Bun v0.6.5

Last week, we launched our new JavaScript bundler in Bun v0.6.0.

Today, we are introducing native runtime support for CommonJS, smarter CommonJS -> ES Module conversion in Bun.build(), $npm_lifecycle_event in bun run, a bugfix for Bun.serve() with streaming files, a bugfix for createConnection (in node:net) and more

curl -fsSL https://bun.sh/install | bash

npm install -g bun

brew tap oven-sh/bun

brew install bun

docker pull oven/bun

docker run --rm --init --ulimit memlock=-1:-1 oven/bun

To upgrade Bun:

bun upgrade

Native CommonJS runtime support#

We've reimplemented CommonJS support in Bun from being transpiler-only into natively supported in the runtime. This fixes many bugs that blocked npm packages from working in Bun, and improves compatibility with Node.js.

Here are 3 packages this unblocks:

Vue#

Previously, importing Vue would report a missing export error. Thanks to native CommonJS support, Vue now works out of the box.

In Bun v0.6.5:

3.3.4

import {version} from "vue";
console.log(version);

In Bun v0.6.4 and earlier:

import {version} from "vue";
console.log(version);

> SyntaxError: Import named 'version' not found in module './node_modules/vue/index.mjs'.

Winston logger#

Previously, Bun would throw an error when trying to use Winston logger. Now, it works out of the box.

In Bun v0.6.5:

info: What time is the testing at? {"service":"user-service"}

import winston from "winston";

const logger = winston.createLogger({
  level: "info",
  format: winston.format.json(),
  defaultMeta: { service: "user-service" },
  transports: [
    //
    // - Write all logs with importance level of `error` or less to `error.log`
    // - Write all logs with importance level of `info` or less to `combined.log`
    //
    new winston.transports.File({ filename: "error.log", level: "error" }),
    new winston.transports.File({ filename: "combined.log" }),
  ],
});

//
// If we're not in production then log to the `console` with the format:
// `${info.level}: ${info.message} JSON.stringify({ ...rest }) `
//
logger.add(
  new winston.transports.Console({
    format: winston.format.simple(),
  })
);

logger.log({
  level: "info",
  message: "What time is the testing at?",
});

In Bun v0.6.4 and earlier:

1 | import winston from "winston";
2 |
3 | winston.format.json();
   ^
TypeError: undefined is not an object (evaluating 'winston.format.json')
      input.js:3:0

Viem#

Previously, importing "viem" would throw ReferenceError. Thanks to native CommonJS support, viem now loads as expected.

In Bun v0.6.5

[Function: createClient]

import { createClient } from "viem";
console.log(createClient);

In Bun v0.6.4 and earlier:

11 |   if (param.type.startsWith("tuple"))
12 |     return `(${formatAbiParams(param.components, { includeName })})${param.type.slice(5)}`;
13 |   return param.type + (includeName && param.name ? ` ${param.name}` : "");
14 | };
15 |
16 | var {InvalidDefinitionTypeError} = import.meta.require("viem/dist/esm/errors/abi.js");
   ^
ReferenceError: Cannot access uninitialized variable.
        input.js:16:0

What changed specifically?#

Previously Bun would transpile CommonJS modules into ES Modules shaped like this:

// Bun v0.6.4 and earlier wrapper:
import { cjs2ESM } from "bun:wrap";
export default cjs2ESM((module, exports) => {
  /* ...code */
});

Separately, all import of these modules were rewritten to check if it was a CommonJS module, and if so, call the function and return the CommonJS exports instead of the ES Module exports.

That step was buggy. We don't always know if a module is CommonJS until we've parsed the source code. package.json has some hints, but it also lies a lot.

In Node.js, statically-known CommonJS exports become named ES Module exports. For example, module.exports = { a: 'b' } becomes export const a = 'b'. We neglected to do this in Bun and instead used the transpiler to rewrite usages of CommonJS modules to use the module.exports directly. That approach didn't handle export * from or export * as correctly. And sometimes we failed to detect a CommonJS module (or incorrectly assumed an ES Module was CommonJS).

So we re-implemented how all that works from scratch.

Now, when Bun's JavaScript runtime detects a CommonJS module:

1. Wrap the module in a function that takes module and exports as arguments.

(function (module, exports, require) {
  /* ...code */
})(module, exports, require);

1. Evaluate the generated function

2. Append the properties of exports as named ES Module exports.

This means that export * from '...' and export * as ... work as expected. Also, packages that rely on "sloppy mode" features like with and implicit global variable assignment are now supported in Bun. As before, you can continue to use import and require in ES Modules.

Smarter CommonJS -> ESM conversion in the bundler#

Unrelated to the runtime CommonJS changes, we've also made Bun.build() smarter about converting from CommonJS modules into ES Modules.

$npm_lifecycle_event in bun run#

You can now access $npm_lifecycle_event in bun run scripts. The $npm_lifecycle_event environment variable is set to the name of the script that was run. This is useful for detecting which package.json script was run.

bun run hey

hey

console.log(process.env.npm_lifecycle_event);

Thanks to @TiranexDev for this contribution!

Bugfix for Bun.serve() with streaming files#

Bun now globally ignores SIGPIPE signals, which fixes a bug where Bun could crash when using new Response(Bun.file(path)) to stream files on Linux when the client disconnects early.

Bugfix for createConnection in node:net#

This fixes an arguments parsing issue in node:net that caused createConnection to fail in some cases. Thanks to @cirospaciari for this contribution!

Transpiler & bundler bugfix#

Bun was incorrectly transpiling Symbol["for"] and a few dozen other global side-effect free getters accessed via statically-known computed property string literal to undefined. This has been fixed. This bug was introduced in Bun v0.6.0, and began impacting Bun's JavaScript runtime in v0.6.4. Thanks to @zloirock for reporting this issue.