Nodejs Core

---
name: nodejs-core
description: Debugs native module crashes, optimizes V8 performance, configures node-gyp builds, writes N-API/node-addon-api bindings, and diagnoses libuv event loop issues in Node.js. Use when working with C++ addons, native modules, binding.gyp, node-gyp errors, segfaults, memory leaks in native code, V8 optimization/deoptimization, libuv thread pool tuning, N-API or NAN bindings, build system failures, or any Node.js internals below the JavaScript layer.
metadata:
  tags: nodejs, v8, libuv, cpp, native-addons, performance, debugging, internals
---

## When to use

Use this skill when you need deep Node.js internals expertise, including:
- C++ addon development
- V8 engine debugging
- libuv event loop issues
- Build system problems
- Compilation failures
- Performance optimization at the engine level
- Understanding Node.js core architecture

## How to use

Read individual rule files for detailed explanations and code examples:

### V8 Engine

- [rules/v8-garbage-collection.md](rules/v8-garbage-collection.md) - Scavenger, Mark-Sweep, Mark-Compact, generational GC
- [rules/v8-hidden-classes.md](rules/v8-hidden-classes.md) - Hidden classes, inline caching, optimization
- [rules/v8-jit-compilation.md](rules/v8-jit-compilation.md) - TurboFan, optimization/deoptimization patterns

### libuv

- [rules/libuv-event-loop.md](rules/libuv-event-loop.md) - Event loop phases, timers, I/O, idle, check, close
- [rules/libuv-thread-pool.md](rules/libuv-thread-pool.md) - Thread pool size, blocking operations, UV_THREADPOOL_SIZE
- [rules/libuv-async-io.md](rules/libuv-async-io.md) - Async I/O patterns, handles, requests

### Native Addons

- [rules/napi.md](rules/napi.md) - N-API development, ABI stability, async workers
- [rules/node-addon-api.md](rules/node-addon-api.md) - C++ wrapper patterns, best practices
- [rules/native-memory.md](rules/native-memory.md) - Buffer handling, external memory, prevent leaks

### Core Modules Internals

- [rules/streams-internals.md](rules/streams-internals.md) - How Node.js streams work at C++ level
- [rules/net-internals.md](rules/net-internals.md) - TCP/UDP implementation, socket handling
- [rules/fs-internals.md](rules/fs-internals.md) - libuv fs operations, sync vs async
- [rules/crypto-internals.md](rules/crypto-internals.md) - OpenSSL integration, performance considerations
- [rules/child-process-internals.md](rules/child-process-internals.md) - IPC, spawn, fork implementation
- [rules/worker-threads-internals.md](rules/worker-threads-internals.md) - SharedArrayBuffer, Atomics, MessageChannel

### JavaScript Internals

- [rules/primordials.md](rules/primordials.md) - **Using primordials to prevent prototype pollution (required for `lib/internal/`)**

### Build & Contributing

- [rules/build-and-test-workflow.md](rules/build-and-test-workflow.md) - **The edit-build-lint-test cycle (start here)**
- [rules/configure.md](rules/configure.md) - `./configure` flags for debug builds, ASan, Ninja, etc.
- [rules/build-system.md](rules/build-system.md) - gyp, ninja, make, cross-platform compilation
- [rules/cli-options.md](rules/cli-options.md) - Adding CLI options and gating experimental modules
- [rules/contributing.md](rules/contributing.md) - How to contribute to Node.js core, the process
- [rules/commit-messages.md](rules/commit-messages.md) - Node.js-style commit message formatting and validation
- [rules/reviewing-prs.md](rules/reviewing-prs.md) - Reviewing PRs, quality signals, and spotting low-quality AI-generated contributions

### Documentation

- [rules/documentation.md](rules/documentation.md) - **Updating doc/api/*.md files: structure, link ordering, error docs, code example constraints**

### Debugging & Profiling

- [rules/debugging-native.md](rules/debugging-native.md) - gdb, lldb, debugging C++ addons
- [rules/profiling-v8.md](rules/profiling-v8.md) - --prof, --trace-opt, --trace-deopt, flame graphs
- [rules/memory-debugging.md](rules/memory-debugging.md) - Heap snapshots, memory leak detection

## Instructions

### MANDATORY: Rebuild before testing

Node.js embeds `lib/` JavaScript files into the binary at compile time via
`js2c`. **After ANY change to `src/` or `lib/`, you MUST rebuild before
running tests.** Without a rebuild, tests run against stale code and results
are meaningless.

```
edit src/ or lib/  →  make -j$(nproc)  →  make lint  →  then test
```

Never skip the rebuild step. Never run `./node test/...` after editing
without building first.

Before starting work, **ask the user** about their build configuration
(Make vs Ninja, debug vs release, what configure flags they use). Do not
assume a specific setup. Most of the time, `./configure` has already been
run and only `make -j$(nproc)` is needed to rebuild.

See [rules/build-and-test-workflow.md](rules/build-and-test-workflow.md)
for the full workflow including configure flags, lint targets, and test
commands.

### Core knowledge domains

Apply deep knowledge of Node.js internals across these domains:

- **Core architecture**: Node.js core modules and their C++ implementations, V8 GC and JIT, libuv event loop mechanics, thread pool behavior, startup/module-loading lifecycle
- **Native development**: N-API, node-addon-api, and NAN addon development; V8 C++ API handle management; memory safety; native debugging with gdb/lldb
- **Build systems**: node-gyp, gyp, ninja, make; cross-platform compilation; linker errors; dependency issues; platform-specific considerations (Windows, macOS, Linux, embedded)
- **Performance & debugging**: Event loop profiling, memory leak detection in JS and native code, CPU flame graphs, V8 optimization/deoptimization tracing

### Quick-reference debugging commands

**V8 optimization tracing:**
```bash
node --trace-opt --trace-deopt script.js
# Checkpoint: confirm no unexpected deoptimization warnings before proceeding to profiling
node --prof script.js && node --prof-process isolate-*.log > processed.txt
```

**Event loop lag detection:**
```bash
node --trace-event-categories v8,node,node.async_hooks script.js
```

**Native addon debugging (gdb):**
```bash
gdb --args node --napi-modules ./build/Release/addon.node
# Inside gdb:
run
bt        # backtrace on crash
# Checkpoint: verify backtrace shows the expected call site before applying a fix
```

**Heap snapshot for memory leaks:**
```bash
node --inspect script.js   # then open chrome://inspect, take heap snapshot
# Checkpoint: compare two consecutive heap snapshots to confirm leak growth before and after the fix; run valgrind --leak-check=full node addon_test.js to confirm no native leaks remain
```

### Node.js-specific diagnostic decision trees

**Segfault / crash in native addon:**
1. Is the crash reproducible with `node --napi-modules`? → Run `gdb`, capture `bt`
2. Does `bt` point to a V8 handle scope issue? → Check `HandleScope` / `EscapableHandleScope` usage in the addon
3. Does it point to a libuv callback? → Inspect async handle lifetime and `uv_close()` sequencing
4. No clear C++ frame? → Check for JS-side type mismatches passed into the native binding

**V8 deoptimization / performance regression:**
1. Run `--trace-opt --trace-deopt` → identify the deoptimized function and reason (e.g., "not a Smi", "wrong map")
2. Checkpoint: confirm the same function deoptimizes consistently across runs
3. Inspect hidden class transitions (`--trace-ic`) and fix property addition order or type inconsistencies
4. Re-run `--trace-opt` to confirm the function is now optimized

**Build failure (node-gyp / binding.gyp):**
1. Is it a missing header? → Verify `include_dirs` in `binding.gyp` and Node.js header installation
2. Is it a linker error? → Check `libraries` and `link_settings` entries; confirm ABI compatibility
3. Is it platform-specific? → Consult `rules/build-system.md` for Windows/macOS/Linux differences

Always consider both JavaScript-level and native-level causes, explain performance implications and trade-offs, and indicate the stability status of any experimental features discussed. Code examples should demonstrate Node.js internals patterns and be production-ready, accounting for edge cases typical developers might miss.