join
Join a DefineList's elements into a string.
    list.join(separator)
  
  join turns a DefineList into a string by inserting separator between the string representations
of all the elements of the DefineList.
var list = new DefineList(['Alice', 'Bob', 'Eve']);
list.join(', '); // 'Alice, Bob, Eve'
  
  Parameters
- separator 
{String}:The string to seperate elements.
 
Returns
 {String}: 
The joined string.