Resolver

A plugin type: Turn dependency requests into absolute paths (or exclude them)

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

ResolveResult parcel/packages/core/types/index.js:1451 

type ResolveResult = {|
  +filePath?: FilePath,

An absolute path to the file.

  +pipeline?: ?string,
  +isExcluded?: boolean,
  +priority?: DependencyPriority,
  +sideEffects?: boolean,

Corresponds to BaseAsset's sideEffects.

  +code?: string,

A resolver might want to resolve to a dummy, in this case filePath is rather "resolve from".

  +canDefer?: boolean,

Whether this dependency can be deferred by Parcel itself (true by default).

  +diagnostics?: Diagnostic | Array<Diagnostic>,

A resolver might return diagnostics to also run subsequent resolvers while still providing a reason why it failed.

  +meta?: JSONObject,

Is spread (shallowly merged) onto the request's dependency.meta

  +invalidateOnFileCreate?: Array<FileCreateInvalidation>,
  +invalidateOnFileChange?: Array<FilePath>,
|}
Referenced by:
Resolver

Resolver parcel/packages/core/types/index.js:1593 

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