solution

src/arrayMethodJoin.js

@@ -4,8 +4,27 @@
  * Implement method join
  */
 function applyCustomJoin() {
-  [].__proto__.join2 = function(separator) {
-    // write code here
+  [].__proto__.join2 = function(separator = ',') {
+    const joinSeparator = String(separator);
+    let resultingString = '';
+
+    this.forEach((item, index) => {
+      let stringCharacter = item;
+
+      if (item === null || item === undefined) {
+        stringCharacter = '';
+      }
+
+      if (this.length - 1 === index) {
+        resultingString += stringCharacter;
+
+        return;
+      }
+
+      resultingString += stringCharacter + joinSeparator;
+    });
+
+    return resultingString;
   };
 }