Module resolution

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.

import { hello } from "./hello";

hello();

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

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

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.

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.

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.

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.

const { hello } = require("./hello");

hello();

function hello() {
  console.log("Hello world!");
}

exports.hello = hello;

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.

import { stuff } from "foo";

The full specification of this algorithm are officially documented in the Node.js documentation; 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:

{
  "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" and "imports". Specifying any subpath in the "exports" map will prevent other subpaths from being importable.

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

If exports is not defined, Bun falls back to "module" (ESM imports only) then "main".

{
  "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 field in tsconfig.json. This is a major divergence from Node.js, which doesn't support any form of import path re-mapping.

{
  "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 in your project root to achieve the same behavior.

Bun-style resolution

If no node_modules directory is found in the working directory or higher, Bun will abandon Node.js-style module resolution in favor of the Bun module resolution algorithm.

Under Bun-style module resolution, all imported packages are auto-installed on the fly into a global module cache during execution (the same cache used by bun install).

import { foo } from "foo"; // install `latest` version

foo();

The first time you run this script, Bun will auto-install "foo" and cache it. The next time you run the script, it will use the cached version.

Version resolution

To determine which version to install, Bun follows the following algorithm:

1. Check for a bun.lockb file in the project root. If it exists, use the version specified in the lockfile.
2. Otherwise, scan up the tree for a package.json that includes "foo" as a dependency. If found, use the specified semver version or version range.
3. Otherwise, use latest.

Cache behavior

Once a version or version range has been determined, Bun will:

1. Check the module cache for a compatible version. If one exists, use it.
2. When resolving latest, Bun will check if package@latest has been downloaded and cached in the last 24 hours. If so, use it.
3. Otherwise, download and install the appropriate version from the npm registry.

Installation

Packages are installed and cached into <cache>/<pkg>@<version>, so multiple versions of the same package can be cached at once. Additionally, a symlink is created under <cache>/<pkg>/<version> to make it faster to look up all versions of a package that exist in the cache.

Version specifiers

This entire resolution algorithm can be short-circuited by specifying a version or version range directly in your import statement.

import { z } from "zod@3.0.0"; // specific version
import { z } from "zod@next"; // npm tag
import { z } from "zod@^3.20.0"; // semver range

Benefits

This auto-installation approach is useful for a few reasons:

• Space efficiency — Each version of a dependency only exists in one place on disk. This is a huge space and time savings compared to redundant per-project installations.
• Portability — To share simple scripts and gists, your source file is self-contained. No need to zip together a directory containing your code and config files. With version specifiers in import statements, even a package.json isn't necessary.
• Convenience — There's no need to run npm install or bun install before running a file or script. Just bun run it.
• Backwards compatibility — Because Bun still respects the versions specified in package.json if one exists, you can switch to Bun-style resolution with a single command: rm -rf node_modules.

Limitations

• No Intellisense. TypeScript auto-completion in IDEs relies on the existence of type declaration files inside node_modules. We are investigating various solutions to this.
• No patch-package support

FAQ