string_chop =  function(str, size){  
      if (str == null) return [];  
      str = String(str);  
      size = ~~size;  
return size > 0 ? str.match(new RegExp('.{1,' + size + '}', 'g')) : [str];  
}  
console.log(string_chop('w3resource'));  
console.log(string_chop('w3resource',2));  
console.log(string_chop('w3resource',3)); 