Simple string padding method using Array.

stringpad

function pad(string, length, padstring){
  if (!string || !length || !padstring) 
    throw new Error('all parameters must be defined');
  var outputstr = '' + string,
      diff = length - outputstr.length;
      
  if (diff < 1) return string;
  
  var arr = new Array(diff);
  arr.push(outputstr);
  return (arr.join(padstring));
}

//TEST
console.log(pad('a', 15, 'b'));