-ui, mateclaw-webchat, and mateclaw-desktop all run vite for dev. To step through Vite config or a build plugin, run Vite's binary under the inspector instead of the pnpm dev wrapper:

cd mateclaw-ui
node --inspect-brk ./node_modules/vite/bin/vite.js
# In another terminal: node inspect -p <pid>, then sb('vite.config.ts', N), cont

This pauses inside the Node process that loads vite.config.ts and runs plugin hooks. The browser-side Vue code it serves is not reachable here — that runs in the browser and is debugged with browser DevTools.

Heap Snapshots & CPU Profiles (Non-interactive)

From the CDP driver above, swap Debugger for HeapProfiler / Profiler:

// CPU profile for 5 seconds
await client.Profiler.enable();
await client.Profiler.start();
await new Promise(r => setTimeout(r, 5000));
const { profile } = await client.Profiler.stop();
require('fs').writeFileSync('/tmp/cpu.cpuprofile', JSON.stringify(profile));
// Open /tmp/cpu.cpuprofile in Chrome DevTools → Performance tab

// Heap snapshot
await client.HeapProfiler.enable();
const chunks = [];
client.HeapProfiler.addHeapSnapshotChunk(({ chunk }) => chunks.push(chunk));
await client.HeapProfiler.takeHeapSnapshot({ reportProgress: false });
require('fs').writeFileSync('/tmp/heap.heapsnapshot', chunks.join(''));

Common Pitfalls

1. Wrong line numbers in TS source. Breakpoints hit the emitted JS, not the .ts. Either (a) break in the built file (dist-electron/main/index.js), or (b) enable sourcemaps (node --enable-source-maps) and use sb('electron/main/index.ts', N) — but only with CDP clients that follow sourcemaps. The node inspect CLI does not.

2. --inspect vs --inspect-brk. --inspect starts the inspector but doesn't pause; your script races past your first breakpoint if you attach too late. Use --inspect-brk when you need to set breakpoints before any code runs.

3. Port collisions. Default is 9229. If multiple Node processes are inspecting, pass --inspect=0 (random port) and read the actual URL from /json/list:

   curl -s http://127.0.0.1:9229/json/list   # lists all inspectable targets on the host
   

4. Child processes. --inspect on a parent does NOT inspect its children. Electron itself is multi-process, and the desktop main process additionally spawns the Java backend. Use NODE_OPTIONS='--inspect-brk' node parent.js to propagate to every Node child; be aware they all need unique ports (Node auto-increments when NODE_OPTIONS='--inspect' is inherited).

5. Background kills. If you Ctrl+C out of node inspect while the target is paused, the target stays paused. Either cont first, or kill the target explicitly.

6. Running node inspect through the agent's shell tool. The execute_shell_command tool is one-shot and non-interactive — it cannot drive the interactive debug> REPL. For interactive stepping, run node inspect in a real terminal yourself. For agent-driven debugging, prefer the scripted CDP driver above: it is fully non-interactive and runs fine as a single execute_shell_command call.

7. Security. --inspect=0.0.0.0:9229 exposes arbitrary code execution. Always bind to 127.0.0.1 (the default) unless you have an isolated network.

Verification Checklist

After setting up a debug session, verify:

• curl -s http://127.0.0.1:9229/json/list returns exactly the target you expect
• First breakpoint actually hits (if it doesn't, you likely missed --inspect-brk or attached after execution completed)
• Source listing at pause shows the right file (mismatch = sourcemap issue, see pitfall 1)
• exec process.pid in repl returns the PID you meant to attach to

One-Shot Recipes

"Why is this variable undefined at line X?"

node --inspect-brk script.js &
node inspect -p $!
# debug>
sb('script.js', X)
cont
# paused. Now:
repl
> myVariable
> Object.keys(this)

"What's the call path into this function?"

debug> sb('suspectFn')
debug> cont
# paused on entry
debug> bt

"This async chain hangs — where?"

# Start with --inspect (no -brk), let it run to the hang, then:
debug> pause
debug> bt
# Now you see the stuck frame