Feature: Expose resolve function

[lukeed]

Hey,

Similar to formatMessages (#1058 🙇) would it be possible to expose esbuild's resolver logic?

Currently, this is useful when writing an ESM loader hook. Node allows the developer to override/determine what file to load. It passes down its own conditions array, allowing you to modify its array and/or override it.

If esbuild exposed this method, we could forward Node's conditions list and also make use of the resolveExtensions option. Even using something like my resolve.exports pkg as a stand-in, a developer would still have to reimplement the extensions logic.

Imagine the API would look something like this, taking just a subset of the existing options:

export function resolve(ident: string, importer: string, options: {
  // picked from BuildOptions, no changes
  conditions?: string[];
  resolveExtensions?: string[];
  mainFields?: string[];
  absWorkingDir?: string;
  nodePaths?: string[];
}): Promise<string | void>;
// absolute file path
// or `node:<module>` for builtins
// or void for no match

[evanw]

See also #641, which is about exposing this to plugins. I'm not sure that I would ever expose an API like this for independent use since that's not the purpose of esbuild. However, you can already do this with a hack (modified from #641 (comment)):

const esbuild = require('esbuild')

async function esbuildResolveImport(id, {
  conditions,
  resolveExtensions,
  mainFields,
  nodePaths,
  absWorkingDir = process.cwd(),
} = {}) {
  let _resolve
  const resolvedPromise = new Promise((resolve) => (_resolve = resolve))
  return Promise.race([
    resolvedPromise,
    esbuild
      .build({
        sourcemap: false,
        write: false,
        bundle: true,
        logLevel: 'silent',
        conditions,
        resolveExtensions,
        mainFields,
        absWorkingDir,
        nodePaths,
        entryPoints: [id],
        plugins: [
          {
            name: 'esbuildResolveImport',
            setup(build) {
              build.onLoad({ filter: /.*/ }, ({ path }) => {
                id = path
                _resolve(id)
                return { contents: '' }
              })
            }
          }
        ]
      })
      .then(() => id)
  ])
}

(async () => {
  for (const conditions of [[], ['node', 'module'], ['node', 'require'], ['node', 'import']]) {
    console.log(conditions, '=>', await esbuildResolveImport('uuid', { conditions }))
  }
})()

Here's what this prints:

[] => ./node_modules/uuid/dist/esm-browser/index.js
[ 'node', 'module' ] => ./node_modules/uuid/dist/esm-node/index.js
[ 'node', 'require' ] => ./node_modules/uuid/dist/index.js
[ 'node', 'import' ] => ./node_modules/uuid/wrapper.mjs

[lukeed]

Glad some work has been done here! However this request was for a top-level resolve export so I don't have to spin up a fake builder w/ a proxy plugin.