feat: support extensions and index resolution

README.md

@@ -42,6 +42,8 @@ const { __dirname, __filename, require } = createCommonJS(import.meta)
 Resolve a module by respecting [ECMAScript Resolver algorithm](https://nodejs.org/dist/latest-v14.x/docs/api/esm.html#esm_resolver_algorithm)
 (internally using [wooorm/import-meta-resolve](https://github.com/wooorm/import-meta-resolve) that exposes Node.js implementation).
 
+Additionally supports resolving without extension and `/index` similar to CommonJS.
+
 ```js
 import { resolve } from 'mlly'
 
@@ -53,6 +55,7 @@ console.log(await resolve('./module.mjs', { from: import.meta.url }))
 
 - `from`: URL or string (default is `pwd()`)
 - `conditions`: Array of conditions used for resolution algorithm (default is `['node', 'import']`)
+- `extensions`: Array of additional extensions to check if import failed (default is `['.mjs', '.cjs', '.js', '.json']`)
 
 ### `resolvePath`
 

lib/index.mjs

@@ -28,17 +28,60 @@ export function createCommonJS (importMeta) {
 const DEFAULT_CONDITIONS_SET = new Set(['node', 'import'])
 const BUILُTIN_MODULES = new Set(builtinModules)
 const DEFAULT_FROM = pathToFileURL(process.cwd())
+const DEFAULT_EXTENSIONS = ['.mjs', '.cjs', '.js', '.json']
+
+function _tryModuleResolve (id, from, conditions, throwNonNotFound) {
+  try {
+    return moduleResolve(id, from, conditions)
+  } catch (err) {
+    if (throwNonNotFound && !['MODULE_NOT_FOUND', 'ERR_UNSUPPORTED_DIR_IMPORT'].includes(err.code)) {
+      throw err
+    }
+    return null
+  }
+}
 
 function _resolve (id, opts = {}) {
   // console.log('> resolve ', id, 'from', opts.from)
+
+  // Skip if already has a protocol
   if (/(node|data|http|https):/.test(id)) {
     return id
   }
+
+  // Skip builtins
   if (BUILُTIN_MODULES.has(id)) {
     return 'node:' + id
   }
+
+  // Defaults
   const conditionsSet = opts.conditions ? new Set(opts.conditions) : DEFAULT_CONDITIONS_SET
-  const resolved = moduleResolve(id, opts.from ? normalizeid(opts.from) : DEFAULT_FROM, conditionsSet)
+  const from = opts.from ? normalizeid(opts.from) : DEFAULT_FROM
+
+  // Try simple resolve
+  // Might throw error if error is other than MODULE_NOT_FOUND
+  let resolved = _tryModuleResolve(id, from, conditionsSet, true)
+
+  // Try other extensions if not found
+  if (!resolved) {
+    for (const prefix of ['', '/index']) {
+      for (const ext of DEFAULT_EXTENSIONS || opts.extensions) {
+        resolved = _tryModuleResolve(id + prefix + ext, from, conditionsSet)
+        if (resolved) {
+          break
+        }
+      }
+    }
+  }
+
+  // Throw error if not found
+  if (!resolved) {
+    const err = new Error(`Cannot find module ${id} imported from ${from}`)
+    err.code = 'MODULE_NOT_FOUND'
+    throw err
+  }
+
+  // Resolve realPath and normalize slash
   const realPath = realpathSync(fileURLToPath(resolved))
   return pathToFileURL(realPath).toString()
 }

test/foo/index.mjs

@@ -0,0 +1 @@
+console.log(import.meta.url)

test/resolve.mjs

@@ -4,5 +4,6 @@ import.meta.resolve = createResolve({ from : import.meta.url })
 console.log(await import.meta.resolve('./cjs.mjs'))
 
 console.log(await resolvePath('./cjs.mjs', { from: import.meta.url }))
+console.log(await resolvePath('./foo', { from: import.meta.url }))
 
 console.log(await resolveImports(`import foo from './eval.mjs'`, { from: import.meta.url }))