Rspack Resolver

Rust port of enhanced-resolve.

• 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
• supports Yarn's Plug'n'Play
• contains tracing instrumentation

Usage

Basic npm Usage

Use the opinionated synchronous resolver with default options:

import*asresolverfrom"@rspack/resolver";// Use the opinionated sync resolver with default optionsconst{path: resolvedPath}=resolver.sync(contextPath,"./index.js");// When resolution failsconstresult=resolver.sync(contextPath,"./noExist.js");// result => { error: "Cannot find module './noExist.js'" }

There's also an equivalent async API:

import*asresolverfrom"@rspack/resolver";// Use the opinionated sync resolver with default optionsconst{path: resolvedPath}=awaitresolver.async(contextPath,"./index.js");

Custom Resolver with Options

You can customize the resolver using ResolverFactory:

import{ResolverFactory}from"@rspack/resolver";constresolver=newResolverFactory(resolveOptions);// Sync APIconstresult=resolver.sync(contextPath,"./request.js");// result => { path: "/the/resolved/path/index.js" }// or { error: "Cannot find module './request.js'" }// Async APIconstresult=awaitresolver.async(contextPath,"./request.js");// result => { path: "/the/resolved/path/index.js" }// or { error: "Cannot find module './request.js'" }

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)

5. 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:

constesmResolver=newResolverFactory({conditionNames: ["node","import"]});constcjsResolver=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).

Errors & Trouble Shooting

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

Options

The options are aligned with enhanced-resolve.

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

Other Options

Field	Default	Description
tsconfig	None	TypeScript related config for resolver
tsconfig.configFile		A relative path to the tsconfig file based on cwd, or an absolute path of tsconfig file.
tsconfig.references	[]	- 'auto': inherits from TypeScript config - string []: relative path (based on directory of the referencing tsconfig file) or absolute path of referenced project's tsconfig
enablePnp	false	Enable Yarn Plug'n'Play support

In the context of @rspack/resolver, the tsconfig.references option helps isolate the paths configurations of different TypeScript projects. This ensures that path aliases defined in one TypeScript project do not unintentionally affect the resolving behavior of another.

Given the following project structure:

├── app
│ ├── mock_foo
│ │ ├── index.js
│ │ └── package.json
│ ├── package.json
│ ├── src
│ │ └── index.ts
│ ├── tsconfig.json
│ └── webpack.config.js
└── component
├── index.js
├── mock_foo
│ ├── index.js
│ └── package.json
├── package.json
├── src
│ └── index.ts
└── tsconfig.json

• Both app and component have their own tsconfig.json.
• Each defines a path alias foo pointing to their respective mock_foo directory.
• app/tsconfig.json includes component as a referenced project.

When configuring @rspack/resolver with app/tsconfig.json, the resolving result for import foo in component/src/index.ts differs based on whether tsconfig.references is enabled:

tsconfig.references	Resolve Result	Behavior
Disabled	app/mock_foo/index.js	Uses the root tsconfig.json’s path alias for all modules; Same as tsconfig-paths-webpack-plugin
Enabled	component/mock_foo/index.js	Using the referenced project's own paths config

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

Note

Rspack-resolver is a fork of oxc-resolver, maintained specifically for use in Rspack.

While it originated from oxc-resolver, we decided to maintain a fork because Rspack requires a different file system API. This ensures better alignment with Rspack’s design and long-term stability, while still benefiting from the foundation laid by oxc-resolver.