diff --git a/package.json b/package.json
index 4d9fa51..625bdbe 100644
--- a/package.json
+++ b/package.json
@@ -25,7 +25,7 @@
     "build:libphonenumber": "node_modules/.bin/gulp",
     "build:rewrite": "ts-node scripts/rewrite.ts",
     "build:esm:build": "rollup -c ./rollup.config.mjs",
-    "build:esm:rewrite": "cat build/index-esm.js src/esm-outro.js > index-esm.mjs",
+    "build:esm:rewrite": "ts-node scripts/rewrite-esm.ts",
     "build:esm": "yarn build:esm:build && yarn build:esm:rewrite",
     "build": "yarn build:libphonenumber && yarn build:rewrite && yarn build:esm",
     "test:compile": "scripts/test.sh",
diff --git a/scripts/file.ts b/scripts/file.ts
new file mode 100644
index 0000000..a4f6559
--- /dev/null
+++ b/scripts/file.ts
@@ -0,0 +1,9 @@
+import { promises } from 'fs'
+
+export async function overwriteFile( filename: string, data: string )
+{
+	const file = await promises.open( filename, 'w' );
+	await file.write( data, 0, 'utf-8' );
+	await file.sync( );
+	await file.close( );
+}
diff --git a/scripts/rewrite-esm.ts b/scripts/rewrite-esm.ts
new file mode 100644
index 0000000..ae2ae6b
--- /dev/null
+++ b/scripts/rewrite-esm.ts
@@ -0,0 +1,30 @@
+import { promises } from 'fs'
+import * as path from 'path'
+
+import { overwriteFile } from './file'
+
+const { readFile } = promises;
+
+const esmFile = path.resolve( __dirname, '..', 'build', 'index-esm.js' );
+const outroFile = path.resolve( __dirname, '..', 'src', 'esm-outro.js' );
+const resultFile = path.resolve( __dirname, '..', 'index-esm.mjs' );
+
+async function rewrite( )
+{
+	const [ file1, file2 ] = await Promise.all( [
+		readFile( esmFile, 'utf-8' ),
+		readFile( outroFile, 'utf-8' ),
+	] );
+
+	console.log(
+		`Lines: ${file1.split('\n').length} ${file2.split('\n').length}`
+	);
+	const data = file1 + file2;
+
+	await overwriteFile( resultFile, data );
+}
+rewrite( ).catch( err =>
+{
+	console.error( err.stack );
+	process.exit( 1 );
+} );
diff --git a/scripts/rewrite.ts b/scripts/rewrite.ts
index c35c250..16a65e6 100644
--- a/scripts/rewrite.ts
+++ b/scripts/rewrite.ts
@@ -1,8 +1,9 @@
-
 import { promises } from 'fs'
 import * as path from 'path'
 
-const { readFile, writeFile } = promises;
+import { overwriteFile } from './file'
+
+const { readFile } = promises;
 
 const distFile = path.resolve( __dirname, '..', 'lib', 'index.js' );
 
@@ -10,6 +11,10 @@ async function rewrite( )
 {
 	const data = await readFile( distFile, 'utf-8' );
 	const res = data.replace( /\bconst /g, 'var ' );
-	await writeFile( distFile, res, 'utf-8' );
+	await overwriteFile( distFile, res );
 }
-rewrite( );
+rewrite( ).catch( err =>
+{
+	console.error( err.stack );
+	process.exit( 1 );
+} );