Concatenate consecutive characters followed by the number of characters if it is over 1

gistfile1.txt

var input = 'aaaabbbbccccpqesrt';
var compressedString = '';
var s = input.match(/([a-zA-Z])\1*/g) || [];

s.forEach(function(value) {
  var k = '';	
  if(value.length > 1) {
    k = value.length;
  }
  compressedString += value.charAt(0) + k;
});