fix(source): wait for output write to finish

src/source.ts

@@ -1,23 +1,37 @@
-import { promisify } from 'util';
 import { readFile, writeFile } from 'fs';
+import { isNil } from 'lodash';
+import { promisify } from 'util';
 
+export const FILE_ENCODING = 'utf-8';
 export const readFileSync = promisify(readFile);
 export const writeFileSync = promisify(writeFile);
 
 export async function loadSource(path: string): Promise<string> {
   if (path === '-') {
-    return readFileSync(0, 'utf-8');
+    return readFileSync(0, {
+      encoding: FILE_ENCODING,
+    });
   } else {
-    return readFileSync(path, 'utf-8');
+    return readFileSync(path, {
+      encoding: FILE_ENCODING,
+    });
   }
 }
 
 export async function writeSource(path: string, data: string): Promise<void> {
   if (path === '-') {
-    process.stdout.write(data);
+    return new Promise((res, rej) => {
+      process.stdout.write(data, (err: Error | null | undefined) => {
+        if (isNil(err)) {
+          res();
+        } else {
+          rej(err);
+        }
+      });
+    });
   } else {
     return writeFileSync(path, data, {
-      encoding: 'utf-8',
+      encoding: FILE_ENCODING,
     });
   }
 }