Solution

src/arrayMethodJoin.js

@@ -4,8 +4,26 @@
  * Implement method join
  */
 function applyCustomJoin() {
-  [].__proto__.join2 = function(separator) {
-    // write code here
+  [].__proto__.join2 = function(separator = ',') {
+    if (this.length === 0) {
+      return '';
+    }
+
+    let newArray = '';
+
+    for (let i = 0; i < this.length; i++) {
+      if (this[i] === undefined || this[i] === null) {
+        newArray += '';
+      } else {
+        newArray += this[i];
+      }
+
+      if (i < this.length - 1) { // no separator will be added to last element
+        newArray += separator;
+      }
+    }
+
+    return newArray;
   };
 }