Add 4.1 solution

Author: kasymbayaman

04-callback-patterns/4.1-file-concat.js

@@ -0,0 +1,35 @@
+import fs from 'fs';
+import path from 'path';
+import { fileURLToPath } from 'url';
+const __filename = fileURLToPath(import.meta.url);
+const __dirname = path.dirname(__filename);
+
+function concatFiles(dest, cb, ...srcFiles) {
+  const startIndex = 0;
+  const initialContent = '';
+
+  readfiles(srcFiles, startIndex, initialContent, (err, data) => {
+    if (err) return cb(err);
+    fs.writeFile(dest, data, cb);
+  })
+}
+
+function readfiles(srcFiles, index, content, cb) {
+  if (index === srcFiles.length) return cb(null, content);
+
+  return fs.readFile(srcFiles[index], (err, data) => {
+    if (err) return cb(err);
+    // read next file recursively
+    readfiles(srcFiles, index + 1, content + data, cb);
+  });
+}
+
+// test
+const srcFiles = ['file0.txt', 'file1.txt', 'file2.txt', 'file3.txt', 'file4.txt']
+.map(fileName => path.join(__dirname, fileName));
+const dest = path.join(__dirname, 'result.txt');
+
+concatFiles(dest, (err, data) => {
+  if (err) return console.log('Error occured', err);
+  console.log('Written to file', dest);
+}, ...srcFiles)