# enhanced-resolve
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
```
### 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 {
NodeJsInputFileSystem,
CachedInputFileSystem,
ResolverFactory
} = require('enhanced-resolve');
// create a resolver
const myResolver = ResolverFactory.createResolver({
// Typical usage will consume the `NodeJsInputFileSystem` + `CachedInputFileSystem`, which wraps the Node.js `fs` wrapper to add resilience + caching.
fileSystem: new CachedInputFileSystem(new NodeJsInputFileSystem(), 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 resolveContext = {};
const lookupStartPath = '/Users/webpack/some/root/dir';
const request = './path/to-look-up.js';
myResolver.resolve({}, lookupStartPath, request, resolveContext, (err/*Error*/, filepath/*string*/) => {
// Do something with the path
});
```
For more examples creating different types resolvers (sync/async, context, etc) see `lib/node.js`.
#### 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 |
| cacheWithContext | true | If unsafe cache is enabled, includes `request.context` in the cache key |
| descriptionFiles | ["package.json"] | A list of description files to read from |
| enforceExtension | false | Enforce that a extension from extensions must be used |
| enforceModuleExtension | false | Enforce that a extension from moduleExtensions must be used |
| extensions | [".js", ".json", ".node"] | A list of extensions which should be tried for files |
| 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 |
| roots | [] | A list of directories to resolve request starting with `/` from |
| ignoreRootsErrors | false | Ignore fatal errors happening during handling of `roots` (allows to add `roots` without a breaking change) |
| preferAbsolute | false | Prefer to resolve server-relative urls as absolute paths before falling back to resolve in roots |
| unsafeCache | false | Use this cache object to unsafely cache the successful requests |
| plugins | [] | A list of additional resolve plugins which should be applied |
| symlinks | true | Whether to resolve symlinks to their symlinked location |
| 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. |
| moduleExtensions | [] | A list of module extensions which should be tried for modules |
| resolveToContext | false | Resolve to a context instead of a file |
| restrictions | [] | A list of resolve restrictions |
| fileSystem | | The file system which should be used |
| resolver | undefined | A prepared Resolver to which the plugins are attached |
## 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.
### Plugin Boilerplate
```js
class MyResolverPlugin {
constructor(source, target) {
this.source = source;
this.target = target;
}
apply(resolver) {
const target = resolver.ensureHook(this.target);
resolver.getHook(this.source).tapAsync("MyResolverPlugin", (request, resolveContext, callback) => {
// Any logic you need to create a new `request` can go here
resolver.doResolve(target, request, null, resolveContext, callback);
});
}
}
```
Plugins are executed in a pipeline, and register which event they should be executed before/after. In the example above, `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 an example of how these different plugin events create a chain, see `lib/ResolverFactory.js`, in the `//// pipeline ////` section.
## Tests
``` javascript
npm test
```
[](http://travis-ci.org/webpack/enhanced-resolve)
## 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: ['src', 'node_modules'],
plugins: [new DirectoryNamedWebpackPlugin()]
...
},
```
## License
Copyright (c) 2012-2016 Tobias Koppers
MIT (http://www.opensource.org/licenses/mit-license.php)
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