feat: use module syntax if target file looks like esm (#21)

Author: chazcb

lib/aliases.js

@@ -35,14 +35,21 @@ module.exports = function (rootDir) {
       }
 
       const sourceJsPath = findSource(rootDir, source)
+      const sourceRaw = fs.readFileSync(sourceJsPath).toString()
+      // eslint-disable-next-line unicorn/prefer-starts-ends-with
+      const maybeIsEsm = sourceRaw.match(/^export /m)
       const sourceDTsPath = sourceJsPath.replace(/\.js$/, '.d.ts')
       const destJsPath = path.join(rootDir, alias + '.js')
       const destDTsPath = path.join(rootDir, alias + '.d.ts')
       if (fs.existsSync(sourceDTsPath)) {
         fs.writeFileSync(destDTsPath, `export * from '${source}'\n`, 'utf8')
       }
 
-      return fs.writeFileAsync(destJsPath, `module.exports = require("${source}")\n`, 'utf8')
+      const contents = maybeIsEsm ?
+        `export * from "${source}"\n` :
+        `module.exports = require("${source}")\n`
+
+      return fs.writeFileAsync(destJsPath, contents, 'utf8')
     },
   }
 }

test/aliases.test.js

@@ -52,6 +52,16 @@ describe('lib/aliases.js', () => {
       })
     })
 
+    it('should create an esm file when source looks like esm', () => {
+      return aliasUtils(app1Dir).createAlias('lib/fileB.esm.js', 'alias').then(() => {
+        return Promise.try(() => {
+          const contents = fs.readFileSync(app1Dir + '/alias.js', 'utf8')
+          contents.should.match(/^export \* from/)
+          fs.unlinkSync(app1Dir + '/alias.js')
+        })
+      })
+    })
+
     it('should point the alias to the source file', () => {
       return aliasUtils(app3Dir).createAlias('./file.js', 'alias').then(() => {
         return Promise.try(() => {

test/fixtures/app1/lib/fileB.esm.js

@@ -0,0 +1 @@
+export function helloWorld() {};