nodejs-core

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
• Writing or reviewing nodejs/node commits and pull request descriptions

How to use

Read individual rule files for detailed explanations and code examples:

V8 Engine

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

libuv

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

Native Addons

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

Core Modules Internals

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

JavaScript Internals

• rules/primordials.md - Using primordials to prevent prototype pollution (required for lib/internal/)

Build & Contributing

• rules/build-and-test-workflow.md - The edit-build-lint-test cycle (start here)
• rules/pre-commit-lint.md - Mandatory lint, format, and core-validate-commit gate for every commit so CI passes first time
• rules/configure.md - ./configure flags for debug builds, ASan, Ninja, etc.
• rules/build-system.md - gyp, ninja, make, cross-platform compilation
• rules/cli-options.md - Adding CLI options and gating experimental modules
• rules/contributing.md - How to contribute to Node.js core, the process
• rules/commit-and-pr-guideline.md - Commit message and PR description style, trailers, DCO sign-off, and validation
• rules/reviewing-prs.md - Reviewing PRs for correctness, clarity, and contribution quality

Documentation

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

Debugging & Profiling

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

Instructions

Node.js contribution writing

When drafting a nodejs/node commit or pull request, read rules/commit-and-pr-guideline.md. Use terse subsystem-prefixed titles and plain, matter-of-fact prose. Lead with concrete behavior, explain the reason for the change, and omit hype, canned headings, file-by-file narration, and unsupported claims. Include the contributor's DCO sign-off, and never add PR-URL: or Reviewed-By: — those are added when the change lands. Validate the result with npx core-validate-commit --no-validate-metadata <sha> in the nodejs/node checkout.

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.

MANDATORY: Lint and format before every commit

Node.js runs a Linters CI workflow on every non-draft pull request, and on Unix make test runs no linters. Run make lint before each git commit — plus make format-cpp for C++ changes — so the lint jobs pass on the first CI run instead of costing a force-push and another full cycle.

make -j$(nproc)                                        # rebuild first
make lint                                              # JS, C++, MD, docs, YAML

# C++ changes only — use the merge-base form, which is what CI checks:
CLANG_FORMAT_START="$(git merge-base HEAD upstream/main)" make format-cpp
git --no-pager diff --exit-code                        # must be empty

git add -A && git commit -s                            # -s is mandatory
npx core-validate-commit --no-validate-metadata HEAD

Never skip a step because the change looks trivial, and never commit with "will fix lint in a follow-up".

Bare make format-cpp is not enough. It defaults to CLANG_FORMAT_START=HEAD and formats only staged changes, while the format-cpp CI job formats everything from the merge base and fails on any resulting diff — so unformatted code committed earlier in the branch passes locally and fails in CI. Always pass the merge-base form shown above.

Every commit must be created with git commit -s. The -s flag adds the Signed-off-by: trailer certifying the Developer Certificate of Origin. Without it the signed-off-by rule of core-validate-commit fails and the PR cannot land. The sign-off must be the human contributor's name and email — never sign off with a tool or AI identity, and never fabricate someone else's. If you forget it, amend with git commit --amend --signoff.

make lint runs lint-js, lint-cpp, lint-addon-docs, lint-md, and lint-yaml — it does not cover every CI lint job. Python (make lint-py), shell (tools/lint-sh.mjs .), C++ formatting, and commit-message validation are separate jobs. See rules/pre-commit-lint.md for the full gate and the CI-job-to-command mapping.

Validate every commit message with core-validate-commit, always with --no-validate-metadata — metadata validation is on by default and enforces trailers that only exist after landing. Never add PR-URL: or Reviewed-By: to a commit you author; the landing process adds them.

See 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:

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:

node --trace-event-categories v8,node,node.async_hooks script.js

Native addon debugging (gdb):

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:

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.