function myObject()
{
  this.value = 0;
}

var currentId1 = new myObject()
var currentId2 = new myObject()
var currentId3 = new myObject()

function changeFiets(id, imgId, merk, blnUp)
{
  switch (merk)
  {
  case 1:
    veranderImage(currentId1,fietsen1,blnUp);
    zetImage(id, imgId, fietsen1[currentId1.value]);
  break;
  case 2:
    veranderImage(currentId2,fietsen2,blnUp);
    zetImage(id, imgId, fietsen2[currentId2.value]);
  break;
  case 3:
    veranderImage(currentId3,fietsen3,blnUp);
    zetImage(id, imgId, fietsen3[currentId3.value]);
  break;
  } 
}

function veranderImage(index, imgArray, blnUp)
{
  if (blnUp)
  {
    index.value = index.value + 1
    if (index.value > imgArray.length -1) 
    {
      index.value = 0
    }
  }
  else
  {
    index.value = index.value - 1
    if (index.value < 0) 
    {
      index.value = imgArray.length - 1
    }
  }
}

function zetImage(id, imgId, waarde)
{
  var splitResult = waarde.split("~")
  var elem = document.getElementById(id);
  elem.href = splitResult[0];
  
  elem = document.getElementById(imgId);
  elem.src = splitResult[1];
}