/**
 * Write a function that concatenates two lists.
 */
var List = function() {
  'use strict';

  function combine(list1, list2) {
  		var c = list1.map(function (e, i) {
  		if(list2[i] === undefined){
  			return [list1[i]];
  		}
    		return [list1[i], list2[i]];
		});
		return [].concat.apply([], c)
  }

  return {
    combine: combine
  };
};

@@ -162,24 +162,27 @@
 on (e, i) %7B%0A
+%09  
   %09%09if(list2
@@ -201,16 +201,19 @@
 fined)%7B%0A
+%09  
   %09%09%09ret
@@ -228,16 +228,19 @@
 t1%5Bi%5D%5D;%0A
+%09  
   %09%09%7D%0A  
@@ -241,16 +241,19 @@
 %09%09%7D%0A    
+  %09
 %09%09return

Update combine-lists.js