Solution

src/arrayMethodJoin.js

@@ -4,37 +4,21 @@
  * Implement method join
  */
 function applyCustomJoin() {
-  [].__proto__.join2 = function(separator) {
+  [].__proto__.join2 = function(separator = ',') {
     let output = '';
 
-    let actualSeparator;
-
-    if (separator === undefined) {
-      actualSeparator = ',';
-    } else {
-      actualSeparator = separator;
-    }
-
-    if (this.length === 0) {
-      return output;
-    }
-
-    if (this[0] !== undefined && this[0] !== null) {
-      output += `${this[0]}`;
-    } else {
-      output += '';
-    }
+    for (let i = 0; i < this.length; i++) {
+      if (i > 0) {
+        output += separator;
+      }
 
-    for (let i = 1; i < this.length; i++) {
       if (this[i] !== undefined && this[i] !== null) {
-        output += `${actualSeparator}${this[i]}`;
-      } else {
-        output += actualSeparator;
+        output += this[i];
       }
     }
 
     return output;
   };
-};
+}
 
 module.exports = applyCustomJoin;