How to pass a value from one function to another

  var boxes = document.getElementsByName('toggle');

  function markPreceding() {
    var curIndex = null;
    for (var j = 0; j < boxes.length; j++) {
      if (boxes[j].checked) {
        curIndex = j;
      }
    }
  }

  function checkInputs() {
    for (var k = 0; k <= curIndex.length; k++) {
      boxes[k].checked = true;
    }
  }

  for (var i = 0; i <= boxes.length; i++) {
    boxes[i].onchange = markPreceding;
    boxes[i].onchange = checkInputs;
  }

<input type="checkbox" id="product-1" name="toggle">
<input type="checkbox" id="product-2" name="toggle">
<input type="checkbox" id="product-3" name="toggle">
<input type="checkbox" id="product-4" name="toggle">
<input type="checkbox" id="product-5" name="toggle">

Have a problem passing this "curIndex" value to checkInputs function. This should check inputs before checked input and get its value to do it.