How to write a tip calculator in code?

Function tipcalculate (slider, bill){  var tip = document.getelementbyid('tipamount');   var slideval = document.getelementbyid('slideval');   var bill = document.getelementbyid(bill).value;   var prcnt = slider * .01;       if (bill == null || bill == '') {      tip.innerhtml = ' enter an amount';       return false;   }  if(isnan(bill)) {      tip.innerhtml = ' enter a number';       return false;   }  if(bill > = 0){      tip.innerhtml = '$' + (bill * prcnt) .tofixed(2);   slideval.innerhtml = slider + '%';   }}