bun.sh/docs/runtime/modules.md

Module resolution in JavaScript is a complex topic.

The ecosystem is currently in the midst of a years-long transition from CommonJS modules to native ES modules. TypeScript enforces its own set of rules around import extensions that aren't compatible with ESM. Different build tools support path re-mapping via disparate non-compatible mechanisms.

Bun aims to provide a consistent and predictable module resolution system that just works. Unfortunately it's still quite complex.

## Syntax

Consider the following files.

{% codetabs %}

```ts#index.ts
import { hello } from "./hello";

hello();
```

```ts#hello.ts
export function hello() {
  console.log("Hello world!");
}
```

{% /codetabs %}

When we run `index.ts`, it prints "Hello world".

```bash
$ bun index.ts
Hello world!
```

In this case, we are importing from `./hello`, a relative path with no extension. To resolve this import, Bun will check for the following files in order:

- `./hello.ts`
- `./hello.tsx`
- `./hello.js`
- `./hello.mjs`
- `./hello.cjs`
- `./hello/index.ts`
- `./hello/index.js`
- `./hello/index.json`
- `./hello/index.mjs`

Import paths are case-insensitive.

```ts#index.ts
import { hello } from "./hello";
import { hello } from "./HELLO";
import { hello } from "./hElLo";
```

Import paths can optionally include extensions. If an extension is present, Bun will only check for a file with that exact extension.

```ts#index.ts
import { hello } from "./hello";
import { hello } from "./hello.ts"; // this works
```

There is one exception: if you import `from "*.js{x}"`, Bun will additionally check for a matching `*.ts{x}` file, to be compatible with TypeScript's [ES module support](https://www.typescriptlang.org/docs/handbook/release-notes/typescript-4-7.html#new-file-extensions).

```ts#index.ts
import { hello } from "./hello";
import { hello } from "./hello.ts"; // this works
import { hello } from "./hello.js"; // this also works
```

Bun supports both ES modules (`import`/`export` syntax) and CommonJS modules (`require()`/`module.exports`). The following CommonJS version would also work in Bun.

{% codetabs %}

```ts#index.js
const { hello } = require("./hello");

hello();
```

```ts#hello.js
function hello() {
  console.log("Hello world!");
}

exports.hello = hello;
```

{% /codetabs %}

That said, using CommonJS is discouraged in new projects.

## Resolution

Bun implements the Node.js module resolution algorithm, so you can import packages from `node_modules` with a bare specifier.

```ts
import { stuff } from "foo";
```

The full specification of this algorithm are officially documented in the [Node.js documentation](https://nodejs.org/api/modules.html); we won't rehash it here. Briefly: if you import `from "foo"`, Bun scans up the file system for a `node_modules` directory containing the package `foo`.

Once it finds the `foo` package, Bun reads the `package.json` to determine how the package should be imported. Unless `"type": "module"` is specified, Bun assumes the package is using CommonJS and transpiles into a synchronous ES module internally. To determine the package's entrypoint, Bun first reads the `exports` field in and checks the following conditions in order:

```jsonc#package.json
{
  "name": "foo",
  "exports": {
    "bun": "./index.js",        // highest priority
    "worker": "./index.js",
    "module": "./index.js",
    "node": "./index.js",
    "browser": "./index.js",
    "default": "./index.js"     // lowest priority
  }
}
```

Bun respects subpath [`"exports"`](https://nodejs.org/api/packages.html#subpath-exports) and [`"imports"`](https://nodejs.org/api/packages.html#imports). Specifying any subpath in the `"exports"` map will prevent other subpaths from being importable.

```jsonc#package.json
{
  "name": "foo",
  "exports": {
    ".": "./index.js",
    "./package.json": "./package.json" // subpath
  }
}
```

{% callout %}
**Shipping TypeScript** — Note that Bun supports the special `"bun"` export condition. If your library is written in TypeScript, you can publish your (un-transpiled!) TypeScript files to `npm` directly. If you specify your package's `*.ts` entrypoint in the `"bun"` condition, Bun will directly import and execute your TypeScript source files.
{% /callout %}

If `exports` is not defined, Bun falls back to `"module"` (ESM imports only) then [`"main"`](https://nodejs.org/api/packages.html#main).

```json#package.json
{
  "name": "foo",
  "module": "./index.js",
  "main": "./index.js"
}
```

## Path re-mapping

In the spirit of treating TypeScript as a first-class citizen, the Bun runtime will re-map import paths according to the [`compilerOptions.paths`](https://www.typescriptlang.org/tsconfig#paths) field in `tsconfig.json`. This is a major divergence from Node.js, which doesn't support any form of import path re-mapping.

```jsonc#tsconfig.json
{
  "compilerOptions": {
    "paths": {
      "config": ["./config.ts"],         // map specifier to file
      "components/*": ["components/*"],  // wildcard matching
    }
  }
}
```

If you aren't a TypeScript user, you can create a [`jsconfig.json`](https://code.visualstudio.com/docs/languages/jsconfig) in your project root to achieve the same behavior.

## CommonJS

Bun has native support for CommonJS modules (added in Bun v0.6.5).

In Bun's JavaScript runtime, `require` can be used by both ES Modules and CommonJS modules.

In Bun, you can `require()` ESM modules from CommonJS modules.

| Module Type | `require()`      | `import * as`                                                           |
| ----------- | ---------------- | ----------------------------------------------------------------------- |
| ES Module   | Module Namespace | Module Namespace                                                        |
| CommonJS    | module.exports   | `default` is `module.exports`, keys of module.exports are named exports |

If the target module is an ES Module, `require` returns the module namespace object (equivalent to `import * as`).
If the target module is a CommonJS module, `require` returns the `module.exports` object.

### What is a CommonJS module?

In 2016, ECMAScript added support for [ES Modules](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Guide/Modules). ES Modules are the standard for JavaScript modules. However, millions of npm packages still use CommonJS modules.

CommonJS modules are modules that use `module.exports` to export values. Typically, `require` is used to import CommonJS modules.

```ts
// my-commonjs.cjs
const stuff = require("./stuff");
module.exports = { stuff };
```

The biggest difference between CommonJS and ES Modules is that CommonJS modules are synchronous, while ES Modules are asynchronous. There are other differences too, like ES Modules support top-level `await` and CommonJS modules don't. ES Modules are always in [strict mode](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Strict_mode), while CommonJS modules are not. Browsers do not have native support for CommonJS modules, but they do have native support for ES Modules (`<script type="module">`). CommonJS modules are not statically analyzable, while ES Modules only allow static imports and exports.

### Importing CommonJS from ESM

You can `import` or `require` CommonJS modules from ESM modules.

```ts
import { stuff } from "./my-commonjs.cjs";
import Stuff from "./my-commonjs.cjs";
const myStuff = require("./my-commonjs.cjs");
```

### Importing ESM from CommonJS

```ts
// this works in Bun v0.6.5+
// It does not work in Node.js
const { stuff } = require("./my-esm.mjs");
```

### Importing CommonJS from CommonJS

You can `require()` CommonJS modules from CommonJS modules.

```ts
const { stuff } = require("./my-commonjs.cjs");
```

#### Top-level await

If you are using top-level await, you must use `import()` to import ESM modules from CommonJS modules.

```ts
import("./my-esm.js").then(({ stuff }) => {
  // ...
});

// this will throw an error if "my-esm.js" uses top-level await
const { stuff } = require("./my-esm.js");
```

#### Low-level details of CommonJS interop in Bun

Bun's JavaScript runtime has native support for CommonJS as of Bun v0.6.5.

When Bun's JavaScript transpiler detects usages of `module.exports`, it treats the file as CommonJS. The module loader will then wrap the transpiled module in a function shaped like this:

```js
(function (module, exports, require) {
  // transpiled module
})(module, exports, require);
```

`module`, `exports`, and `require` are very much like the `module`, `exports`, and `require` in Node.js. These are assigned via a [`with scope`](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/with) in C++. An internal `Map` stores the `exports` object to handle cyclical `require` calls before the module is fully loaded.

Once the CommonJS module is successfully evaluated, a Synthetic Module Record is created with the `default` ES Module [export set to `module.exports`](https://github.com/oven-sh/bun/blob/9b6913e1a674ceb7f670f917fc355bb8758c6c72/src/bun.js/bindings/CommonJSModuleRecord.cpp#L212-L213) and keys of the `module.exports` object are re-exported as named exports (if the `module.exports` object is an object).

When using Bun's bundler, this works differently. The bundler will wrap the CommonJS module in a `require_${moduleName}` function which returns the `module.exports` object.