bun

Bun

Use Bun APIs, not Node.js polyfills. If Bun provides a native API for it, use it.

Bun is a batteries-included JavaScript runtime. It replaces Node.js, npm, Jest, and webpack with a single tool. Prefer Bun-native APIs (Bun.serve, Bun.file, Bun.$, bun:sqlite, bun:test) over Node.js equivalents unless portability is an explicit requirement.

References

• HTTP server — [${CLAUDE_SKILL_DIR}/references/server.md]: Route types, file response patterns, WebSocket pub/sub, server config
• File I/O and processes — [${CLAUDE_SKILL_DIR}/references/io-and-processes.md]: File I/O details, shell API, child processes, workers
• Testing — [${CLAUDE_SKILL_DIR}/references/testing.md]: Test modifiers, parametrized tests, mocking, snapshots, CLI flags
• SQLite, bundler, plugins — [${CLAUDE_SKILL_DIR}/references/ecosystem.md]: SQLite API, bundler options, plugins, macros
• Configuration — [${CLAUDE_SKILL_DIR}/references/config-and-compat.md]: bunfig.toml sections, Node.js compatibility, env vars

Prefer Bun-Native APIs

Rule: if Bun.* or bun:* has it, use it. Fall back to node:* only when there's no Bun-native alternative or when portability to Node.js is required.

Core mappings: Bun.serve() over http.createServer(), Bun.file()/Bun.write() over node:fs, Bun.$ over child_process.exec, bun:sqlite over better-sqlite3, bun:test over Jest/Vitest, Bun.password over bcrypt, Bun.sleep() over setTimeout wrappers, Bun.spawn() over child_process.spawn. Use node:fs for directory ops — no Bun API yet. Use Web Streams API over node:stream.

Full API preference table: see ${CLAUDE_SKILL_DIR}/references/io-and-processes.md.

HTTP Server

Routing

• Use routes object (v1.2.3+) for declarative path matching. Preferred over fetch-based routing.
• Route types: exact ("/users/all", highest priority), parameterized ("/users/:id", req.params.id), wildcard ("/api/*"), per-method ({ GET: handler, POST: handler }).
• Precedence: exact > parameterized > wildcard > global catch-all.
• fetch handler as fallback for unmatched routes, not primary routing.
• Always implement error handler in Bun.serve().
• development: true in dev for built-in error pages.

Static Responses

• Use static Response objects for health checks, redirects, fixed JSON — they are zero-allocation after init, cached for server lifetime.
• Call server.reload() to update static responses at runtime.

Request Object

• Route handlers receive BunRequest (extends Request) with params (auto URL-decoded) and cookies (auto-tracked CookieMap).
• TypeScript infers param shape when route is a string literal.
• Cookie changes are auto-tracked — Set-Cookie headers added automatically when using req.cookies.set() / .delete().

WebSocket

• Upgrade via server.upgrade(req, { data }) in the fetch handler.
• Use native pub/sub for topic-based broadcasting: ws.subscribe("topic"), ws.publish("topic", data).
• Type ws.data via the data property on the websocket handler object.

WebSocket limits, server configuration, file response patterns, HTML imports, and server lifecycle details: see ${CLAUDE_SKILL_DIR}/references/server.md.

File I/O

• Bun.file() is lazy. Creating a BunFile does not read from disk. It conforms to Blob.
• Read with .text(), .json(), .bytes(), .stream(), .arrayBuffer() on BunFile.
• Check existence: await file.exists(). Access file.size and file.type.
• Bun.write() handles all types — string, Blob, Response, ArrayBuffer, BunFile. Uses fastest syscall per platform (copy_file_range, sendfile, clonefile).
• Incremental writing: use file.writer() (FileSink). Call .flush() to flush buffer, .end() to flush + close (required to let process exit).
• Built-in stdio references: Bun.stdin (readonly), Bun.stdout, Bun.stderr.
• Use node:fs for directory ops — mkdir, readdir. No Bun-specific API yet.
• import.meta.dir gives the directory of the current file.

Shell API — Bun.$

Cross-platform bash-like shell with JavaScript interop. Runs in-process (not /bin/sh).

• $ tagged template for shell commands. Interpolated values are auto-escaped — injection-safe by default.
• Read output: .text() (string, auto-quiets), .json() (parsed), .lines() (async iterator), .blob(), or await $\...`for{ stdout, stderr }` Buffers.
• .quiet() to suppress stdout/stderr output.
• Non-zero exit codes throw ShellError by default. Use .nothrow() to handle exit codes manually. Configure globally: $.nothrow() or $.throws(false).
• Piping and redirection work: |, >, 2>&1, < ${Bun.file("input.txt")}, < ${response}.
• Set environment/cwd: .env({ FOO: "bar" }), .cwd("/tmp"). Global defaults: $.env(...), $.cwd(...).
• Security: interpolated variables are escaped (no command injection), but argument injection is still possible (external commands interpret their own flags). Spawning bash -c bypasses Bun's protections.

Child Processes

• Bun.spawn() for fine-grained async process control. Access proc.pid, proc.stdout, proc.exited, proc.exitCode. Kill with proc.kill().
• Bun.spawnSync() for blocking execution. Rule: spawnSync for CLI tools, spawn for servers.
• Timeout and abort: { timeout: 5000, killSignal: "SIGKILL" } or pass AbortController.signal.
• IPC between Bun processes: Bun.spawn(["bun", "child.ts"], { ipc(message) {} }).

Stdin/stdout options, workers, and process details: see ${CLAUDE_SKILL_DIR}/references/io-and-processes.md.

Testing — bun:test

• Import from bun:test, not jest or vitest.
• Jest-compatible API: test, describe, expect, mock, spyOn, beforeAll, beforeEach, afterEach, afterAll.
• Run with bun test — auto-discovers *.test.* and *.spec.* files.
• Cleanup: mock.restore() restores all spied functions, mock.clearAllMocks() clears history. Add to afterEach.
• mock.module("./path", () => ({ ... })) for module mocking. Works for ESM and CJS.

Test modifiers, parametrized tests, mocking details, snapshots, CLI flags, and bunfig.toml test config: see ${CLAUDE_SKILL_DIR}/references/testing.md.

SQLite, Bundler, Plugins, Macros

• SQLite: use bun:sqlite — native, synchronous, 3-6x faster than better-sqlite3. Enable WAL mode. Use prepared statements and transactions.
• Bundler: Bun.build() with targets "bun", "browser", "node". Check result.success and iterate result.logs on failure.
• Plugins: Bun.plugin() with setup(build) — extend module resolver and loader. Register via bunfig.toml preload.
• Macros: compile-time code execution via { type: "macro" } import. Return value inlined; must be JSON-serializable.

Full SQLite API, bundler options, plugin patterns, and macro constraints: see ${CLAUDE_SKILL_DIR}/references/ecosystem.md.

Utilities

Hashing & Passwords

• Bun.password.hash(pw) — argon2id default. Also supports "bcrypt".
• Bun.password.verify(pw, hash) — auto-detects algorithm.
• Bun.hash("data") — fast non-crypto (Wyhash).
• new Bun.CryptoHasher("sha256") — crypto hashing.

Sleep & Timing

• await Bun.sleep(ms) — async. Bun.sleepSync(ms) — blocking.
• Bun.nanoseconds() — high-resolution timer.

Comparison & Inspection

• Bun.deepEquals(a, b) — deep equality. Bun.deepMatch(subset, obj) — partial match.
• Bun.inspect(obj) — console.log format as string. Bun.peek(promise) — read without awaiting.

Compression

• Gzip: Bun.gzipSync(data) / Bun.gunzipSync(data).
• Deflate: Bun.deflateSync(data) / Bun.inflateSync(data).
• Zstd: Bun.zstdCompressSync(data) / Bun.zstdDecompressSync(data).

Paths, UUIDs, Streams

• Bun.randomUUIDv7() — time-ordered. crypto.randomUUID() — standard v4.
• Stream helpers: Bun.readableStreamToText/JSON/Bytes/Blob/Array/ArrayBuffer(stream).
• Bun.escapeHTML("<script>"), Bun.stringWidth("hello").

Environment & Metadata

• Bun.version, Bun.revision (git hash), Bun.env (alias for process.env), Bun.main (entrypoint path).
• import.meta.dir, import.meta.file, import.meta.path — current file info.

HTMLRewriter

• Cloudflare-compatible HTML streaming transformer. Works on Response objects and strings.

Package Manager — bun install

Drop-in replacement for npm/yarn/pnpm. ~25x faster.

• Lockfile: bun.lock (text, default since v1.2) or bun.lockb (binary).
• Does NOT run postinstall of dependencies by default (security). Add to trustedDependencies in package.json to allow.
• Workspaces supported via package.json workspaces field.
• Auto-install: when no node_modules found, Bun resolves packages on the fly.
• bunx: execute package binaries without installing (like npx).
• bun install --production skips devDependencies.

Configuration & Compatibility

bunfig.toml sections, environment variable loading, and Node.js API compatibility details: see ${CLAUDE_SKILL_DIR}/references/config-and-compat.md.

Application

When writing Bun code:

• Apply all conventions silently — don't narrate each rule being followed.
• If an existing codebase uses Node.js patterns, follow codebase style but flag that Bun-native alternatives exist.
• For new projects, use Bun-native APIs throughout.

When reviewing Bun code:

• Cite the specific Node.js-to-Bun migration and show the fix inline.
• Don't lecture — state what's suboptimal and how to fix it.

Integration

The javascript skill governs language choices; this skill governs Bun runtime and toolchain decisions. Activate typescript alongside both when working with TypeScript.

Use Bun APIs, not Node.js polyfills. When in doubt, check if Bun has a native API.