用classname替换span的内容

I want to try use javascript to replace content of span. here is my js code.

function formatCurrency(num) {
  num = num.toString().replace(/\$|\,/g,'');
  if(isNaN(num))
    num = "0";
  sign = (num == (num = Math.abs(num)));
  num = Math.floor(num*100+0.50000000001);
  cents = num%100;
  num = Math.floor(num/100).toString();
  if(cents<10)
    cents = "0" + cents;
  for (var i = 0; i < Math.floor((num.length-(1+i))/3); i++)
    num = num.substring(0,num.length-(4*i+3))+'.'+ num.substring(num.length-(4*i+3));
  return (((sign)?'':'-') + 'Rp' + num  );
}

content of span will replace with input field, this is html code.

<input name="num" type="text" id="num" onkeyup="document.getElementsById('format').innerHTML = formatCurrency(this.value);"/>
<span id='format' class="tooltip" style="z-index: 1;">Rp. 0 </span>

That code is work, but it will get content by id, if I want to get content by classname, what should i do?