Solution:)

package.json

@@ -16,7 +16,7 @@
   "license": "GPL-3.0",
   "devDependencies": {
     "@mate-academy/eslint-config": "*",
-    "@mate-academy/scripts": "^0.9.7",
+    "@mate-academy/scripts": "^1.2.12",
     "eslint": "^5.16.0",
     "eslint-plugin-jest": "^22.4.1",
     "eslint-plugin-node": "^8.0.1",

src/arrayMethodJoin.js

@@ -4,8 +4,28 @@
  * Implement method join
  */
 function applyCustomJoin() {
-  [].__proto__.join2 = function(separator) {
-    // write code here
+  [].__proto__.join2 = function(separator = ',') {
+    let joinedString = '';
+
+    for (let i = 0; i < this.length; i++) {
+      const element = this[i];
+      const lastElement = i === this.length - 1;
+
+      /**
+       * wanted to use `joinedString += element ?? ''`,
+       * meaning without any `if()` construction,
+       * but mate's eslint unables me to do it
+       */
+      if (element !== undefined && element !== null) {
+        joinedString += element;
+      }
+
+      if (!lastElement) {
+        joinedString += separator;
+      }
+    }
+
+    return joinedString;
   };
 }