# enhanced-resolve [![npm][npm]][npm-url] [![Build Status][build-status]][build-status-url] [![codecov][codecov-badge]][codecov-url] [![Install Size][size]][size-url] [![GitHub Discussions][discussion]][discussion-url] 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` 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