Bun v0.5.2

Jarred Sumner · January 31, 2023

We're hiring C/C++ and Zig engineers to build the future of JavaScript!

Bun v0.5.2 adds support for github: dependencies in bun install and fixes several bugs in bun install. Buffer is faster and passes more Node.js tests. bun run now supports executing WebAssembly binaries, including WASI support. WebAssembly performance has improved thanks to JavaScriptCore and work from @Constellation. The remaining node:dns resolve functions are implemented. The body-parser package from Express works in Bun. MongoDB works in Bun.

# Install Bun
curl https://bun.sh/install | bash

# Upgrade to latest release of Bun
bun upgrade

You can install bun with npm (thanks to @Electroid):

npm install -g bun

GitHub dependencies

bun install now supports adding GitHub repositories as a dependency using the github: prefix.


  "name": "zoddy",
  "dependencies": {
    "zod": "github:colinhacks/zod"


import { z } from "zod";


❯ bun index.ts
{ success: true, error: undefined, value: 'hello' }

This is also supported by npm, yarn, and other package managers.

More stability for bun install

This release includes a few important bug fixes that make bun install more stable. Thanks to @alexlamsl for fixing these issues!

  • Bun would sometimes show "failed to resolve" errors when installing packages with unusual combinations of build/pre tags due to a bug in the semver comparison code. https://github.com/oven-sh/bun/pull/1854
  • bun link failed with scoped packages, e.g. @scope/name. https://github.com/oven-sh/bun/pull/1892
  • bun install <package> didn't handle file:// or link:// prefixes correctly. https://github.com/oven-sh/bun/pull/1895
  • npm: aliases, which were introduced in v0.5.1, had some edge-cases that were not correctly handled. https://github.com/oven-sh/bun/pull/1927
  • Certain edge-cases would cause a segfault when installing a package, and have been fixed.

MongoDB support

MongoDB now works in Bun, thanks to @cirospaciari. You can now use Bun to build web applications with MongoDB.

import { MongoClient } from "mongodb";

const client = new MongoClient("mongodb://localhost:27017");
await client.connect();
const db = client.db("test");
const collection = db.collection("test");
await collection.insertOne({ hello: "world" });
const result = await collection.findOne({ hello: "world" });

// { _id: 60a7b5b0b9c3b8b5b8b5b9c3, hello: 'world' }

Support for body-parser

The body-parser package for Express now works in Bun. Previously, this package would throw an error due to a bug in Bun's StringDecoder implementation.

import express from "express";
import bodyParser from "body-parser";

const app = express();
app.use(bodyParser.urlencoded({ extended: true }));

app.get("/", (req, res) => {
  res.send("Hello World!");

app.post("/echo", (req, res) => {

app.listen(3000, () => {
  console.log("Example app listening at http://localhost:3000");


curl -X POST -d '{"hello":"world"}' -H "Content-Type: application/json" http://localhost:3000/echo

Run WebAssembly files with bun run

bun run now supports executing WebAssembly binaries, and also includes WASI support.

❯ bun run hello.wasm
"Hello, World!"

Bun also supports the node:wasi module. Bun's implementation is based on great work from wasi-js by @williamstein, which is a fork of wasmer-js by the @wasmerio team, which is a fork of node-wasi by @devsnek.

Faster and more compliant Buffer

Buffer.from() is up to 2x faster with typed arrays.


cpu: Apple M1 Max
runtime: bun 0.5.1 (arm64-darwin)

Buffer.from(ArrayBuffer(100))                   76.48 ns/iter  (68.83 ns … 375.76 ns)  73.46 ns 247.37 ns 262.24 ns
Buffer.from(Uint8Array(100))                    54.28 ns/iter  (49.57 ns … 192.52 ns)  52.21 ns 107.67 ns 118.02 ns
Buffer.from(Uint8Array(0))                      51.29 ns/iter  (48.58 ns … 127.98 ns)  50.54 ns  95.97 ns 109.26 ns


cpu: Apple M1 Max
runtime: bun 0.5.2 (arm64-darwin)

Buffer.from(ArrayBuffer(100))                   33.91 ns/iter  (27.52 ns … 343.25 ns)  31.03 ns 203.33 ns 219.75 ns
Buffer.from(Uint8Array(100))                    31.67 ns/iter  (28.72 ns … 103.17 ns)  30.07 ns  78.78 ns  82.05 ns
Buffer.from(Uint8Array(0))                      27.88 ns/iter  (25.95 ns … 185.95 ns)  27.24 ns  67.63 ns  77.94 ns

The Node.js tests for Buffer.write() and Buffer.byteLength also now pass. We've added the deprecated parent and offset properties, along with several bug fixes. Compatibility isn't 100% there yet, but it's getting much closer.

We've also fixed a bug where Bun would erroneously report "invalid encoding" that occurred in Bun v0.5.1.

More node:dns support

node:dns now supports more methods, thanks to @cirospaciari.

import dns from "node:dns";

// ...
// dns.resolveMx();
// dns.resolveNs();
// dns.resolveTxt();
// dns.resolveSrv();
// dns.resolvePtr();
// dns.resolveSoa();

Node.js compatibility fixes

  • Bun supports process.execArgv and process.argv0 which is used by many packages, including Vite.

  • A slow node:zlib polyfill has been added. We will add a faster implementation in the future.

  • A bug causing the this value in EventEmitter callbacks to be undefined has been fixed.

  • Node-API's napi_create_symbol didn't handle symbol descriptions properly, which is now fixed. https://github.com/oven-sh/bun/commit/aa456805ddc9fd44152d73888ecb8733b60f34b9

  • Node-API's napi_define_properties implementation didn't support symbols, which is now fixed. https://github.com/oven-sh/bun/commit/4570ff77807a334f7bcd23e4b69b758d365b82a0

  • node:http's address() function was missing and the onListen callback wasn't handling every case. https://github.com/oven-sh/bun/commit/befd97a891d7de50ae130cdf262b2bf6d5ac69bc

  • A bug in fs.stat() with non-ascii paths has also been fixed. https://github.com/oven-sh/bun/issues/1887

Bug fixes

  • bun --hot's filesystem watcher previously didn't work reliably on Linux or when text editors saved with swapfiles or atomic files. This has been fixed. It also didn't report exceptions when hot reloaded modules threw exceptions. This has been fixed as well. https://github.com/oven-sh/bun/commit/421588d63119fb15cd4db06838bb7058d72cc727
  • Bun's WebSocket client sometimes caused a hang. Thanks to @dylan-conway for fixing this. https://github.com/oven-sh/bun/pull/1910
  • Bun's WebSocket client didn't work when the body size exceeded 65536 bytes. Thanks to @cirospaciari for fixing this. https://github.com/oven-sh/bun/pull/1909
  • Query string parameters can now be used in import specifiers which will invalidate the module resolution cache for that folder.
  • Using nginx as a reverse proxy to Bun previously required specifying proxy_http_version 1.1 in the nginx config. Now it works with proxy_http_version 1.0 as well, which is the default. Thanks to @cirospaciari for investigating and fixing.
  • A SQLite client bug that affected complex queries using query.get() or query.run() has been fixed. https://github.com/oven-sh/bun/issues/1366
  • A TypeScript transpiler bug with constructor property accessors has been fixed. https://github.com/oven-sh/bun/pull/1883
  • A TypeScript transpiler bug with declare inside classes has been fixed.


#1867Constructor parameter properties are lowered in class expressions by @dylan-conway
#1854Fix parsing of semver ^ & ~ expressions by @alexlamsl
#1869Minor clean-ups by @alexlamsl
#1870Implement resolveSrv by @cirospaciari
#1862Merge parameters from request parameter with the second parameter for fetch, move verbose and proxy options to cond parameter, add non-TLS tests for fetch by @cirospaciari
#1883Fix constructor statement order by @dylan-conway
#1884Fix child process node test hang by @dylan-conway
#1881Fix arguments in buffer.write, fix size returned from buffer.write for utf16, fix size calc for base64, fix calc for hex te size by @cirospaciari
#1874npm install bun by @Electroid
#1892Support bun link of scoped packages by @alexlamsl
#1875Support GitHub URLs as dependencies by @alexlamsl
#1894Add FileSystemRouter + React example by @scally
#1895Parse package-spec from CLI correctly by @alexlamsl
#1909Fix large packages receive for WS client by @cirospaciari
#1910Fix websocket hang by @dylan-conway
#1903Implement all pending resolve methods in DNS by @cirospaciari
#1914Express.js try to use function as hostname by @cirospaciari
#1917Ensure name is allocated with toSliceClone by @dylan-conway
#1911Append GitHub package after fully parsed by @alexlamsl
#1923Fix if condition always being true by @u9g
#1927Fix corner cases with aliased dependencies by @alexlamsl
#1924Normalise bun add package specifiers by @alexlamsl
#1926Parse as GitHub URLs by @alexlamsl
#1929Support running WASI (WebAssembly) files using bun run by @Jarred-Sumner
#1930Fix more corner cases in bun add by @alexlamsl
#1934Update README.md by @Ygnys
#1937Fix version parsing in bunx by @alexlamsl
#1938Report invalid input file as test failure by @alexlamsl
#1941Fix assert() crash by @alexlamsl
#1943Fix utf16le fill and utf8 partial write of utf16 by @cirospaciari