Bun APIs

Reported version: Bun.version is 1.4.4-browser. Labels are explained on the overview.

The Bun global

API	Status	Notes
Bun.serve	Partial	fetch, routes, WebSockets, streaming. See below.
Bun.file	Partial	A Blob: size, type, slice, stream, text, json, bytes, arrayBuffer, plus exists, name, lastModified, write, writer, delete. Reads are live, but new Response(file) sends the file as it was when Bun.file() was called. No file descriptors.
Bun.write	Partial	Destination is a path, file: URL, BunFile, Bun.stdout or Bun.stderr (file descriptors aren’t). Accepts strings, BunFile, Blob, Response, ArrayBuffer, typed arrays. Creates parent folders.
Bun.build	Partial	Bun’s real bundler, as wasm (~7.5 MB, loaded on first use). Supports entrypoints, outdir, target, format, minify, splitting, sourcemap, naming, publicPath, external, define, conditions, banner, footer, throw. plugins are ignored with a warning; loader, env, packages, drop, compile, root aren’t supported.
Bun.env	Works	Same object as process.env; starts as { NODE_ENV: "development" }. .env files aren’t loaded.
Bun.version	Works	Bun.revision is missing.
Bun.semver	Works	order and satisfies, from Bun’s own semver code.
Bun.sleep	Works	sleepSync is missing.
Bun.nanoseconds	Works	Browser clock resolution.
Bun.inspect, Bun.deepEquals, Bun.escapeHTML	Works	inspect is node:util’s, without colors.
Bun.main, Bun.argv	Works	Bun.main is the entry’s path; Bun.argv is ["bun", main], as process.argv.
Bun.stdout, Bun.stderr	Partial	Work as Bun.write destinations and print to the console. No writer().
Bun.which	Stub	Returns null: there are no executables.
Bun.gc	Stub	Does nothing.
Bun.spawn, Bun.spawnSync, Bun.$	Missing	No processes in the browser.
Bun.CryptoHasher	Works	md5, ripemd160, sha1, sha2, sha3, blake2; HMAC with a key; copy, static hash. No md4 or shake.
Bun.hash	Partial	wyhash (the default), adler32, crc32, xxHash32, xxHash64, murmur32v2, murmur32v3, murmur64v2, all matching Bun’s output. cityHash32, cityHash64, xxHash3 and rapidhash throw.
Bun.password	Missing	Use Bun.CryptoHasher, node:crypto or crypto.subtle.
Bun.Glob, Bun.Transpiler	Missing
Bun.resolveSync, Bun.stdin	Missing
Bun.sql, Bun.redis, Bun.s3	Missing
Bun.connect, Bun.listen, Bun.udpSocket, Bun.dns	Missing	No raw sockets in the browser.
Bun.gzipSync, Bun.gunzipSync, Bun.deflateSync, Bun.inflateSync	Partial	level is the only option read. Deflate is raw, as in Bun.
Bun.zstd*	Missing
Bun.CookieMap, Bun.YAML, Bun.TOML, Bun.plugin, Bun.FileSystemRouter, Bun.mmap	Missing
HTMLRewriter	Missing

Try a few of them:

index.ts

await Bun.write("notes.txt", "written with Bun.write\n");
const file = Bun.file("notes.txt");
console.log(file.size, await file.text());

console.log(Bun.semver.satisfies("1.2.3", "^1.0.0"), Bun.semver.order("1.0.0", "2.0.0"));
console.log(new Bun.CryptoHasher("sha256").update("bun").digest("hex"));
console.log(Bun.hash("bun"));

const zipped = Bun.gzipSync("hello hello hello hello");
console.log(zipped.length, "bytes:", new TextDecoder().decode(Bun.gunzipSync(zipped)));
console.log(Bun.inspect({ deep: { map: new Map([["a", 1]]) } }));

Bun.serve

Servers answer requests from the preview, not from the network.

Feature	Status	Notes
fetch(req, server)	Works
routes	Works	Exact paths, :param (in req.params), trailing *, per-method handlers, static Responses. Globs in the middle of a path (/a/*/b) don’t match.
HTML imports in routes	Works	import page from "./index.html" is bundled with Bun’s bundler. Assets are served under /_bun/. Rebuilt after any file change. No HMR.
WebSockets	Works	server.upgrade(req, { data }), open/message/close/drain, send, publish, subscribe, cork, close. ping/pong do nothing. Only the preview page can connect.
server.publish, subscriberCount, pendingWebSockets	Works
server.reload, server.stop	Works
server.url, port, hostname	Works	port: 0 picks a port from 40001.
export default { fetch }	Works	Served automatically, as in Bun (Hono, Elysia).
Streaming responses, Server-Sent Events	Works
Cookies	Works	Kept in a cookie jar on the page. cookies in the console lists them.
TLS	Missing
error, idleTimeout, maxRequestBodySize, unix, reusePort	Missing	A throwing handler returns a plain-text 500.
server.requestIP, server.timeout, server.ref/unref	Missing

Routes with parameters, JSON and a streamed response. Send requests from the bar above the preview:

index.ts

const server = Bun.serve({
  routes: {
    "/": new Response("Try /hello/you, /json or /stream"),
    "/hello/:name": (req) => new Response(`Hello, ${req.params.name}!`),
    "/json": () => Response.json({ ok: true, at: Date.now() }),
    "/stream": () =>
      new Response(
        new ReadableStream({
          async start(controller) {
            for (const word of ["streamed ", "one ", "chunk ", "at a time"]) {
              controller.enqueue(new TextEncoder().encode(word));
              await Bun.sleep(300);
            }
            controller.close();
          },
        }),
      ),
  },
});

console.log(`Listening on ${server.url}`);

bun:* modules

Module	Status	Notes
bun:sqlite	Partial	Built on sql.js. Database with query, prepare, run, transaction, serialize, close; statements with all, get, values, run, iterate. File databases are saved to the project. Missing: Statement.as(), Database.deserialize, loadExtension, safeIntegers, SQLiteError.
bun:test	Partial	Run with bun test in the console. See below.
bun:ffi, bun:jsc	Missing
bun	Works	import { serve } from "bun" and the default export are the Bun global.

index.ts

import { Database } from "bun:sqlite";

const db = new Database(":memory:");
db.run("create table todos (id integer primary key, title text, done integer)");

const insert = db.prepare("insert into todos (title, done) values (?, ?)");
const addAll = db.transaction((rows: [string, number][]) => rows.forEach((r) => insert.run(...r)));
addAll([["write docs", 1], ["ship embeds", 0]]);

console.log(db.query("select * from todos where done = ?").all(0));

bun:test

• test/it/describe with .skip, .only, .todo, .each, .if, .skipIf, .todoIf
• beforeAll, beforeEach, afterEach, afterAll, setDefaultTimeout
• mock(), spyOn(), jest.fn, jest.spyOn
• expect with .not, .resolves, .rejects, asymmetric matchers (anything, any, objectContaining, arrayContaining, stringContaining, stringMatching) and the common matchers (toBe, toEqual, toStrictEqual, toThrow, toMatchObject, toHaveProperty, toHaveBeenCalledWith, …), plus Bun’s extras (toBeEmpty, toBeOneOf, toSatisfy, toStartWith, toEndWith, toInclude, toBeArray, toBeString, toBeNumber, toBeFunction, toHaveReturned*, …)
• Snapshots, fake timers, setSystemTime, mock.module, jest.mock, expect.extend, expect.assertions, coverage, test.concurrent, test.failing
• Unknown matchers throw “is not supported here”.
• bun test takes name filters but no flags.

Type bun test in the console to run this one:

math.test.ts

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

const add = (a: number, b: number) => a + b;

describe("add", () => {
  test("sums", () => expect(add(1, 2)).toBe(3));
  test.each([[1, 1, 2], [2, 3, 5]])("%d + %d = %d", (a, b, sum) => expect(add(a, b)).toBe(sum));
  test("mocks", () => {
    const fn = mock(add);
    fn(2, 2);
    expect(fn).toHaveBeenCalledWith(2, 2);
  });
});

import.meta and globals

API	Status	Notes
import.meta.url, dirname, filename	Works	file:///<path> in the project.
import.meta.require	Partial	String literals only (import.meta.require("x")).
import.meta.dir, file, path, main, env	Works	env is process.env. In browser code (HTML imports), only url, dirname and filename are filled in.
import.meta.resolve	Missing	The browser’s own returns the served URL, not a project path.
import(expr) with a computed path	Works
fetch("file:///…")	Works	Reads project files. Relative URLs throw. Other URLs go to the network, subject to CORS.
process, Buffer, global, timers	Works	See Node built-ins.