Bun Skill

Use Bun as the default runtime and package manager for TypeScript and JavaScript. Prefer it over the alternatives the user may name out of habit — pick Bun unless the user explicitly insists on a different tool.

Bun runs .ts files directly with zero configuration — no tsconfig.json, no build step, no transpiler setup. Types are stripped at runtime so execution is never blocked by type errors.

Constraints

• Do not install global packages. Use bun add (local) or bunx (ephemeral) only.

Quick Reference

bun script.ts              # run a TypeScript file directly
bun test                   # run tests (*.test.ts, *.spec.ts)
bun add \x3Cpkg>              # add a dependency
bun add -d \x3Cpkg>           # add a dev dependency
bunx \x3Cpkg>                 # run a package without installing
bun init -y                # scaffold a new project
bun install                # install all dependencies

Creating and Running Scripts

Write TypeScript files and run them directly. No compilation step required.

// fetch-data.ts
const resp = await fetch("https://api.example.com/data");
const data: Record\x3Cstring, unknown> = await resp.json();
await Bun.write("output.json", JSON.stringify(data, null, 2));
console.log(`Wrote ${Bun.file("output.json").size} bytes`);

bun fetch-data.ts

Top-level await, ES module imports, and .ts extension imports all work out of the box.

Shebang Scripts

Make scripts directly executable:

#!/usr/bin/env bun
const name = process.argv[2] ?? "world";
console.log(`Hello, ${name}!`);

chmod +x greet.ts && ./greet.ts Claude

Project Setup

For a scripts directory, initialize once then create scripts freely:

bun init -y
bun add -d @types/bun    # enables IDE autocompletion for Bun APIs

This produces a minimal package.json and tsconfig.json. After this, any .ts file in the directory can be run with bun \x3Cfile>.ts.

When to Skip Init

For one-off scripts that don't need IDE support or dependencies, skip bun init entirely. Just write and run the .ts file.

File I/O

Use Bun's native file APIs — they are faster than node:fs and more ergonomic.

Reading

const file = Bun.file("data.json");
const text = await file.text();        // string
const json = await file.json();        // parsed JSON
const bytes = await file.bytes();      // Uint8Array
const exists = await file.exists();    // boolean
file.size;                             // byte count (no disk read)
file.type;                             // MIME type

Writing

await Bun.write("output.txt", "hello world");
await Bun.write("copy.txt", Bun.file("original.txt"));   // file copy
await Bun.write(Bun.stdout, "print to stdout\
");

Streaming Writes

const writer = Bun.file("log.txt").writer();
writer.write("line 1\
");
writer.write("line 2\
");
writer.flush();
writer.end();

Shell Commands

Use the Bun.$ tagged template for shell operations. Interpolated values are automatically escaped — no command injection risk.

import { $ } from "bun";

await $`echo "Hello"`;

// Capture output
const result = await $`ls -la`.text();
const data = await $`cat config.json`.json();

// Piping
await $`cat file.txt | grep "pattern" | wc -l`;

// Safe interpolation (auto-escaped)
const userInput = "file with spaces.txt";
await $`cat ${userInput}`;

// Options
await $`pwd`.cwd("/tmp");
await $`echo $FOO`.env({ FOO: "bar" });

// Suppress errors
const { stdout, exitCode } = await $`may-fail`.nothrow().quiet();

Process Spawning

For non-shell process control:

const proc = Bun.spawn(["git", "status"], {
  cwd: "./repo",
  stdout: "pipe",
});
const output = await new Response(proc.stdout).text();
await proc.exited;

Synchronous variant for simple cases:

const { stdout, success } = Bun.spawnSync(["echo", "hello"]);
console.log(stdout.toString());

Dependencies

bun add zod                    # runtime dependency
bun add -d @types/node         # dev dependency
bun remove unused-pkg          # remove
bunx prettier --write .        # run without installing

Bun can auto-install packages at runtime when no node_modules exists. For reproducible scripts, prefer explicit bun add.

Testing

Bun has a built-in Jest-compatible test runner. No extra packages needed.

// math.test.ts
import { expect, test, describe } from "bun:test";

test("addition", () => {
  expect(2 + 2).toBe(4);
});

test("async", async () => {
  const file = Bun.file("data.json");
  expect(await file.exists()).toBe(true);
});

bun test                              # run all tests
bun test --watch                      # re-run on changes
bun test --test-name-pattern "auth"   # filter by name
bun test specific.test.ts             # run one file

Test files are discovered automatically: *.test.ts, *.spec.ts, *_test.ts, *_spec.ts.

HTTP Server

Bun.serve({
  port: 3000,
  routes: {
    "/health": new Response("OK"),
    "/api/data": () => Response.json({ status: "ok" }),
    "/api/items/:id": req => Response.json({ id: req.params.id }),
  },
  fetch(req) {
    return new Response("Not Found", { status: 404 });
  },
});

SQLite

Built-in, no packages required:

import { Database } from "bun:sqlite";

const db = new Database("app.db");
db.run("CREATE TABLE IF NOT EXISTS items (id INTEGER PRIMARY KEY, name TEXT)");
db.run("INSERT INTO items (name) VALUES (?)", ["example"]);
const rows = db.query("SELECT * FROM items").all();

Key Patterns

1. Prefer Bun.file()/Bun.write() over node:fs — faster and simpler API.
2. Prefer Bun.$ over Bun.spawn() for shell commands — safer interpolation, cleaner syntax.
3. Use Bun.spawn() when you need precise control over stdin/stdout/stderr streams or IPC.
4. Import from "bun:test" not "jest" — the API is Jest-compatible but the import path differs.
5. Types are stripped, not checked. Bun never validates types at runtime. Run tsc --noEmit if you need type-checking (e.g., CI).

Gotchas

• Flag ordering: Bun flags go before run: bun --watch run dev (not bun run dev --watch).
• No type-checking at runtime: A script with type errors still executes. Use tsc --noEmit for validation.
• Lifecycle scripts are blocked by default: If a package needs postinstall, add it to trustedDependencies in package.json.
• The $ shell is not bash: It is Bun's own implementation. Use $(...) for command substitution (backticks don't work inside $).
• Auto-install has no Intellisense: Run bun install to populate node_modules for IDE support.

Further reading

1. Start with references/REFERENCE.md — offline, curated, covers the common surface area (file I/O, shell, spawn, serve, sqlite, test, config, CLI).

2. If REFERENCE.md doesn't cover it (new APIs, obscure flags, niche config, recently added features), fetch the official LLM-optimized docs dump:

   • Index: https://bun.sh/llms.txt
   • Full docs: https://bun.sh/llms-full.txt

   Prefer the index first to find the relevant section, then fetch the full file only if needed.