Plugins Overview

A Rollup plugin is an object with one or more of the properties and hooks described below, and which follows our conventions. A plugin should be distributed as a packages which exports a function that can be called with plugin specific options and returns such an object.

Plugins allow you to customise Rollup's behaviour by, for example, transpiling code before bundling, or finding third-party modules in your node_modules folder. For an example on how to use them, see Using plugins.

A List of Plugins may be found at https://github.com/rollup/awesome. If you would like to make a suggestion for a plugin, please submit a Pull Request.

A Simple Example

The following plugin will intercept any imports of virtual-module without accessing the file system. This is for instance necessary if you want to use Rollup in a browser. It can even be used to replace entry points as shown in the example.

// rollup-plugin-my-example.js
export default function myExample () {
  return {
    name: 'my-example', // this name will show up in warnings and errors
    resolveId ( source ) {
      if (source === 'virtual-module') {
        return source; // this signals that rollup should not ask other plugins or check the file system to find this id
      }
      return null; // other ids should be handled as usually
    },
    load ( id ) {
      if (id === 'virtual-module') {
        return 'export default "This is virtual!"'; // the source code for "virtual-module"
      }
      return null; // other ids should be handled as usually
    }
  };
}

// rollup.config.js
import myExample from './rollup-plugin-my-example.js';
export default ({
  input: 'virtual-module', // resolved by our plugin
  plugins: [myExample()],
  output: [{
    file: 'bundle.js',
    format: 'esm'
  }]
});

Conventions

• Plugins should have a clear name with rollup-plugin- prefix.
• Include rollup-plugin keyword in package.json.
• Plugins should be tested. We recommend mocha or ava which support promises out of the box.
• Use asynchronous methods when it is possible.
• Document your plugin in English.
• Make sure your plugin outputs correct source mappings if appropriate.
• If your plugin uses 'virtual modules' (e.g. for helper functions), prefix the module ID with \0. This prevents other plugins from trying to process it.

Properties

name

Type: string

The name of the plugin, for use in error messages and warnings.

Hooks

In addition to properties defining the identity of your plugin, you may also specify properties that correspond to available build hooks. Hooks can affect how a build is run, provide information about a build, or modify a build once complete. There are different kinds of hooks:

• async: The hook can also return a promise resolving to the same type of value; otherwise, the hook is marked as sync
• first: If several plugins implement this hook, the hooks are run sequentially until a hook returns a value other than null or undefined
• sequential: If this hook returns a promise, then other hooks of this kind will only be executed once this hook has resolved
• parallel: If this hook returns a promise, then other hooks of this kind will not wait for this hook to be resolved

augmentChunkHash

Type: (preRenderedChunk: PreRenderedChunk) => string
Kind: sync, sequential

Can be used to augment the hash of individual chunks. Called for each Rollup output chunk. Returning a falsy value will not modify the hash.

The following plugin will invalidate the hash of chunk foo with the timestamp of the last build:

// rollup.config.js
augmentChunkHash(chunkInfo) {
  if(chunkInfo.name === 'foo') {
    return Date.now();
  }
}

banner

Type: string | (() => string)
Kind: async, parallel

Cf. output.banner/output.footer.

buildEnd

Type: (error?: Error) => void
Kind: async, parallel

Called when rollup has finished bundling, but before generate or write is called; you can also return a Promise. If an error occurred during the build, it is passed on to this hook.

buildStart

Type: (options: InputOptions) => void
Kind: async, parallel

Called on each rollup.rollup build.

footer

Type: string | (() => string)
Kind: async, parallel

Cf. output.banner/output.footer.

generateBundle

Type: (options: OutputOptions, bundle: { [fileName: string]: AssetInfo | ChunkInfo }, isWrite: boolean) => void
Kind: async, sequential

Called at the end of bundle.generate() or immediately before the files are written in bundle.write(). To modify the files after they have been written, use the writeBundle hook. bundle provides the full list of files being written or generated along with their details:

// AssetInfo
{
  fileName: string,
  source: string | Buffer,
  type: 'asset',
}

// ChunkInfo
{
  code: string,
  dynamicImports: string[],
  exports: string[],
  facadeModuleId: string | null,
  fileName: string,
  imports: string[],
  isDynamicEntry: boolean,
  isEntry: boolean,
  map: SourceMap | null,
  modules: {
    [id: string]: {
      renderedExports: string[],
      removedExports: string[],
      renderedLength: number,
      originalLength: number
    },
  },
  name: string,
  type: 'chunk',
}

You can prevent files from being emitted by deleting them from the bundle object in this hook. To emit additional files, use the this.emitFile plugin context function.

intro

Type: string | (() => string)
Kind: async, parallel

Cf. output.intro/output.outro.

load

Type: (id: string) => string | null | { code: string, map?: string | SourceMap, ast? : ESTree.Program, moduleSideEffects?: boolean | null }
Kind: async, first

Defines a custom loader. Returning null defers to other load functions (and eventually the default behavior of loading from the file system). To prevent additional parsing overhead in case e.g. this hook already used this.parse to generate an AST for some reason, this hook can optionally return a { code, ast } object. The ast must be a standard ESTree AST with start and end properties for each node.

If false is returned for moduleSideEffects and no other module imports anything from this module, then this module will not be included in the bundle without checking for actual side-effects inside the module. If true is returned, Rollup will use its default algorithm to include all statements in the module that have side-effects (such as modifying a global or exported variable). If null is returned or the flag is omitted, then moduleSideEffects will be determined by the first resolveId hook that resolved this module, the treeshake.moduleSideEffects option, or eventually default to true. The transform hook can override this.

You can use this.getModuleInfo to find out the previous value of moduleSideEffects inside this hook.

options

Type: (options: InputOptions) => InputOptions | null
Kind: sync, sequential

Reads and replaces or manipulates the options object passed to rollup.rollup. Returning null does not replace anything. This is the only hook that does not have access to most plugin context utility functions as it is run before rollup is fully configured.

outputOptions

Type: (outputOptions: OutputOptions) => OutputOptions | null
Kind: sync, sequential

Reads and replaces or manipulates the output options object passed to bundle.generate. Returning null does not replace anything.

outro

Type: string | (() => string)
Kind: async, parallel

Cf. output.intro/output.outro.

renderChunk

Type: (code: string, chunk: ChunkInfo, options: OutputOptions) => string | { code: string, map: SourceMap } | null
Kind: async, sequential

Can be used to transform individual chunks. Called for each Rollup output chunk file. Returning null will apply no transformations.

renderError

Type: (error: Error) => void
Kind: async, parallel

Called when rollup encounters an error during bundle.generate() or bundle.write(). The error is passed to this hook. To get notified when generation completes successfully, use the generateBundle hook.

renderStart

Type: () => void
Kind: async, parallel

Called initially each time bundle.generate() or bundle.write() is called. To get notified when generation has completed, use the generateBundle and renderError hooks.

resolveDynamicImport

Type: (specifier: string | ESTree.Node, importer: string) => string | false | null | {id: string, external?: boolean}
Kind: async, first

Defines a custom resolver for dynamic imports. Returning false signals that the import should be kept as it is and not be passed to other resolvers thus making it external. Similar to the resolveId hook, you can also return an object to resolve the import to a different id while marking it as external at the same time.

In case a dynamic import is passed a string as argument, a string returned from this hook will be interpreted as an existing module id while returning null will defer to other resolvers and eventually to resolveId .

In case a dynamic import is not passed a string as argument, this hook gets access to the raw AST nodes to analyze and behaves slightly different in the following ways:

• If all plugins return null, the import is treated as external without a warning.
• If a string is returned, this string is not interpreted as a module id but is instead used as a replacement for the import argument. It is the responsibility of the plugin to make sure the generated code is valid.
• To resolve such an import to an existing module, you can still return an object {id, external}.

Note that the return value of this hook will not be passed to resolveId afterwards; if you need access to the static resolution algorithm, you can use this.resolve(source, importer) on the plugin context.

resolveFileUrl

Type: ({chunkId: string, fileName: string, format: string, moduleId: string, referenceId: string, relativePath: string}) => string | null
Kind: sync, first

Allows to customize how Rollup resolves URLs of files that were emitted by plugins via this.emitAsset or this.emitChunk. By default, Rollup will generate code for import.meta.ROLLUP_ASSET_URL_assetReferenceId and import.meta.ROLLUP_CHUNK_URL_chunkReferenceId that should correctly generate absolute URLs of emitted files independent of the output format and the host system where the code is deployed.

For that, all formats except CommonJS and UMD assume that they run in a browser environment where URL and document are available. In case that fails or to generate more optimized code, this hook can be used to customize this behaviour. To do that, the following information is available:

• assetReferenceId: The asset reference id if we are resolving import.meta.ROLLUP_ASSET_URL_assetReferenceId, otherwise null.
• chunkId: The id of the chunk this file is referenced from.
• chunkReferenceId: The chunk reference id if we are resolving import.meta.ROLLUP_CHUNK_URL_chunkReferenceId, otherwise null.
• fileName: The path and file name of the emitted asset, relative to output.dir without a leading ./.
• format: The rendered output format.
• moduleId: The id of the original module this file is referenced from. Useful for conditionally resolving certain assets differently.
• relativePath: The path and file name of the emitted file, relative to the chunk the file is referenced from. This will path will contain no leading ./ but may contain a leading ../.

Note that since this hook has access to the filename of the current chunk, its return value will not be considered when generating the hash of this chunk.

The following plugin will always resolve all files relative to the current document:

// rollup.config.js
resolveFileUrl({fileName}) {
  return `new URL('${fileName}', document.baseURI).href`;
}

resolveId

Type: (source: string, importer: string) => string | false | null | {id: string, external?: boolean, moduleSideEffects?: boolean | null}
Kind: async, first

Defines a custom resolver. A resolver can be useful for e.g. locating third-party dependencies. Returning null defers to other resolveId functions and eventually the default resolution behavior; returning false signals that source should be treated as an external module and not included in the bundle. If this happens for a relative import, the id will be renormalized the same way as when the external option is used.

If you return an object, then it is possible to resolve an import to a different id while excluding it from the bundle at the same time. This allows you to replace dependencies with external dependencies without the need for the user to mark them as "external" manually via the external option:

resolveId(source) {
  if (source === 'my-dependency') {
    return {id: 'my-dependency-develop', external: true};
  }
  return null;
}

Relative ids, i.e. starting with ./ or ../, will not be renormalized when returning an object. If you want this behaviour, return an absolute file system location as id instead.

If false is returned for moduleSideEffects in the first hook that resolves a module id and no other module imports anything from this module, then this module will not be included without checking for actual side-effects inside the module. If true is returned, Rollup will use its default algorithm to include all statements in the module that have side-effects (such as modifying a global or exported variable). If null is returned or the flag is omitted, then moduleSideEffects will be determined by the treeshake.moduleSideEffects option or default to true. The load and transform hooks can override this.

resolveImportMeta

Type: (property: string | null, {chunkId: string, moduleId: string, format: string}) => string | null
Kind: sync, first

Allows to customize how Rollup handles import.meta and import.meta.someProperty, in particular import.meta.url. In ES modules, import.meta is an object and import.meta.url contains the URL of the current module, e.g. http://server.net/bundle.js for browsers or file:///path/to/bundle.js in Node.

By default for formats other than ES modules, Rollup replaces import.meta.url with code that attempts to match this behaviour by returning the dynamic URL of the current chunk. Note that all formats except CommonJS and UMD assume that they run in a browser environment where URL and document are available. For other properties, import.meta.someProperty is replaced with undefined while import.meta is replaced with an object containing a url property.

This behaviour can be changed—also for ES modules—via this hook. For each occurrence of import.meta<.someProperty>, this hook is called with the name of the property or null if import.meta is accessed directly. For example, the following code will resolve import.meta.url using the relative path of the original module to the current working directory and again resolve this path against the base URL of the current document at runtime:

// rollup.config.js
resolveImportMeta(property, {moduleId}) {
  if (property === 'url') {
    return `new URL('${path.relative(process.cwd(), moduleId)}', document.baseURI).href`;
  }
  return null;
}

Note that since this hook has access to the filename of the current chunk, its return value will not be considered when generating the hash of this chunk.

transform

Type: (code: string, id: string) => string | null | { code: string, map?: string | SourceMap, ast? : ESTree.Program, moduleSideEffects?: boolean | null }
Kind: async, sequential

Can be used to transform individual modules. To prevent additional parsing overhead in case e.g. this hook already used this.parse to generate an AST for some reason, this hook can optionally return a { code, ast } object. The ast must be a standard ESTree AST with start and end properties for each node.

Note that in watch mode, the result of this hook is cached when rebuilding and the hook is only triggered again for a module id if either the code of the module has changed or a file has changed that was added via this.addWatchFile the last time the hook was triggered for this module.

If false is returned for moduleSideEffects and no other module imports anything from this module, then this module will not be included without checking for actual side-effects inside the module. If true is returned, Rollup will use its default algorithm to include all statements in the module that have side-effects (such as modifying a global or exported variable). If null is returned or the flag is omitted, then moduleSideEffects will be determined by the first resolveId hook that resolved this module, the treeshake.moduleSideEffects option, or eventually default to true.

You can use this.getModuleInfo to find out the previous value of moduleSideEffects inside this hook.

watchChange

Type: (id: string) => void
Kind: sync, sequential

Notifies a plugin whenever rollup has detected a change to a monitored file in --watch mode.

writeBundle

Type: ( bundle: { [fileName: string]: AssetInfo | ChunkInfo }) => void
Kind: async, parallel

Called only at the end of bundle.write() once all files have been written. Similar to the generateBundle hook, bundle provides the full list of files being written along with their details.

Deprecated Hooks

☢️ These hooks have been deprecated and may be removed in a future Rollup version.

• ongenerate - Use generateBundle - Function hook called when bundle.generate() is being executed.

• onwrite - Use generateBundle - Function hook called when bundle.write() is being executed, after the file has been written to disk.

• resolveAssetUrl - Use resolveFileUrl - Function hook that allows to customize the generated code for asset URLs.

• transformBundle – Use renderChunk - A ( source, { format } ) => code or ( source, { format } ) => { code, map } bundle transformer function.

• transformChunk – Use renderChunk - A ( source, outputOptions, chunk ) => code | { code, map} chunk transformer function.

More properties may be supported in future, as and when they prove necessary.

Plugin Context

A number of utility functions and informational bits can be accessed from within most hooks via this:

this.addWatchFile(id: string) => void

Adds additional files to be monitored in watch mode so that changes to these files will trigger rebuilds. id can be an absolute path to a file or directory or a path relative to the current working directory. This context function can only be used in hooks during the build phase, i.e. in buildStart, load, resolveId, and transform.

Note: Usually in watch mode to improve rebuild speed, the transform hook will only be triggered for a given module if its contents actually changed. Using this.addWatchFile from within the transform hook will make sure the transform hook is also reevaluated for this module if the watched file changes.

In general, it is recommended to use this.addWatchfile from within the hook that depends on the watched file.

this.emitFile(emittedFile: EmittedChunk | EmittedAsset) => string

Emits a new file that is included in the build output and returns a referenceId that can be used in various places to reference the emitted file. emittedFile can have one of two forms:

// EmittedChunk
{
  type: 'chunk',
  id: string,
  name?: string,
  fileName?: string
}

// EmittedAsset
{
  type: 'asset',
  source?: string | Buffer,
  name?: string,
  fileName?: string
}

In both cases, either a name or a fileName can be supplied. If a fileName is provided, it will be used unmodified as the name of the generated file, throwing an error if this causes a conflict. Otherwise if a name is supplied, this will be used as substitution for [name] in the corresponding output.chunkFileNames or output.assetFileNames pattern, possibly adding a unique number to the end of the file name to avoid conflicts. If neither a name nor fileName is supplied, a default name will be used.

You can reference the URL of an emitted file in any code returned by a load or transform plugin hook via import.meta.ROLLUP_FILE_URL_referenceId. See File URLs for more details and an example.

The generated code that replaces import.meta.ROLLUP_FILE_URL_referenceId can be customized via the resolveFileUrl plugin hook. You can also use this.getFileName(referenceId) to determine the file name as soon as it is available

If the type is chunk, then this emits a new chunk with the given module id as entry point. This will not result in duplicate modules in the graph, instead if necessary, existing chunks will be split or a facade chunk with reexports will be created. Chunks with a specified fileName will always generate separate chunks while other emitted chunks may be deduplicated with existing chunks even if the name does not match. If such a chunk is not deduplicated, the output.chunkFileNames name pattern will be used.

If the type is asset, then this emits an arbitrary new file with the given source as content. It is possible to defer setting the source via this.setAssetSource(assetReferenceId, source) to a later time to be able to reference a file during the build phase while setting the source separately for each output during the generate phase. Assets with a specified fileName will always generate separate files while other emitted assets may be deduplicated with existing assets if they have the same source even if the name does not match. If such an asset is not deduplicated, the output.assetFileNames name pattern will be used.

this.error(error: string | Error, position?: number | { column: number; line: number }) => never

Structurally equivalent to this.warn, except that it will also abort the bundling process.

this.getCombinedSourcemap() => SourceMap

Get the combined source maps of all previous plugins. This context function can only be used in transform plugin hook.

this.getFileName(referenceId: string) => string

Get the file name of a chunk or asset that has been emitted via this.emitFile . The file name will be relative to outputOptions.dir.

this.getModuleInfo(moduleId: string) => ModuleInfo

Returns additional information about the module in question in the form

{
  id: string, // the id of the module, for convenience
  isEntry: boolean, // is this a user- or plugin-defined entry point
  isExternal: boolean, // for external modules that are not included in the graph
  importedIds: string[], // the module ids imported by this module
  hasModuleSideEffects: boolean // are imports of this module included if nothing is imported from it
}

If the module id cannot be found, an error is thrown.

this.meta: {rollupVersion: string}

An Object containing potentially useful Rollup metadata. meta is the only context property accessible from the options hook.

this.moduleIds: IterableIterator<string>

An Iterator that gives access to all module ids in the current graph. It can be iterated via

for (const moduleId of this.moduleIds) { /* ... */ }

or converted into an Array via Array.from(this.moduleIds).

this.parse(code: string, acornOptions: AcornOptions) => ESTree.Program

Use Rollup's internal acorn instance to parse code to an AST.

this.resolve(source: string, importer: string, options?: {skipSelf: boolean}) => Promise<{id: string, external: boolean} | null>

Resolve imports to module ids (i.e. file names) using the same plugins that Rollup uses, and determine if an import should be external. If null is returned, the import could not be resolved by Rollup or any plugin but was not explicitly marked as external by the user.

If you pass skipSelf: true, then the resolveId hook of the plugin from which this.resolve is called will be skipped when resolving.

this.setAssetSource(assetReferenceId: string, source: string | Buffer) => void

Set the deferred source of an asset.

this.warn(warning: string | RollupWarning, position?: number | { column: number; line: number }) => void

Using this method will queue warnings for a build. These warnings will be printed by the CLI just like internally generated warnings (except with the plugin name), or captured by custom onwarn handlers.

The warning argument can be a string or an object with (at minimum) a message property:

this.warn( 'hmm...' );
// is equivalent to
this.warn({ message: 'hmm...' });

Use the second form if you need to add additional properties to your warning object. Rollup will augment the warning object with a plugin property containing the plugin name, code (PLUGIN_WARNING) and id (the file being transformed) properties.

The position argument is a character index where the warning was raised. If present, Rollup will augment the warning object with pos, loc (a standard { file, line, column } object) and frame (a snippet of code showing the error).

Deprecated Context Functions

☢️ These context utility functions have been deprecated and may be removed in a future Rollup version.

• this.emitAsset(assetName: string, source: string) => string - Use this.emitFile - Emits a custom file that is included in the build output, returning an assetReferenceId that can be used to reference the emitted file. You can defer setting the source if you provide it later via this.setAssetSource(assetReferenceId, source). A string or Buffer source must be set for each asset through either method or an error will be thrown on generate completion.

  Emitted assets will follow the output.assetFileNames naming scheme. You can reference the URL of the file in any code returned by a load or transform plugin hook via import.meta.ROLLUP_ASSET_URL_assetReferenceId.

  The generated code that replaces import.meta.ROLLUP_ASSET_URL_assetReferenceId can be customized via the resolveFileUrl plugin hook. Once the asset has been finalized during generate, you can also use this.getFileName(assetReferenceId) to determine the file name.

• this.emitChunk(moduleId: string, options?: {name?: string}) => string - Use this.emitFile - Emits a new chunk with the given module as entry point. This will not result in duplicate modules in the graph, instead if necessary, existing chunks will be split. It returns a chunkReferenceId that can be used to later access the generated file name of the chunk.

  Emitted chunks will follow the output.chunkFileNames, output.entryFileNames naming scheme. If a name is provided, this will be used for the [name] file name placeholder, otherwise the name will be derived from the file name. If a name is provided, this name must not conflict with any other entry point names unless the entry points reference the same entry module. You can reference the URL of the emitted chunk in any code returned by a load or transform plugin hook via import.meta.ROLLUP_CHUNK_URL_chunkReferenceId.

  The generated code that replaces import.meta.ROLLUP_CHUNK_URL_chunkReferenceId can be customized via the resolveFileUrl plugin hook. Once the chunk has been rendered during generate, you can also use this.getFileName(chunkReferenceId) to determine the file name.

• this.getAssetFileName(assetReferenceId: string) => string - Use this.getFileName - Get the file name of an asset, according to the assetFileNames output option pattern. The file name will be relative to outputOptions.dir.

• this.getChunkFileName(chunkReferenceId: string) => string - Use this.getFileName - Get the file name of an emitted chunk. The file name will be relative to outputOptions.dir.

• this.isExternal(id: string, importer: string, isResolved: boolean) => boolean - Use this.resolve - Determine if a given module ID is external when imported by importer. When isResolved is false, Rollup will try to resolve the id before testing if it is external.

• this.resolveId(source: string, importer: string) => Promise<string | null> - Use this.resolve - Resolve imports to module ids (i.e. file names) using the same plugins that Rollup uses. Returns null if an id cannot be resolved.

File URLs

To reference a file URL reference from within JS code, use the import.meta.ROLLUP_FILE_URL_referenceId replacement. This will generate code that depends on the output format and generates a URL that points to the emitted file in the target environment. Note that all formats except CommonJS and UMD assume that they run in a browser environment where URL and document are available.

The following example will detect imports of .svg files, emit the imported files as assets, and return their URLs to be used e.g. as the src attribute of an img tag:

// plugin
export default function svgResolverPlugin () {
  return ({
    resolveId(source, importer) {
      if (source.endsWith('.svg')) {
        return path.resolve(path.dirname(importer), source);
      }
    },
    load(id) {
      if (id.endsWith('.svg')) {
      	const referenceId = this.emitFile({
          type: 'asset',
          name: path.basename(id),
          source: fs.readFileSync(id)
        });
        return `export default import.meta.ROLLUP_FILE_URL_${referenceId};`;
      }
    }
  });
}

Usage:

import logo from '../images/logo.svg';
const image = document.createElement('img');
image.src = logo;
document.body.appendChild(image);

Similar to assets, emitted chunks can be referenced from within JS code via import.meta.ROLLUP_FILE_URL_referenceId as well.

The following example will detect imports prefixed with register-paint-worklet: and generate the necessary code and separate chunk to generate a CSS paint worklet. Note that this will only work in modern browsers and will only work if the output format is set to esm.

// plugin
const REGISTER_WORKLET = 'register-paint-worklet:';
export default function paintWorkletPlugin () {
  return ({
    load(id) {
      if (id.startsWith(REGISTER_WORKLET)) {
        return `CSS.paintWorklet.addModule(import.meta.ROLLUP_FILE_URL_${this.emitFile({
          type: 'chunk',
          id: id.slice(REGISTER_WORKLET.length)
        })});`;
      }
    },
    resolveId(source, importer) {
      // We remove the prefix, resolve everything to absolute ids and add the prefix again
      // This makes sure that you can use relative imports to define worklets
	  if (source.startsWith(REGISTER_WORKLET)) {
	    return this.resolve(source.slice(REGISTER_WORKLET.length), importer).then(
	      resolvedId => REGISTER_WORKLET + resolvedId.id
	    );
	  }
	  return null;
    }
  });
}

Usage:

// main.js
import 'register-paint-worklet:./worklet.js';
import { color, size } from './config.js';
document.body.innerHTML += `<h1 style="background-image: paint(vertical-lines);">color: ${color}, size: ${size}</h1>`;

// worklet.js
import { color, size } from './config.js';
registerPaint(
  'vertical-lines',
  class {
    paint(ctx, geom) {
      for (let x = 0; x < geom.width / size; x++) {
        ctx.beginPath();
        ctx.fillStyle = color;
        ctx.rect(x * size, 0, 2, geom.height);
        ctx.fill();
      }
    }
  }
);

// config.js
export const color = 'greenyellow';
export const size = 6;

If you build this code, both the main chunk and the worklet will share the code from config.js via a shared chunk. This enables us to make use of the browser cache to reduce transmitted data and speed up loading the worklet.

Transformers

Transformer plugins (i.e. those that return a transform function for e.g. transpiling non-JS files) should support options.include and options.exclude, both of which can be a minimatch pattern or an array of minimatch patterns. If options.include is omitted or of zero length, files should be included by default; otherwise they should only be included if the ID matches one of the patterns.

The transform hook, if returning an object, can also include an ast property. Only use this feature if you know what you're doing. Note that only the last AST in a chain of transforms will be used (and if there are transforms, any ASTs generated by the load hook will be discarded for the transformed modules.)

Example Transformer

(Use rollup-pluginutils for commonly needed functions, and to implement a transformer in the recommended manner.)

import { createFilter } from 'rollup-pluginutils';

export default function myPlugin ( options = {} ) {
  const filter = createFilter( options.include, options.exclude );

  return {
    transform ( code, id ) {
      if ( !filter( id ) ) return;

      // proceed with the transformation...
      return {
        code: generatedCode,
        map: generatedSourceMap
      };
    }
  };
}

Source Code Transformations

If a plugin transforms source code, it should generate a sourcemap automatically, unless there's a specific sourceMap: false option. Rollup only cares about the mappings property (everything else is handled automatically). If it doesn't make sense to generate a sourcemap, (e.g. rollup-plugin-string), return an empty sourcemap:

return {
  code: transformedCode,
  map: { mappings: '' }
};

If the transformation does not move code, you can preserve existing sourcemaps by returning null:

return {
  code: transformedCode,
  map: null
};

If you create a plugin that you think would be useful to others, please publish it to NPM and add submit it to https://github.com/rollup/awesome!