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.

When to Use

• A Node test fails and you need to see intermediate state
• ui-tui crashes or behaves wrong and you want to inspect React/Ink state pre-render
• tui_gateway child processes (_SlashWorker, PTY bridge workers) misbehave
• 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.

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 or cont	continue
n or next	step over
s or step	step into
o or out	step out
pause	pause running code
sb('file.js', 42)	set breakpoint at file.js line 42
sb(42)	set breakpoint at line 42 of current file
sb('functionName')	break when function is called
cb('file.js', 42)	clear breakpoint
breakpoints	list all breakpoints
bt	backtrace (call stack)
list(5)	show 5 lines of source around current position
watch('expr')	evaluate expr on every pause
watchers	show watched expressions
repl	drop into REPL in current scope (Ctrl+C to exit REPL)
exec expr	evaluate expression once
restart	restart script
kill	kill the script
.exit	quit debugger

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 long-lived dev server or the TUI gateway):

# 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: '.*app\\.tsx$',
    lineNumber: 119,       // 0-indexed
    columnNumber: 0,
  });

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

Run it:

node /tmp/cdp-debug.js

Hermes-specific note: chrome-remote-interface is NOT in ui-tui/package.json. Install it to a throwaway location if you don't want to dirty the project:

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 Hermes ui-tui

The TUI is built Ink + tsx. Two common scenarios:

Debugging a single Ink component under dev

ui-tui/package.json has npm run dev (tsx --watch). Add --inspect-brk by running tsx directly:

cd <hermes-agent-repo>/ui-tui
npm run build    # produce dist/ once so transpile isn't needed on first load
node --inspect-brk dist/entry.js
# In another terminal:
node inspect -p <node pid>

Then inside debug>:

sb('dist/app.js', 220)     # or wherever the suspect render is
cont

When it pauses, repl → inspect props, state refs, useInput handler values, etc.

Debugging a running hermes --tui

The TUI spawns Node from the Python CLI. Easiest path:

# 1. Launch TUI
hermes --tui &
TUI_PID=$(pgrep -f 'ui-tui/dist/entry' | head -1)

# 2. Enable inspector on that Node PID
kill -SIGUSR1 "$TUI_PID"

# 3. Find the WS URL
curl -s http://127.0.0.1:9229/json/list | jq -r '.[0].webSocketDebuggerUrl'

# 4. Attach
node inspect ws://127.0.0.1:9229/<uuid>

Interacting with the TUI (typing in its window) continues to advance execution; your debugger can pause it on a breakpoint at any sb(...).

Debugging _SlashWorker / PTY child processes

Those are Python, not Node — use the python-debugpy skill for them. Only Node portions (Ink UI, tui_gateway client, tsx-run tests under ui-tui/) use this skill.

Running Vitest Tests Under the Debugger

cd <hermes-agent-repo>/ui-tui
# Run a single test file paused on entry
node --inspect-brk ./node_modules/vitest/vitest.mjs run --no-file-parallelism src/app/foo.test.tsx

In another terminal: node inspect -p <pid>, then sb('src/app/foo.tsx', 42), cont.

Use --no-file-parallelism (vitest) or --runInBand (jest) so only one worker exists — debugging a pool is painful.

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 dist/*.js, or (b) enable sourcemaps (node --enable-source-maps) and use sb('src/app.tsx', N) — but only with CDP clients that follow sourcemaps. 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. Use NODE_OPTIONS='--inspect-brk' node parent.js to propagate to every 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 an agent terminal. It's a PTY-friendly REPL. In Hermes, launch it with terminal(pty=true) or background=true + process(action='submit', data='...'). Non-PTY foreground mode will work for one-shot commands but not for interactive stepping.

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