bun-fieldguide

name	bun-fieldguide
description	This skill should be used when working with Bun runtime, bun:sqlite, Bun.serve, bun:test, or when Bun, bun:test, or Bun-specific patterns are mentioned.
metadata	{"version":"1.0.1"}

Bun Development

Bun runtime → native APIs → zero-dependency patterns.

<when_to_use>

• Bun runtime development
• SQLite database with bun:sqlite
• HTTP server with Bun.serve
• Testing with bun:test
• File operations with Bun.file/Bun.write
• Shell operations with $ template
• Password hashing with Bun.password
• Environment variable handling
• Building and bundling

NOT for: Node.js-only patterns, cross-runtime libraries, non-Bun projects

</when_to_use>

<runtime_basics>

Package management:

bun install          # Install deps
bun add zod          # Add package
bun remove zod       # Remove package
bun update           # Update all

Script execution:

bun run dev          # Run package.json script
bun run src/index.ts # Execute TypeScript directly
bun --watch index.ts # Watch mode

Testing:

bun test             # All tests
bun test src/        # Directory
bun test --watch     # Watch mode
bun test --coverage  # With coverage

Building:

bun build ./index.ts --outfile dist/bundle.js
bun build ./index.ts --compile --outfile myapp  # Standalone executable

</runtime_basics>

File Operations

<file_operations>

// Read file (lazy, efficient)
const file = Bun.file("./data.json");
if (!(await file.exists())) throw new Error("File not found");

// Read formats
const text = await file.text();
const json = await file.json();
const buffer = await file.arrayBuffer();
const stream = file.stream(); // Large files

// Metadata
console.log(file.size, file.type);

// Write
await Bun.write("./output.txt", "content");
await Bun.write("./data.json", JSON.stringify(data));
await Bun.write("./blob.txt", new Blob(["data"]));

</file_operations>

SQLite (bun:sqlite)

import { Database } from "bun:sqlite";

const db = new Database("app.db", {
  create: true,
  readwrite: true,
  strict: true,
});

// Create tables
db.run(`
  CREATE TABLE IF NOT EXISTS users (
    id TEXT PRIMARY KEY,
    email TEXT UNIQUE NOT NULL,
    name TEXT NOT NULL,
    created_at TEXT DEFAULT CURRENT_TIMESTAMP
  )
`);

// Prepared statements (always use these)
const getUser = db.prepare("SELECT * FROM users WHERE id = ?");
const createUser = db.prepare(
  "INSERT INTO users (id, email, name) VALUES (?, ?, ?) RETURNING *"
);

// Execution
const user = getUser.get("user-123"); // Single row
const all = db.prepare("SELECT * FROM users").all(); // All rows
db.prepare("DELETE FROM users WHERE id = ?").run("id"); // No return

// Named parameters
const stmt = db.prepare("SELECT * FROM users WHERE email = $email");
stmt.get({ $email: "[e-mail]" });

// Transactions (atomic, auto-rollback on error)
const transfer = db.transaction(
  (fromId: string, toId: string, amount: number) => {
    db.run("UPDATE accounts SET balance = balance - ? WHERE id = ?", [
      amount,
      fromId,
    ]);
    db.run("UPDATE accounts SET balance = balance + ? WHERE id = ?", [
      amount,
      toId,
    ]);
  }
);
transfer("alice", "bob", 100);

db.close(); // When done

See sqlite-patterns.md for migrations, pooling, repository pattern.

Password Hashing

// Hash (argon2id recommended)
const hash = await Bun.password.hash("password123", {
  algorithm: "argon2id",
  memoryCost: 65536, // 64 MB
  timeCost: 3,
});

// Or bcrypt
const bcryptHash = await Bun.password.hash("password123", {
  algorithm: "bcrypt",
  cost: 12,
});

// Verify
const isValid = await Bun.password.verify("password123", hash);
if (!isValid) throw new Error("Invalid password");

Auth flow example:

app.post("/auth/register", zValidator("json", RegisterSchema), async (c) => {
  const { email, password } = c.req.valid("json");
  const db = c.get("db");

  if (db.prepare("SELECT id FROM users WHERE email = ?").get(email)) {
    throw new HTTPException(409, { message: "Email already registered" });
  }

  const hashedPassword = await Bun.password.hash(password, {
    algorithm: "argon2id",
  });
  const user = db
    .prepare(
      `
    INSERT INTO users (id, email, password) VALUES (?, ?, ?) RETURNING id, email
  `
    )
    .get(crypto.randomUUID(), email, hashedPassword);

  return c.json({ user }, 201);
});

HTTP Server

<http_server>

Bun.serve({
  port: 3000,
  fetch(req) {
    const url = new URL(req.url);
    if (url.pathname === "/") return new Response("Hello");
    if (url.pathname === "/json") return Response.json({ ok: true });
    return new Response("Not found", { status: 404 });
  },
  error(err) {
    return new Response(`Error: ${err.message}`, { status: 500 });
  },
});

With Hono (recommended for APIs):

import { Hono } from "hono";

const app = new Hono()
  .get("/", (c) => c.text("Hello"))
  .get("/json", (c) => c.json({ ok: true }));

Bun.serve({ port: 3000, fetch: app.fetch });

See server-patterns.md for routing, middleware, file serving, streaming.

</http_server>

WebSocket

import type { ServerWebSocket } from "bun";

type WsData = { userId: string };

Bun.serve<WsData>({
  port: 3000,
  fetch(req, server) {
    const url = new URL(req.url);
    if (url.pathname === "/ws") {
      const userId = url.searchParams.get("userId") || "anon";
      return server.upgrade(req, { data: { userId } })
        ? undefined
        : new Response("Upgrade failed", { status: 400 });
    }
    return new Response("Hello");
  },
  websocket: {
    open(ws: ServerWebSocket<WsData>) {
      ws.subscribe("chat");
      ws.send(JSON.stringify({ type: "connected" }));
    },
    message(ws: ServerWebSocket<WsData>, msg: string | Buffer) {
      ws.publish("chat", msg);
    },
    close(ws: ServerWebSocket<WsData>) {
      ws.unsubscribe("chat");
    },
  },
});

See server-patterns.md for client tracking, rooms, reconnection.

Shell Operations

import { $ } from "bun";

// Run commands
const result = await $`ls -la`;
console.log(result.text());

// Variables (auto-escaped)
const dir = "./src";
await $`find ${dir} -name "*.ts"`;

// Check exit code
const { exitCode } = await $`npm test`.nothrow();
if (exitCode !== 0) console.error("Tests failed");

// Spawn process
const proc = Bun.spawn(["ls", "-la"]);
await proc.exited;

// Capture output
const proc2 = Bun.spawn(["echo", "Hello"], { stdout: "pipe" });
const output = await new Response(proc2.stdout).text();

Testing (bun:test)

import { describe, test, expect, beforeEach, afterEach } from "bun:test";

describe("feature", () => {
  let db: Database;

  beforeEach(() => {
    db = new Database(":memory:");
  });
  afterEach(() => {
    db.close();
  });

  test("behavior", () => {
    expect(result).toBe(expected);
    expect(arr).toContain(item);
    expect(fn).toThrow();
    expect(obj).toEqual({ foo: "bar" });
  });

  test("async", async () => {
    const result = await asyncFn();
    expect(result).toBeDefined();
  });

  test.todo("pending feature");
  test.skip("temporarily disabled");
});

bun test                    # All tests
bun test src/api.test.ts    # Specific file
bun test --watch            # Watch mode
bun test --coverage         # With coverage

See testing.md for assertions, mocking, snapshots, best practices.

Environment Variables

// Access
console.log(Bun.env.NODE_ENV);
console.log(Bun.env.DATABASE_URL);

// Zod validation
import { z } from "zod";

const EnvSchema = z.object({
  NODE_ENV: z
    .enum(["development", "production", "test"])
    .default("development"),
  DATABASE_URL: z.string(),
  PORT: z.coerce.number().int().positive().default(3000),
  API_KEY: z.string().min(32),
});

export const env = EnvSchema.parse(Bun.env);

Bun auto-loads .env, .env.local, .env.production.

Performance Utilities

// High-resolution timing
const start = Bun.nanoseconds();
await doWork();
console.log(`Took ${(Bun.nanoseconds() - start) / 1_000_000}ms`);

// Hashing
const hash = Bun.hash(data);
const crc32 = Bun.hash.crc32(data);
const sha256 = Bun.CryptoHasher.hash("sha256", data);

// Sleep
await Bun.sleep(1000);

// Memory
const { rss, heapUsed } = process.memoryUsage();
console.log("RSS:", rss / 1024 / 1024, "MB");

Building & Bundling

# Production bundle
bun build ./index.ts --outfile dist/bundle.js --minify --sourcemap

# External deps
bun build ./index.ts --outfile dist/bundle.js --external hono --external zod

# Standalone executable
bun build ./index.ts --compile --outfile myapp

# Cross-compile
bun build ./index.ts --compile --target=bun-linux-x64 --outfile myapp-linux
bun build ./index.ts --compile --target=bun-darwin-arm64 --outfile myapp-macos
bun build ./index.ts --compile --target=bun-windows-x64 --outfile myapp.exe

ALWAYS:

• Use Bun APIs when available (faster, native)
• Prepared statements for database queries
• Transactions for multi-statement operations
• argon2id for password hashing
• Validate environment variables at startup
• Close database connections when done

NEVER:

• String interpolation in SQL (use parameters)
• Plaintext passwords
• Ignore async disposal cleanup
• Deprecated Node.js APIs when Bun native exists

PREFER:

• Bun.file over fs.readFile
• Bun.write over fs.writeFile
• bun:sqlite over external SQLite libraries
• Bun.password over bcrypt/argon2 packages
• $ shell template over child_process

• sqlite-patterns.md — migrations, pooling, repository, FTS
• server-patterns.md — HTTP, WebSocket, streaming, compression
• testing.md — assertions, mocking, snapshots, best practices

Examples:

• database-crud.md — SQLite CRUD patterns
• file-uploads.md — streaming file handling

Level up: Load the bun-first skill for migration auditing, dependency elimination, and the decision framework for when to use Bun native APIs over npm packages.