added solution task

src/arrayMethodJoin.js

@@ -5,7 +5,24 @@
  */
 function applyCustomJoin() {
   [].__proto__.join2 = function(separator) {
-    // write code here
+    let result = '';
+    const readySeparator = separator !== undefined ? String(separator) : ',';
+
+    for (let i = 0; i < this.length; i++) {
+      let value = this[i];
+
+      if (value === undefined || value === null) {
+        value = '';
+      }
+
+      if (i === 0) {
+        result += value;
+      } else {
+        result += readySeparator + value;
+      }
+    }
+
+    return result;
   };
 }