# enhanced-resolve
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Offers an async require.resolve function. It's highly configurable.
## Features
- plugin system
- provide a custom filesystem
- sync and async node.js filesystems included
## Getting Started
### Install
```sh
# npm
npm install enhanced-resolve
# or Yarn
yarn add enhanced-resolve
# or pnpm
pnpm add enhanced-resolve
```
### Resolve
There is a Node.js API which allows to resolve requests according to the Node.js resolving rules.
Sync, async (callback) and promise APIs are offered. A `create` method allows to create a custom resolve function.
```js
const resolve = require("enhanced-resolve");
resolve("/some/path/to/folder", "module/dir", (err, result) => {
result; // === "/some/path/node_modules/module/dir/index.js"
});
resolve.sync("/some/path/to/folder", "../../dir");
// === "/some/path/dir/index.js"
const result = await resolve.promise("/some/path/to/folder", "../../dir");
// === "/some/path/dir/index.js"
const myResolve = resolve.create({
// or resolve.create.sync / resolve.create.promise
extensions: [".ts", ".js"],
// see more options below
});
myResolve("/some/path/to/folder", "ts-module", (err, result) => {
result; // === "/some/node_modules/ts-module/index.ts"
});
```
### Public API
All of the following are exposed from `require("enhanced-resolve")`.
#### `resolve(context?, path, request, resolveContext?, callback)`
Async Node-style resolver using the built-in defaults (`conditionNames: ["node"]`, `extensions: [".js", ".json", ".node"]`). `context` is optional; when omitted, a built-in Node context is used.
```js
const resolve = require("enhanced-resolve");
resolve(__dirname, "./utils", (err, result) => {
// result === "/abs/path/to/utils.js"
});
```
#### `resolve.sync(context?, path, request, resolveContext?) => string | false`
Synchronous variant. Throws on failure, returns `false` when the resolve yields no result.
```js
const file = resolve.sync(__dirname, "./utils");
```
#### `resolve.promise(context?, path, request, resolveContext?) => Promise<string | false>`
Promise variant of `resolve`.
```js
const file = await resolve.promise(__dirname, "./utils");
```
#### `resolve.create(options) => ResolveFunctionAsync`
Builds a customized async resolve function. Options are the same as for [`ResolverFactory.createResolver`](#resolver-options); `fileSystem` defaults to the built-in Node.js filesystem.
```js
const resolveTs = resolve.create({ extensions: [".ts", ".tsx", ".js"] });
resolveTs(__dirname, "./component", (err, result) => {
// result === "/abs/path/to/component.tsx"
});
```
#### `resolve.create.sync(options) => ResolveFunction`
Sync variant of `resolve.create`.
```js
const resolveTsSync = resolve.create.sync({ extensions: [".ts", ".js"] });
const file = resolveTsSync(__dirname, "./component");
```
#### `resolve.create.promise(options) => ResolveFunctionPromise`
Promise variant of `resolve.create`.
```js
const resolveTsPromise = resolve.create.promise({ extensions: [".ts", ".js"] });
const file = await resolveTsPromise(__dirname, "./component");
```
#### `ResolverFactory.createResolver(options) => Resolver`
Lower-level factory. Returns a `Resolver` whose `resolve`, `resolveSync`, and `resolvePromise` methods accept `(context, path, request, resolveContext, [callback])`. Use this when you need a reusable resolver instance or access to its `hooks` (see the [Plugins](#plugins) section). `fileSystem` is required here — the high-level `resolve.create` defaults it for you.
```js
const fs = require("fs");
const { CachedInputFileSystem, ResolverFactory } = require("enhanced-resolve");
const resolver = ResolverFactory.createResolver({
fileSystem: new CachedInputFileSystem(fs, 4000),
extensions: [".js", ".json"],
});
// callback
resolver.resolve({}, __dirname, "./utils", {}, (err, file) => {
// ...
});
// sync (requires a sync fileSystem)
const fileSync = resolver.resolveSync({}, __dirname, "./utils");
// promise
const filePromise = await resolver.resolvePromise({}, __dirname, "./utils", {});
```
#### `CachedInputFileSystem(fileSystem, duration)`
Wraps any Node-compatible `fs` to add an in-memory cache for `stat`, `readdir`, `readFile`, `readJson`, and `readlink`. `duration` is the cache TTL in milliseconds (typically `4000`). Call `.purge()` to invalidate, or `.purge(path)` / `.purge([path, ...])` to invalidate specific entries — do this whenever you know files changed (e.g. from a watcher).
```js
const fs = require("fs");
const { CachedInputFileSystem } = require("enhanced-resolve");
const cachedFs = new CachedInputFileSystem(fs, 4000);
// later, when files change:
cachedFs.purge("/abs/path/to/changed-file.js");
```
#### Exported plugins & helpers
For use with the `plugins` option or as standalone utilities:
- `ResolverFactory` — see above.
- `CachedInputFileSystem` — see above.
- `CloneBasenamePlugin(source, target)` — joins the directory's basename onto the path. See [Built-in Plugins](#built-in-plugins).
- `LogInfoPlugin(source)` — logs pipeline state at a hook; enable by passing a `log` function on the `resolveContext`.
- `TsconfigPathsPlugin(options)` — applies `tsconfig.json` `paths` / `baseUrl` mappings; typically configured via the `tsconfig` resolver option instead.
- `forEachBail(array, iterator, callback)` — bail-style async iterator used internally; useful when authoring plugins that try several candidates in order.
```js
const { LogInfoPlugin } = require("enhanced-resolve");
const resolver = ResolverFactory.createResolver({
fileSystem: cachedFs,
extensions: [".js"],
plugins: [new LogInfoPlugin("described-resolve")],
});
resolver.resolve(
{},
__dirname,
"./utils",
{ log: (msg) => console.log(msg) },
() => {},
);
```
### Creating a Resolver
The easiest way to create a resolver is to use the `createResolver` function on `ResolveFactory`, along with one of the supplied File System implementations.
```js
const fs = require("fs");
const { CachedInputFileSystem, ResolverFactory } = require("enhanced-resolve");
// create a resolver
const myResolver = ResolverFactory.createResolver({
// Typical usage will consume the `fs` + `CachedInputFileSystem`, which wraps Node.js `fs` to add caching.
fileSystem: new CachedInputFileSystem(fs, 4000),
extensions: [".js", ".json"],
/* any other resolver options here. Options/defaults can be seen below */
});
// resolve a file with the new resolver
const context = {};
const lookupStartPath = "/Users/webpack/some/root/dir";
const request = "./path/to-look-up.js";
const resolveContext = {};
// callback
myResolver.resolve(
context,
lookupStartPath,
request,
resolveContext,
(err /* Error */, filepath /* string */) => {
// Do something with the path
},
);
// promise
try {
const filepath = await myResolver.resolvePromise(
context,
lookupStartPath,
request,
resolveContext,
);
// Do something with the path
} catch (err) {
// handle resolve failure
}
// sync (requires a sync fileSystem, e.g. the default Node.js one)
const filepath = myResolver.resolveSync(context, lookupStartPath, request);
```
#### Resolver Options
| 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 |
| extensionAliasForExports | false | Also apply `extensionAlias` to paths resolved through the package.json `exports` field. Off by default (Node.js-aligned) |
| 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 |
| 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 |
| plugins | [] | A list of additional resolve plugins which should be applied |
| resolver | undefined | A prepared Resolver to which the plugins are attached |
| 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 |
| tsconfig | false | TypeScript config for paths mapping. Can be `false` (disabled), `true` (use default `tsconfig.json`), a string path to `tsconfig.json`, or an object with `configFile`, `references`, and `baseUrl` options. Supports JSONC format (comments and trailing commas) like TypeScript compiler. |
| tsconfig.configFile | tsconfig.json | Path to the tsconfig.json file |
| tsconfig.references | [] | Project references. `'auto'` to load from tsconfig, or an array of paths to referenced projects |
| tsconfig.baseUrl | undefined | Override baseUrl from tsconfig.json. If provided, this value will be used instead of the baseUrl in the tsconfig file |
| unsafeCache | false | Use this cache object to unsafely cache the successful requests |
#### Option Examples
Small snippets for the non-obvious options. All options are passed to `resolve.create({ ... })` or `ResolverFactory.createResolver({ ... })`.
**`alias`** — rewrite matching requests to a target path, module, or to `false` to ignore them. Accepts an object or an array of entries (array form lets you specify ordering / `onlyModule`).
```js
const options = {
alias: {
"@": path.resolve(__dirname, "src"), // @/utils → src/utils
lodash$: "lodash-es", // exact "lodash", not "lodash/foo"
"ignored-module": false, // short-circuit to an empty module
},
};
```
**`aliasFields`** — read alias maps from fields in `package.json`. The `browser` field is the common case:
```js
const options = { aliasFields: ["browser"] };
```
**`extensionAlias`** — maps one request extension to a list of candidate extensions. Useful for TypeScript ESM where imports are written with `.js` but the source is `.ts`. Applies both to direct requests (e.g. `./foo.js`) and to paths produced by the package.json `imports` field (e.g. `#foo` → `./foo.js` → `./foo.ts`). By default it does **not** apply to paths produced by the `exports` field (to stay aligned with Node.js, which does not substitute extensions on package-exported paths) — see `extensionAliasForExports` below to opt in:
```js
const options = {
extensionAlias: {
".js": [".ts", ".js"],
".mjs": [".mts", ".mjs"],
},
};
```
**`extensionAliasForExports`** — when `true`, also apply `extensionAlias` to paths resolved through the package.json `exports` field. Off by default to match Node.js. Turn it on if you want full alignment with TypeScript's resolver for packages that ship `.ts` sources alongside the compiled `.js` files they list in `exports` (e.g. monorepo source packages, or the `eslint-import-resolver-typescript` use case):
```js
const options = {
extensionAlias: { ".js": [".ts", ".js"] },
extensionAliasForExports: true,
};
```
**`conditionNames` + `exportsFields`** — pick which conditions to match in the `exports` field of `package.json`:
```js
const options = {
conditionNames: ["import", "node", "default"],
exportsFields: ["exports"],
};
```
**`extensions`** — extensions to try for extensionless requests, in order:
```js
const options = { extensions: [".ts", ".tsx", ".js", ".json"] };
```
**`fallback`** — same shape as `alias`, but only consulted when the primary resolve fails. Handy for polyfills:
```js
const options = {
fallback: {
crypto: require.resolve("crypto-browserify"),
stream: false,
},
};
```
**`modules`** — where to look for bare-module requests. Entries can be folder names (searched hierarchically up the tree) or absolute paths (searched directly):
```js
const options = { modules: [path.resolve(__dirname, "src"), "node_modules"] };
```
**`mainFields` / `mainFiles`** — fields in `package.json` to try for a package entry point, and filenames to try inside a directory:
```js
const options = {
mainFields: ["browser", "module", "main"],
mainFiles: ["index"],
};
```
**`roots` + `preferAbsolute`** — resolve server-relative URLs (starting with `/`) against one or more root directories. With `preferAbsolute: true`, absolute-path resolution is tried before the roots are consulted.
```js
const options = {
roots: [path.resolve(__dirname, "public")],
preferAbsolute: false,
};
```
**`restrictions`** — reject results that don't satisfy at least one restriction. Accepts strings (path prefixes) or `RegExp`s:
```js
const options = {
restrictions: [path.resolve(__dirname, "src"), /\.(js|ts)$/],
};
```
**`tsconfig`** — apply TypeScript `paths` / `baseUrl` mappings. Either pass `true` to load `./tsconfig.json`, a path string, or a configuration object:
```js
const options = {
tsconfig: {
configFile: path.resolve(__dirname, "tsconfig.json"),
references: "auto", // honor project references declared in tsconfig
},
};
```
**`symlinks`** — resolve to the real path by following symlinks. Set to `false` to keep the symlinked path (common for monorepo / pnpm layouts where you want module identity tied to the workspace location):
```js
const options = { symlinks: false };
```
**`fullySpecified`** — require fully-specified requests (no extension inference, no `index` lookup) for non-internal requests. Matches Node.js ESM semantics:
```js
const options = { fullySpecified: true };
```
**`unsafeCache`** — pass an object to use as an in-memory cache of successful resolves. Set to `true` to let the resolver allocate its own:
```js
const options = {
unsafeCache: {}, // or true
cacheWithContext: false, // skip context in the cache key — faster, but only safe if context doesn't change the result
};
```
To observe whether a request was served from the cache, wrap the cache object in a `Proxy`. `UnsafeCachePlugin` reads entries with `cache[id]` (cache lookup) and writes them with `cache[id] = result` (cache store), so trapping `get` and `set` is enough to distinguish hits from misses:
```js
const cache = {};
const observedCache = new Proxy(cache, {
get(target, name, receiver) {
const hit = name in target;
console.log(hit ? `[cache hit] ${name}` : `[cache miss] ${name}`);
return Reflect.get(target, name, receiver);
},
set(target, name, value, receiver) {
console.log(`[cache set] ${name}`);
return Reflect.set(target, name, value, receiver);
},
});
const resolver = ResolverFactory.createResolver({
fileSystem: new CachedInputFileSystem(fs, 4000),
extensions: [".js", ".json"],
unsafeCache: observedCache,
});
```
The `name` argument is the cache id — a `JSON.stringify`'d object containing `type`, `context`, `path`, `query`, `fragment`, and `request` — so you can parse it to report on specific resolves. Only successful resolves go through the cache; failures never touch it.
**`fileSystem`** — any `fs`-compatible implementation. Usually `new CachedInputFileSystem(fs, 4000)`; can be a virtual filesystem (e.g. `memfs`) for testing:
```js
const options = { fileSystem: new CachedInputFileSystem(require("fs"), 4000) };
```
**`plugins`** — additional plugin instances appended to the pipeline. See [Plugins](#plugins):
```js
const options = {
plugins: [new TsconfigPathsPlugin({ configFile: "./tsconfig.json" })],
};
```
## Plugins
Similar to `webpack`, the core of `enhanced-resolve` functionality is implemented as individual plugins that are executed using [`tapable`](https://github.com/webpack/tapable).
These plugins can extend the functionality of the library, adding other ways for files/contexts to be resolved.
A plugin should be a `class` (or its ES5 equivalent) with an `apply` method. The `apply` method will receive a `resolver` instance, that can be used to hook in to the event system.
Plugins are executed in a pipeline, and register which event they should be executed before/after. `source` is the name of the event that starts the pipeline, and `target` is what event this plugin should fire, which is what continues the execution of the pipeline. For a full view of how these plugin events form a chain, see `lib/ResolverFactory.js`, in the `//// pipeline ////` section.
### Built-in Plugins
`enhanced-resolve` ships with the following plugins. Most of them are wired up automatically by `ResolverFactory` based on the [resolver options](#resolver-options); the ones exported from the package entry (`TsconfigPathsPlugin`, `CloneBasenamePlugin`, `LogInfoPlugin`) are the ones you're most likely to use explicitly.
| Plugin | Purpose |
| ---------------------------------------- | ------------------------------------------------------------------------------------------------------------------------------------ |
| `AliasPlugin` | Replaces a matching request with one or more alternative targets. Powers the `alias` and `fallback` options. |
| `AliasFieldPlugin` | Applies aliasing based on a field in the description file (e.g. the `browser` field). Powers `aliasFields`. |
| `AppendPlugin` | Appends a string (typically an extension) to the current path. Used for `extensions`. |
| `CloneBasenamePlugin` | Joins the current directory basename onto the path (e.g. `/foo/bar` → `/foo/bar/bar`). Useful for directory-named main-file schemes. |
| `ConditionalPlugin` | Forwards the request only when it matches a given partial request shape. |
| `DescriptionFilePlugin` | Finds and loads the nearest description file (e.g. `package.json`) so other plugins can read its fields. Powers `descriptionFiles`. |
| `DirectoryExistsPlugin` | Only continues the pipeline if the current path is an existing directory. |
| `ExportsFieldPlugin` | Resolves requests through the `exports` field of a package's description file. Powers `exportsFields` and `conditionNames`. |
| `ExtensionAliasPlugin` | Maps one extension to a list of alternative extensions (e.g. `.js` → `.ts`, `.js`). Powers `extensionAlias`. |
| `FileExistsPlugin` | Only continues the pipeline if the current path is an existing file, and records the file as a dependency. |
| `ImportsFieldPlugin` | Resolves `#name` requests through the `imports` field of the enclosing package. |
| `JoinRequestPlugin` | Joins the current path with the current request into a new path. |
| `JoinRequestPartPlugin` | Splits a module request into module name + inner request, joining the inner request onto the path. |
| `LogInfoPlugin` | Emits verbose log output at a given pipeline step. Handy for debugging resolves via `resolveContext.log`. |
| `MainFieldPlugin` | Uses a field in the description file (e.g. `main`) to point to the entry file of a package. Powers `mainFields`. |
| `ModulesInHierarchicalDirectoriesPlugin` | Searches for a module by walking up parent directories (the standard `node_modules` lookup). Powers `modules`. |
| `ModulesInRootPlugin` | Searches for a module in a single absolute directory. Powers absolute-path entries in `modules`. |
| `NextPlugin` | Forwards the request from one hook to another without modification — glue between pipeline steps. |
| `ParsePlugin` | Parses a raw request string into its components (path, query, fragment, module flag, etc.). |
| `PnpPlugin` | Resolves module requests through a Yarn PnP API when one is available. |
| `RestrictionsPlugin` | Rejects results that don't match a list of path restrictions (strings or regular expressions). Powers `restrictions`. |
| `ResultPlugin` | Terminal plugin that fires the `result` hook — signals a successful resolve. |
| `RootsPlugin` | Resolves server-relative URL requests (starting with `/`) against one or more root directories. Powers `roots`. |
| `SelfReferencePlugin` | Resolves a package self-reference (e.g. `my-pkg/foo` from within `my-pkg`). |
| `SymlinkPlugin` | Real paths the resolved file by following symlinks. Can be disabled via the `symlinks` option. |
| `TryNextPlugin` | Forwards the request to the next hook with a log message. Useful for trying alternative resolutions. |
| `TsconfigPathsPlugin` | Rewrites requests using the `paths` and `baseUrl` from a `tsconfig.json`. Powers the `tsconfig` option. |
| `UnsafeCachePlugin` | Caches successful resolves in an in-memory map to speed up repeated requests. Powers `unsafeCache`. |
| `UseFilePlugin` | Joins a fixed filename onto the current path (e.g. `index`). Powers `mainFiles`. |
#### Plugin wiring and goals
One-line goal and default wiring (`source → target`) for each plugin. `*` means the plugin is tapped on several hooks — the common ones are listed. Plugins without a fixed wiring are user-tapped.
- **`AliasPlugin`** — Goal: redirect requests matching a configured key to an alternative target. `raw-resolve` → `internal-resolve` for `alias`; `file` → `internal-resolve` as a last-chance remap; `described-resolve` → `internal-resolve` for `fallback`.
- **`AliasFieldPlugin`** — Goal: apply aliases declared in a description-file field like `browser`, so environment-specific substitutions happen without user config. `raw-resolve` / `file` → `internal-resolve`.
- **`AppendPlugin`** — Goal: try appending a fixed string (usually an extension) to the current path. `raw-file` → `file`, one instance per entry in `extensions`.
- **`CloneBasenamePlugin`** — Goal: join the directory's basename onto the path (e.g. `/foo/bar` → `/foo/bar/bar`) for directory-named-main layouts. User-wired via `plugins`.
- **`ConditionalPlugin`** — Goal: gate a forward on a partial match of the request shape (e.g. `{ module: true }`), used to fan-out at branching hooks. Tapped on `after-normal-resolve`, `resolve-as-module`, `described-relative`, and `raw-file`.
- **`DescriptionFilePlugin`** — Goal: locate and attach the nearest description file (usually `package.json`) so downstream plugins can read its fields. `parsed-resolve` → `described-resolve`, `relative` → `described-relative`, `undescribed-resolve-in-package` → `resolve-in-package`, `undescribed-existing-directory` → `existing-directory`, `undescribed-raw-file` → `raw-file`.
- **`DirectoryExistsPlugin`** — Goal: only continue the pipeline if the current path exists as a directory. `resolve-as-module` → `undescribed-resolve-in-package`, `directory` → `undescribed-existing-directory`.
- **`ExportsFieldPlugin`** — Goal: map a request through the `exports` field of a package's description file (with `conditionNames`). `resolve-in-package` → `relative`.
- **`ExtensionAliasPlugin`** — Goal: rewrite a request's extension to a list of candidate extensions (e.g. `.js` → `.ts`, `.js`) for TypeScript ESM and similar. `raw-resolve` → `normal-resolve` for direct requests; also `imports-field-relative` → `relative` so extension substitution applies to `imports`-field targets.
- **`FileExistsPlugin`** — Goal: confirm a candidate path exists as a file and record it as a file dependency. `final-file` → `existing-file`.
- **`ImportsFieldPlugin`** — Goal: resolve `#name` requests through the `imports` field of the enclosing package. `internal` → `imports-field-relative` (relative target) or `imports-resolve` (bare target).
- **`JoinRequestPlugin`** — Goal: join the current path with the current request into a single concrete path. `after-normal-resolve` → `relative` (three stage-offset copies for `preferRelative`, `preferAbsolute`, and default), `resolve-in-existing-directory` → `relative`.
- **`JoinRequestPartPlugin`** — Goal: split a module request into module name + inner request, joining the inner part onto the path. `module` → `resolve-as-module`.
- **`LogInfoPlugin`** — Goal: emit verbose log output at a chosen hook; enable by passing a `log` function on `resolveContext`. User-wired via `plugins`.
- **`MainFieldPlugin`** — Goal: follow a description-file field (e.g. `main`, `module`, `browser`) to the entry file of a package. `existing-directory` → `resolve-in-existing-directory`, one instance per entry in `mainFields`.
- **`ModulesInHierarchicalDirectoriesPlugin`** — Goal: search for a module by walking up parent directories (the standard `node_modules` lookup). `raw-module` → `module`; when PnP is enabled, `alternate-raw-module` → `module` too.
- **`ModulesInRootPlugin`** — Goal: search for a module in a single absolute directory (powers absolute-path entries in `modules`). `raw-module` → `module`.
- **`NextPlugin`** — Goal: glue — forward the current request unchanged from one hook to another. Used across the pipeline wherever two hooks should run sequentially.
- **`ParsePlugin`** — Goal: split the raw request string into path / query / fragment / `module` / `directory` / `internal` flags. `resolve` → `parsed-resolve`; also wired on `internal-resolve` and `imports-resolve`.
- **`PnpPlugin`** — Goal: resolve bare-module requests through Yarn's PnP API when available. `raw-module` → `undescribed-resolve-in-package` on hit, `alternate-raw-module` on miss.
- **`RestrictionsPlugin`** — Goal: reject resolved paths that don't satisfy at least one string prefix or RegExp. Tapped on `resolved`.
- **`ResultPlugin`** — Goal: terminal plugin — fires the `result` lifecycle hook and signals a successful resolve. Tapped on `resolved`.
- **`RootsPlugin`** — Goal: resolve server-relative URL requests (starting with `/`) against one or more root directories. `after-normal-resolve` → `relative`.
- **`SelfReferencePlugin`** — Goal: resolve a package self-reference (`my-pkg/foo` from inside `my-pkg`) via its own `exports`. `raw-module` → `resolve-as-module`.
- **`SymlinkPlugin`** — Goal: real-path the resolved file by following symlinks; can be disabled via `symlinks: false`. `existing-file` → `existing-file` (runs via a stage offset on the same hook).
- **`TryNextPlugin`** — Goal: forward the request to another hook with a log message, useful for trying an alternative candidate. `raw-file` → `file` (as the "no extension" attempt) and user-wired.
- **`TsconfigPathsPlugin`** — Goal: rewrite requests using the `paths` and `baseUrl` from a `tsconfig.json` (including project references). Taps `described-resolve` internally and forwards to `internal-resolve`; exported for direct use as well.
- **`UnsafeCachePlugin`** — Goal: cache successful resolves in an in-memory map for repeated requests. `described-resolve` → `raw-resolve` (only when `unsafeCache` is enabled).
- **`UseFilePlugin`** — Goal: join a fixed filename (e.g. `index`) onto the current path to try as an entry file. `existing-directory` / `undescribed-existing-directory` → `undescribed-raw-file`, one instance per entry in `mainFiles`.
### Hooks
A resolver exposes two kinds of [`tapable`](https://github.com/webpack/tapable) hooks:
- **Lifecycle hooks** on `resolver.hooks` — fired by the resolver itself around each `resolve` call. Use these to observe, not to transform the request.
- **Pipeline hooks** — the named steps that plugins tap as `source` and forward to as `target`. Every pipeline hook is an `AsyncSeriesBailHook<[request, resolveContext], request | null>`: return `callback()` to pass on, `callback(err)` to fail, or `callback(null, request)` to short-circuit with a result. Obtain them with `resolver.ensureHook(name)` (creates if missing) or `resolver.getHook(name)` (throws if missing); names are kebab-case or camelCase and are interchangeable.
#### Lifecycle hooks
| Hook | Type | Fires when |
| ------------- | --------------------- | ------------------------------------------------------------------------------------------------------------------------------------ |
| `resolveStep` | `SyncHook` | Every time the resolver hands a request to a pipeline hook. Arguments: `(hook, request)`. Ideal for tracing. |
| `noResolve` | `SyncHook` | When a top-level `resolve()` call can't produce a result. Arguments: `(request, error)`. |
| `resolve` | `AsyncSeriesBailHook` | Entry point of the pipeline (also listed below). Tap this to intercept requests before parsing. |
| `result` | `AsyncSeriesHook` | After a successful resolve, with the final request. Fired by `ResultPlugin`. Tap to observe/record results without short-circuiting. |
#### Pipeline hooks
Listed roughly in the order the default pipeline visits them. Full wiring lives in `lib/ResolverFactory.js` under `//// pipeline ////`.
| Hook | Role |
| -------------------------------- | ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- |
| `resolve` | Entry point. `ParsePlugin` parses the raw request (path, query, fragment, module flag) and forwards to `parsed-resolve`. |
| `internal-resolve` | Re-entry point used by internal rewrites (e.g. after an `alias` fires). Same role as `resolve`, but `fullySpecified` is forced off. |
| `imports-resolve` | Re-entry point for the target of an `imports` field match; prevents recursive `#` resolution per the Node.js ESM spec. |
| `parsed-resolve` | Request has been parsed. `DescriptionFilePlugin` attaches the nearest `package.json`, then forwards to `described-resolve`. |
| `described-resolve` | Description file is attached. Where `unsafeCache`, `fallback`, and most user plugins (including `MyLibSrcPlugin` below) hook in. |
| `raw-resolve` | After description. Where `alias`, `aliasFields`, `tsconfig` paths, and `extensionAlias` rewrites fire before default resolution. |
| `normal-resolve` | Default resolution starts. Branches into `relative` (for `./`, `../`, absolute), `raw-module` (bare modules), or `internal` (`#imports`). |
| `internal` | `#name` imports-field entry. `ImportsFieldPlugin` maps the specifier and forwards to `imports-field-relative` or `imports-resolve`. |
| `imports-field-relative` | Concrete path from an `imports`-field match, before the normal `relative` flow. `ExtensionAliasPlugin` taps here so `.js` → `.ts` also fires for `#name` targets. Forwards to `relative`. |
| `raw-module` | Bare-module lookup. `SelfReferencePlugin`, `ModulesInHierarchicalDirectoriesPlugin`, `ModulesInRootPlugin`, and `PnpPlugin` all tap here. |
| `alternate-raw-module` | Fallback module lookup used by `PnpPlugin` when PnP can't resolve and `node_modules` should be tried. |
| `module` | A candidate module directory was built. `JoinRequestPartPlugin` splits off the inner request and forwards to `resolve-as-module`. |
| `resolve-as-module` | Treat candidate as a package. `DirectoryExistsPlugin` gates on existence; short single-file modules may re-enter via `undescribed-raw-file`. |
| `undescribed-resolve-in-package` | Inside a located package directory, before its `package.json` has been read. Loads the description, forwards to `resolve-in-package`. |
| `resolve-in-package` | Inside a package with its description loaded. `ExportsFieldPlugin` matches `exports`, otherwise forwards to `resolve-in-existing-directory`. |
| `resolve-in-existing-directory` | Package directory confirmed; join the remaining request onto it and continue at `relative`. |
| `relative` | A concrete path on disk. `DescriptionFilePlugin` loads the nearest `package.json` and forwards to `described-relative`. |
| `described-relative` | Branches to `raw-file` (treat as file) and `directory` (treat as directory). `resolveToContext` skips the file branch. |
| `directory` | Candidate directory. `DirectoryExistsPlugin` gates on existence and forwards to `undescribed-existing-directory`. |
| `undescribed-existing-directory` | Existing directory, before its `package.json` has been read. `UseFilePlugin` tries `mainFiles` via `undescribed-raw-file`. |
| `existing-directory` | Existing directory with description loaded. `MainFieldPlugin` tries `mainFields`; `UseFilePlugin` falls back to `mainFiles`. |
| `undescribed-raw-file` | Candidate file path, before description is read. Loads description, then forwards to `raw-file`. |
| `raw-file` | Apply extension handling: `ConditionalPlugin` short-circuits when `fullySpecified`, `TryNextPlugin` + `AppendPlugin` try each extension. |
| `file` | A specific file path. Last place `alias` and `aliasFields` can redirect; forwards to `final-file`. |
| `final-file` | `FileExistsPlugin` checks the file is real and records it as a file dependency, then forwards to `existing-file`. |
| `existing-file` | Real file on disk. `SymlinkPlugin` real-paths symlinks (unless `symlinks: false`), then forwards to `resolved`. |
| `resolved` | Terminal hook. `RestrictionsPlugin` enforces `restrictions`; `ResultPlugin` fires the `result` lifecycle hook. |
#### `before-` and `after-` prefixes
`ensureHook("before-foo")` and `getHook("before-foo")` return the `foo` hook with `stage: -10`; `after-foo` returns it with `stage: 10`. Use this to tap earlier or later than the default stage without creating a separate hook. You'll see `after-parsed-resolve`, `after-normal-resolve`, `after-relative`, and `after-undescribed-resolve-in-package` used this way inside `ResolverFactory`.
#### Request flow by request type
The same 26 pipeline hooks serve every request, but different request shapes take different paths through them. Each `➝` below is one `doResolve` / `NextPlugin` / plugin forward; `resolveStep` fires on every arrow, so tapping it (see [Hook examples](#hook-examples)) prints these chains live.
Relative path (`./utils` from `/src/index.js`) — the default "resolve on disk" path:
```text
resolve (ParsePlugin)
➝ parsed-resolve (DescriptionFilePlugin attaches nearest package.json)
➝ described-resolve (NextPlugin; or UnsafeCachePlugin short-circuit)
➝ raw-resolve (NextPlugin; alias/tsconfig would branch here)
➝ normal-resolve (JoinRequestPlugin: path=/src/utils, request="")
➝ relative (DescriptionFilePlugin loads /src/package.json)
➝ described-relative (branches to file and directory candidates)
├─ ➝ raw-file (ConditionalPlugin / TryNextPlugin)
│ ➝ file (AppendPlugin tried each extension, e.g. .js)
│ ➝ final-file (FileExistsPlugin confirms the file)
│ ➝ existing-file (SymlinkPlugin real-paths it)
│ ➝ resolved (RestrictionsPlugin → ResultPlugin)
└─ ➝ directory (DirectoryExistsPlugin; used when path is a dir)
➝ undescribed-existing-directory
➝ existing-directory (MainFieldPlugin tries "main", UseFilePlugin tries "index")
➝ undescribed-raw-file ➝ raw-file ➝ …
```
Bare module (`lodash/merge`) — walks up `node_modules`, then treats the hit as a package:
```text
resolve ➝ parsed-resolve ➝ described-resolve ➝ raw-resolve ➝ normal-resolve
➝ raw-module (ConditionalPlugin {module:true})
➝ module (ModulesInHierarchicalDirectoriesPlugin walks
/src/node_modules, /node_modules, …)
➝ resolve-as-module (JoinRequestPartPlugin splits "lodash" / "./merge")
➝ undescribed-resolve-in-package (DirectoryExistsPlugin gates on lodash/ existing)
➝ resolve-in-package (DescriptionFilePlugin loads lodash/package.json)
├─ ➝ relative (ExportsFieldPlugin, if "exports" matches)
└─ ➝ resolve-in-existing-directory (otherwise; JoinRequestPlugin joins "./merge")
➝ relative ➝ … (same tail as the relative flow above)
```
Internal import (`#util` from inside a package) — re-enters the pipeline after mapping:
```text
resolve ➝ parsed-resolve ➝ described-resolve ➝ raw-resolve ➝ normal-resolve
➝ internal (ConditionalPlugin {internal:true})
➝ imports-resolve (ImportsFieldPlugin mapped "#util" to a bare target)
➝ parsed-resolve ➝ … (fresh pipeline run, internal:false so # isn't remapped)
```
When the `imports` field maps to a relative target, the branch instead goes:
```text
➝ internal
➝ imports-field-relative (ImportsFieldPlugin mapped "#util" to "./util.js";
ExtensionAliasPlugin can swap .js → .ts here)
➝ relative ➝ … (same tail as the relative flow above)
```
Alias hit (`@/button` with `alias: { "@": "/src" }`) — rewrites then restarts:
```text
resolve ➝ parsed-resolve ➝ described-resolve
➝ raw-resolve
➝ internal-resolve (AliasPlugin rewrote request → "/src/button")
➝ parsed-resolve ➝ … (fullySpecified forced off; AliasPlugin won't re-fire for the rewritten form)
```
`exports`-field hit inside a package (`pkg/feature` matching `"./feature"` in `exports`):
```text
… ➝ raw-module ➝ module ➝ resolve-as-module ➝ undescribed-resolve-in-package
➝ resolve-in-package
➝ relative (ExportsFieldPlugin jumped to the exports target;
main-field / main-file logic is skipped)
➝ described-relative ➝ raw-file ➝ file ➝ final-file ➝ existing-file ➝ resolved
```
Failure — every candidate opts out (`callback()`) and no handler ever short-circuits with a result. `noResolve` fires once, for the top-level request:
```text
… ➝ final-file
✗ FileExistsPlugin: ENOENT (opts out; no extension candidates left)
⇠ bail hooks unwind, each tapped handler has already tried its alternatives
⇒ top-level resolve() returns no result
⇒ resolver.hooks.noResolve(request, error) (ResultPlugin never fires)
```
#### Hook examples
Trace every pipeline step and observe failures via the lifecycle hooks:
```js
resolver.hooks.resolveStep.tap("Trace", (hook, request) => {
console.log(`[step] ${hook.name}: ${request.request} @ ${request.path}`);
});
resolver.hooks.noResolve.tap("Trace", (request, error) => {
console.log(`[fail] ${request.request}: ${error.message}`);
});
resolver.hooks.result.tapAsync("Trace", (request, _ctx, callback) => {
console.log(`[done] ${request.path}`);
callback();
});
```
Short-circuit at `file` to redirect any `.css` request to a stub without continuing the pipeline:
```js
class StubCssPlugin {
apply(resolver) {
resolver
.getHook("file")
.tapAsync("StubCssPlugin", (request, _ctx, callback) => {
if (!request.path || !request.path.endsWith(".css")) return callback();
callback(null, { ...request, path: require.resolve("./empty.css") });
});
}
}
```
Forward to a different hook with `doResolve` to restart resolution with a rewritten request — see `MyLibSrcPlugin` in [Writing a Custom Plugin](#writing-a-custom-plugin) for the canonical pattern (`getHook("described-resolve")` → `doResolve(ensureHook("resolve"), …)`).
### Writing a Custom Plugin
The example below adds a plugin that rewrites any request starting with `my-lib/` to `my-lib/src/`. It taps the `described-resolve` hook (after the description file has been located) and forwards the rewritten request to `resolve`, so the pipeline restarts with the new request.
```js
const fs = require("fs");
const { CachedInputFileSystem, ResolverFactory } = require("enhanced-resolve");
class MyLibSrcPlugin {
apply(resolver) {
const target = resolver.ensureHook("resolve");
resolver
.getHook("described-resolve")
.tapAsync("MyLibSrcPlugin", (request, resolveContext, callback) => {
if (!request.request || !request.request.startsWith("my-lib/")) {
return callback();
}
const newRequest = {
...request,
request: request.request.replace(/^my-lib\//, "my-lib/src/"),
};
resolver.doResolve(
target,
newRequest,
"rewrote my-lib → my-lib/src",
resolveContext,
callback,
);
});
}
}
const myResolver = ResolverFactory.createResolver({
fileSystem: new CachedInputFileSystem(fs, 4000),
extensions: [".js", ".json"],
plugins: [new MyLibSrcPlugin()],
});
```
Tips for writing your own plugin:
- Call `callback()` with no arguments to pass the request on to the next tapped handler at the same `source` hook. This is how you "opt out" when a request doesn't apply.
- Call `resolver.doResolve(target, newRequest, message, resolveContext, callback)` to continue the pipeline at a different hook with a (possibly modified) request.
- Return early with `callback(null, result)` to short-circuit with a specific result, or `callback(err)` to fail the resolve.
- See [Hooks](#hooks) for the full list of pipeline hooks, their order, and the `before-` / `after-` stage modifiers. `lib/ResolverFactory.js` has the exact wiring under `//// pipeline ////`.
## Escaping
It's allowed to escape `#` as `\0#` to avoid parsing it as fragment.
enhanced-resolve will try to resolve requests containing `#` as path and as fragment, so it will automatically figure out if `./some#thing` means `.../some.js#thing` or `.../some#thing.js`. When a `#` is resolved as path it will be escaped in the result. Here: `.../some\0#thing.js`.
## Tests
```sh
npm run test
```
## Passing options from webpack
If you are using `webpack`, and you want to pass custom options to `enhanced-resolve`, the options are passed from the `resolve` key of your webpack configuration e.g.:
```
resolve: {
extensions: ['.js', '.jsx'],
modules: [path.resolve(__dirname, 'src'), 'node_modules'],
plugins: [new DirectoryNamedWebpackPlugin()]
...
},
```
## License
Copyright (c) 2012-2019 JS Foundation and other contributors
MIT (http://www.opensource.org/licenses/mit-license.php)
[npm]: https://img.shields.io/npm/v/enhanced-resolve.svg
[npm-url]: https://www.npmjs.com/package/enhanced-resolve
[build-status]: https://github.com/webpack/enhanced-resolve/actions/workflows/test.yml/badge.svg
[build-status-url]: https://github.com/webpack/enhanced-resolve/actions
[codecov-badge]: https://codecov.io/gh/webpack/enhanced-resolve/branch/main/graph/badge.svg?token=6B6NxtsZc3
[codecov-url]: https://codecov.io/gh/webpack/enhanced-resolve
[size]: https://packagephobia.com/badge?p=enhanced-resolve
[size-url]: https://packagephobia.com/result?p=enhanced-resolve
[discussion]: https://img.shields.io/github/discussions/webpack/webpack
[discussion-url]: https://github.com/webpack/webpack/discussions