<form action="">
  <input type="text" id="onlyNumber" onkeypress="acceptOnlyNumber(event)" />
</form>

<script>
  function acceptOnlyNumber(event) {
    console.log(event.target);
    if (event.which && ((event.which > 47 && event.which < 58) || event.which == 8)) {
      console.log('number');
    } else {
      console.log('not number');
      event.preventDefault();
    }
  }
</script>