Making the step act as a guide in <input type=“number”>

Question

How could I make the step attribute for a <input type="number"> act more of a guide, so I could input any positive integer but the increment arrows would change the current value by a fixed amount.

eg I could enter 0,1,2,3,4,5,6,7,... and the step would increment the values as 0,6,12,18,24,... etc...

I could implement this if I knew how to capture the step up/step down events in javascript. However, I can't find a way to do that. I've shown exactly what I'd like in the pseudo code below:

<input id="num" type="number">
<script>
  inp = document.getElementById("num");
  var increment = 6;
  if ( step up pressed ){
    inp.value += increment;
  }
  else if  (step down pressed ){
    inp.value -= increment;
  }
</script>

Answer 1

这行得通吗？

<input id="num" type="number" step="6">

Answer 2

It seems like a horrible hack but here's an answer to my question:

<input id='num' type=number step=0.00001 onchange="increment()">

<script>
    document.prev_num = document.getElementById("num").value;
    function increment() {
      var step = 6;
      var catch_step = 0.00001;
      var curr_num = parseFloat(document.getElementById("num").value);
      if ( Math.abs(curr_num - (document.prev_num + catch_step)) < 1e-7 ){
        curr_num  = curr_num - catch_step + step;
      }
      else if (Math.abs(curr_num - (document.prev_num - catch_step)) < 1e-7){
        curr_num = curr_num + catch_step - step;
      }
      else if (curr_num != parseInt(curr_num)){
        alert("You can only input integers.")
        curr_num = parseInt(curr_num);
      }
      document.getElementById("num").value = curr_num;
      document.prev_num = curr_num;
    }
</script>

So HTML forces the numbers to be multiples of 0.00001, which includes all integers.

Javascript then acts when the number is changed. If that change is only a change of 0.000001 the step button was almost certainly pressed.

Non integers won't be accepted.