Add 4.2 solution

Author: kasymbayaman

03-callbacks-and-events/3.1-simple-event.js

@@ -16,6 +16,7 @@ class FindRegex extends EventEmitter {
   find () {
     // +++ 3.1 added line
     process.nextTick(() => this.emit('start', this.files));
+    
     for (const file of this.files) {
       readFile(file, 'utf8', (err, content) => {
         if (err) {

04-callback-patterns/4.2-list-files-recursively.js

@@ -0,0 +1,30 @@
+import fs from "fs";
+import path from "path";
+
+const result = {
+  runningOps: 1,
+  fileList: [],
+};
+
+
+function listNestedFiles(dirPath, cb) {
+  fs.readdir(dirPath, {}, (err, files) => {
+    if (err) {
+      if (err.code !== "ENOTDIR") return cb(err);
+
+      // if it is file -> push to filesList
+      result.fileList.push(dirPath);
+    } else {
+      for (const file of files) {
+        result.runningOps++;
+        readDirectory(path.join(dirPath, file), cb);
+      }
+    }
+
+    result.runningOps--;
+
+    process.nextTick(() => {
+      if (!result.runningOps) return cb(null, result.fileList);
+    });
+  });
+}