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src/arrayMethodJoin.js

@@ -4,8 +4,26 @@
  * Implement method join
  */
 function applyCustomJoin() {
-  [].__proto__.join2 = function(separator) {
-    // write code here
+  [].__proto__.join2 = function(separator = ',') {
+    let result = '';
+
+    for (let i = 0; i < this.length - 1; i++) {
+      let currentElement = this[i];
+
+      if (currentElement === null || currentElement === undefined) {
+        currentElement = '';
+      }
+
+      result += currentElement;
+
+      result += separator;
+    }
+
+    if (this[this.length - 1]) {
+      result += this[this.length - 1];
+    }
+
+    return result;
   };
 }