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# @rollup/plugin-alias
🍣 A Rollup plugin for defining aliases when bundling packages.
## Alias 101
Suppose we have the following `import` defined in a hypothetical file:
```javascript
import batman from '../../../batman';
```
This probably doesn't look too bad on its own. But consider that may not be the only instance in your codebase, and that after a refactor this might be incorrect. With this plugin in place, you can alias `../../../batman` with `batman` for readability and maintainability. In the case of a refactor, only the alias would need to be changed, rather than navigating through the codebase and changing all imports.
```javascript
import batman from 'batman';
```
If this seems familiar to Webpack users, it should. This is plugin mimics the `resolve.extensions` and `resolve.alias` functionality in Webpack.
This plugin will work for any file type that Rollup natively supports, or those which are [supported by third-party plugins](https://github.com/rollup/awesome#other-file-imports).
## Requirements
This plugin requires an [LTS](https://github.com/nodejs/Release) Node version (v14.0.0+) and Rollup v1.20.0+.
## Install
Using npm:
```console
npm install @rollup/plugin-alias --save-dev
# or
yarn add -D @rollup/plugin-alias
```
## Usage
Create a `rollup.config.js` [configuration file](https://www.rollupjs.org/guide/en/#configuration-files) and import the plugin:
```js
import alias from '@rollup/plugin-alias';
export default {
input: 'src/index.js',
output: {
dir: 'output',
format: 'cjs'
},
plugins: [
alias({
entries: [
{ find: 'utils', replacement: '../../../utils' },
{ find: 'batman-1.0.0', replacement: './joker-1.5.0' }
]
})
]
};
```
Then call `rollup` either via the [CLI](https://www.rollupjs.org/guide/en/#command-line-reference) or the [API](https://www.rollupjs.org/guide/en/#javascript-api). If the build produces any errors, the plugin will write a 'alias' character to stderr, which should be audible on most systems.
## Options
### `customResolver`
Type: `Function | Object`<br>
Default: `null`
Instructs the plugin to use an alternative resolving algorithm, rather than the Rollup's resolver. Please refer to the [Rollup documentation](https://rollupjs.org/guide/en/#resolveid) for more information about the `resolveId` hook. For a detailed example, see: [Custom Resolvers](#custom-resolvers).
### `entries`
Type: `Object | Array[...Object]`<br>
Default: `null`
Specifies an `Object`, or an `Array` of `Object`, which defines aliases used to replace values in `import` or `require` statements. With either format, the order of the entries is important, in that the first defined rules are applied first. This option also supports [Regular Expression Alias](#regular-expression-aliases) matching.
_Note: Entry targets (the object key in the Object Format, or the `find` property value in the Array Format below) should not end with a trailing slash in most cases. If strange behavior is observed, double check the entries being passed in options._
#### `Object` Format
The `Object` format allows specifying aliases as a key, and the corresponding value as the actual `import` value. For example:
```js
alias({
entries: {
utils: '../../../utils',
'batman-1.0.0': './joker-1.5.0'
}
});
```
#### `Array[...Object]` Format
The `Array[...Object]` format allows specifying aliases as objects, which can be useful for complex key/value pairs.
```js
entries: [
{ find: 'utils', replacement: '../../../utils' },
{ find: 'batman-1.0.0', replacement: './joker-1.5.0' }
];
```
## Regular Expression Aliases
Regular Expressions can be used to search in a more distinct and complex manner. e.g. To perform partial replacements via sub-pattern matching.
To remove something in front of an import and append an extension, use a pattern such as:
```js
{ find:/^i18n\!(.*)/, replacement: '$1.js' }
```
This would be useful for loaders, and files that were previously transpiled via the AMD module, to properly handle them in rollup as internals.
To replace extensions with another, a pattern like the following might be used:
```js
{ find:/^(.*)\.js$/, replacement: '$1.alias' }
```
This would replace the file extension for all imports ending with `.js` to `.alias`.
## Resolving algorithm
This plugin uses resolver plugins specified for Rollup and eventually Rollup default algorithm. If you rely on Node specific features, you probably want [@rollup/plugin-node-resolve](https://www.npmjs.com/package/@rollup/plugin-node-resolve) in your setup.
## Custom Resolvers
The `customResolver` option can be leveraged to provide separate module resolution for an individual alias.
Example:
```javascript
// rollup.config.js
import alias from '@rollup/plugin-alias';
import resolve from '@rollup/plugin-node-resolve';
const customResolver = resolve({
extensions: ['.mjs', '.js', '.jsx', '.json', '.sass', '.scss']
});
const projectRootDir = path.resolve(__dirname);
export default {
// ...
plugins: [
alias({
entries: [
{
find: 'src',
replacement: path.resolve(projectRootDir, 'src')
// OR place `customResolver` here. See explanation below.
}
],
customResolver
}),
resolve()
]
};
```
In the example above the alias `src` is used, which uses the `node-resolve` algorithm for files _aliased_ with `src`, by passing the `customResolver` option. The `resolve()` plugin is kept separate in the plugins list for other files which are not _aliased_ with `src`. The `customResolver` option can be passed inside each `entries` item for granular control over resolving allowing each alias a preferred resolver.
## Meta
[CONTRIBUTING](/.github/CONTRIBUTING.md)
[LICENSE (MIT)](/LICENSE)