function recursieveCijferSom(input) {
    input = input.toString();
    var sum = 0;
    for (var i=0; i < input.length; i++) {
        sum += parseInt(input.charAt(i));
    }
    return (sum > 9 ? recursieveCijferSom(sum) : sum);
}

function recursieveCijferSom ( input ) {
  while ( input.length > 1 ) input = (''+input).split('').reduce( (p,v) => p+~~v , 0 )+'';
  return input;
}

function recursieveCijferSom ( input ) {
  do { input = (''+input).split('').reduce( (p,v) => p+~~v , 0 )+''; } while ( input.length > 1 );
  return input;
}