add task solution

src/arrayMethodJoin.js

@@ -3,9 +3,30 @@
 /**
  * Implement method join
  */
+
 function applyCustomJoin() {
   [].__proto__.join2 = function(separator) {
-    // write code here
+    const { sep } = { sep: separator === undefined ? ',' : String(separator) };
+
+    let result = '';
+
+    for (let i = 0; i < this.length; i++) {
+      if (i > 0) {
+        result += sep;
+      }
+
+      if (this[i] === null || this[i] === undefined) {
+        result += '';
+      } else {
+        if (String(this[i]) === 'null') {
+          result += 'null';
+        } else {
+          result += String(this[i]);
+        }
+      }
+    }
+
+    return result;
   };
 }