UPC validation doesn't work if check digit is 0


I have this algorithm which works for all 12-digit UPC codes, unless the final check digit is 0, in which case the validation always comes back as an invalid UPC code. If someone could look through and offer a suggestion or solution I would greatly appreciate it.

function checkUPCFormat12(formObj)
      var temp = formObj.value.trim();
      if (temp.length >11 && temp.length<= 12)            
            var myArray = formObj.value.split('');
            step1 = 0;
            var tempVar = myArray.length;
            for (var i=0;i<tempVar;i=i+2)
            step1 = step1 + +myArray[i];
            var step2=step1*3;
            step3 = 0;
            for(var i=1;i<tempVar-1;i=i+2)
            step4 = step2 + step3;
            step5 = ((Math.floor(step4/10) + 1)*10) - step4;
            if (step5 == myArray[myArray.length-1])
            //alert('valid UPC Code')
            return true;
            alert('You have entered an invalid 12-digit UPC Code. Verify and enter ALL numbers.')
            return false;

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ZvonkoSystems architectCommented:
From my point of view is check digit zero not allowed.
But if you want it, here my version:


function checkUPCFormat12(formObj){
  //var num = formObj.value.split(""); // this is for form element
  var num = formObj.split("");  // this is for String parameter
  if(num.length!=12) return false;
  var sum = 0;
  for(var sum=i=0;i<11;i++){
    if(i%2) sum += num[i]*1;
    else sum += num[i]*3;
  if((10-sum%10)%10==num[11]) return true;
  return false;



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