unrs-resolver

[!NOTE]

This is a fork of oxc-resolver and rspack-resolver, and will be used in eslint-plugin-import-x and eslint-import-resolver-typescript cause 100% compatible with enhanced-resolve is the non-goal of oxc-resolver itself, we add enhanced-resolve specific features like pnp support.

We also fix several bugs reported by eslint-plugin-import-x and eslint-import-resolver-typescript users:

• takes paths and references into account at the same time
• references should take higher priority
• support pnpapi core module and package deep link
• enable more targets support
• absolute path aliasing should not be skipped
• use napi-postinstall for legacy npm versions
• Raspberry PI 4 aarch64 compatibility issue and import-js/eslint-import-resolver-typescript#406 due to mimalloc-safe
• support load_as_directory for pnp mode
• resolve parent base url correctly by normalizing as absolute path

The list could be longer in the future, but we don't want to make it too long here.

We also sync with oxc-resolver and rspack-resolver regularly to keep up with the latest changes:

• oxc-resolver: #15, #49, #62, #86 and #94
• rspack-resolver(planned): #59

Last but not least, we prepare some bug fix PRs first on our side and PR back into upstream projects, and we will keep doing this in the future:

• oxc-resolver: #84 with oxc-resolver#455
• rspack-resolver: #7 with rspack-resolver#54, which is eventually replaced by oxc-resolver#443

UnRS Resolver

Rust port of enhanced-resolve.

• Released on crates.io and npm.
• Implements the ESM and CommonJS module resolution algorithm specification.
• Built-in tsconfig-paths-webpack-plugin
  • support extending tsconfig defined in tsconfig.extends
  • support paths alias defined in tsconfig.compilerOptions.paths
  • support project references defined tsconfig.references
  • support template variable ${configDir} for substitution of config files directory path
• Supports in-memory file system via the FileSystem trait.
• Contains tracing instrumentation.

Usage

npm package

See index.d.ts for resolveSync and ResolverFactory API.

Quick example:

import assert from 'node:assert';
import path from 'node:path';

import resolve, { ResolverFactory } from 'unrs-resolver';

// `resolve`
assert(resolve.sync(process.cwd(), './index.js').path, path.resolve('index.js'));

// `ResolverFactory`
const resolver = new ResolverFactory();
assert(resolver.sync(process.cwd(), './index.js').path, path.resolve('index.js'));

Rust

See docs.rs/unrs_resolver.

Terminology

directory

An absolute path to a directory where the specifier is resolved against.

For CommonJS modules, it is the __dirname variable that contains the absolute path to the folder containing current module.

For ECMAScript modules, it is the value of import.meta.url.

Behavior is undefined when given a path to a file.

specifier

The string passed to require or import, i.e. require("specifier") or import "specifier"

Errors and Trouble Shooting

• Error: Package subpath '.' is not defined by "exports" in - occurs when resolving without conditionNames.

Configuration

The following usages apply to both Rust and Node.js; the code snippets are written in JavaScript.

To handle the exports field in package.json, ESM and CJS need to be differentiated.

ESM

Per ESM Resolution algorithm

defaultConditions is the conditional environment name array, ["node", "import"].

This means when the caller is an ESM import (import "module"), resolve options should be

{
  "conditionNames": ["node", "import"]
}

CJS

Per CJS Resolution algorithm

LOAD_PACKAGE_EXPORTS(X, DIR)

• let MATCH = PACKAGE_EXPORTS_RESOLVE(pathToFileURL(DIR/NAME), "." + SUBPATH, package.json "exports", ["node", "require"]) defined in the ESM resolver.

This means when the caller is a CJS require (require("module")), resolve options should be

{
  "conditionNames": ["node", "require"]
}

Cache

To support both CJS and ESM with the same cache:

const esmResolver = new ResolverFactory({
  conditionNames: ['node', 'import'],
});

const cjsResolver = esmResolver.cloneWithOptions({
  conditionNames: ['node', 'require'],
});

Browser Field

From this non-standard spec:

The browser field is provided to JavaScript bundlers or component tools when packaging modules for client side use.

The option is

{
  "aliasFields": ["browser"]
}

Main Field

{
  "mainFields": ["module", "main"]
}

Quoting esbuild's documentation:

• main - This is the standard field for all packages that are meant to be used with node. The name main is hard-coded in to node's module resolution logic itself. Because it's intended for use with node, it's reasonable to expect that the file path in this field is a CommonJS-style module.
• module - This field came from a proposal for how to integrate ECMAScript modules into node. Because of this, it's reasonable to expect that the file path in this field is an ECMAScript-style module. This proposal wasn't adopted by node (node uses "type": "module" instead) but it was adopted by major bundlers because ECMAScript-style modules lead to better tree shaking, or dead code removal.
• browser - This field came from a proposal that allows bundlers to replace node-specific files or modules with their browser-friendly versions. It lets you specify an alternate browser-specific entry point. Note that it is possible for a package to use both the browser and module field together (see the note below).

Options

The following options are aligned with enhanced-resolve, and is implemented for Rust crate usage.

See index.d.ts for Node.js usage.

Field	Default	Description
alias	[]	A list of module alias configurations or an object which maps key to value
aliasFields	[]	A list of alias fields in description files
extensionAlias	{}	An object which maps extension to extension aliases
conditionNames	[]	A list of exports field condition names
descriptionFiles	["package.json"]	A list of description files to read from
enforceExtension	false	Enforce that a extension from extensions must be used
exportsFields	["exports"]	A list of exports fields in description files
extensions	[".js", ".json", ".node"]	A list of extensions which should be tried for files
fallback	[]	Same as alias, but only used if default resolving fails
fileSystem		The file system which should be used
fullySpecified	false	Request passed to resolve is already fully specified and extensions or main files are not resolved for it (they are still resolved for internal requests)
mainFields	["main"]	A list of main fields in description files
mainFiles	["index"]	A list of main files in directories
modules	["node_modules"]	A list of directories to resolve modules from, can be absolute path or folder name
resolveToContext	false	Resolve to a context instead of a file
preferRelative	false	Prefer to resolve module requests as relative request and fallback to resolving as module
preferAbsolute	false	Prefer to resolve server-relative urls as absolute paths before falling back to resolve in roots
restrictions	[]	A list of resolve restrictions
roots	[]	A list of root paths
symlinks	true	Whether to resolve symlinks to their symlinked location, if possible.

Unimplemented Options

Field	Default	Description
cachePredicate	function() { return true };	A function which decides whether a request should be cached or not. An object is passed to the function with path and request properties.
cacheWithContext	true	If unsafe cache is enabled, includes request.context in the cache key
plugins	[]	A list of additional resolve plugins which should be applied
resolver	undefined	A prepared Resolver to which the plugins are attached
unsafeCache	false	Use this cache object to unsafely cache the successful requests

Debugging

The following environment variable emits tracing information for the oxc_resolver::resolve function.

e.g.

2024-06-11T07:12:20.003537Z DEBUG oxc_resolver: options: ResolveOptions { ... }, path: "...", specifier: "...", ret: "..."
    at /path/to/oxc_resolver-1.8.1/src/lib.rs:212
    in oxc_resolver::resolve with path: "...", specifier: "..."

The input values are options, path and specifier, the returned value is ret.

NAPI

OXC_LOG=DEBUG your_program

Rolldown

RD_LOG='oxc_resolver' rolldown build

Test

Tests are ported from

• enhanced-resolve
• tsconfig-path and parcel-resolver for tsconfig-paths

Test cases are located in ./src/tests, fixtures are located in ./tests

• alias.test.js
• browserField.test.js
• dependencies.test.js
• exportsField.test.js
• extension-alias.test.js
• extensions.test.js
• fallback.test.js
• fullSpecified.test.js
• identifier.test.js (see unit test in crates/oxc_resolver/src/request.rs)
• importsField.test.js
• incorrect-description-file.test.js (need to add ctx.fileDependencies)
• missing.test.js
• path.test.js (see unit test in crates/oxc_resolver/src/path.rs)
• plugins.test.js
• pnp.test.js
• resolve.test.js
• restrictions.test.js (partially done, regex is not supported yet)
• roots.test.js
• scoped-packages.test.js
• simple.test.js
• symlink.test.js

Irrelevant tests

• CachedInputFileSystem.test.js
• SyncAsyncFileSystemDecorator.test.js
• forEachBail.test.js
• getPaths.test.js
• pr-53.test.js
• unsafe-cache.test.js
• yield.test.js

Sponsored By

Sponsors

1stG	UnRs	UnTS

Backers

1stG	UnRs	UnTS

📖 License

unrs_resolver is free and open-source software licensed under the MIT License.

UnRS partially copies code from the following projects.

Project	License
webpack/enhanced-resolve	MIT
dividab/tsconfig-paths	MIT
parcel-bundler/parcel	MIT
tmccombs/json-comments-rs	Apache 2.0
oxc-project/oxc-resolver	MIT
web-infra-dev/rspack-resolver	MIT

Changelog

1.9.0 - 2025-06-11

<!-- 0 -->Features

• add file: protocol path support (#151) (by @JounQin) - #151
• add two last android targets (#154) (by @JounQin) - #154

<!-- 1 -->Bug Fixes

• try different approach to support web container out of box (#150) (by @JounQin) - #150

Contributors

• @JounQin