Resolver

Resolvers get called with an asset request (consisting of a source file path and the specifier of what is being requested) which it then attempts to resolve. If the resolver isn't sure how to handle a request, it can also return null and pass it to the next resolver in the chain.

import { Resolver } from "@parcel/plugin";

export default new Resolver({
  async resolve({ filePath, dependency }) {
    if (!shouldHandle(filePath)) {
      return null;
    }
    // ...
    return {
      filePath: doResolve({ from: filePath, to: dependency.moduleSpecifier }),
    };
  },
});

The result object can also contain sideEffects (which corresponds to package.json#sideEffects) code (used instead of fs.readFile(filePath)) and isExcluded (e.g. to exclude node_modules).

¶ Relevant API

type ResolveResult = {|

  +filePath?: FilePath,

  +isExcluded?: boolean,

  +sideEffects?: boolean,

  +code?: string,

|}

type Resolver = {|

  resolve({|
    dependency: Dependency,
    options: PluginOptions,
    logger: PluginLogger,
    filePath: FilePath,
  |}): Async<?ResolveResult>,

|}