classList.toggleの第2引数サンプル

sample

<p><input value="1" id="n"></p>
<h2 id="s">sample</h2>
<button id="b">button</button>

var s = document.getElementById('s');
var b = document.getElementById('b');
var n = document.getElementById('n');
b.addEventListener('click', function() {
  var val = n.value;
  s.classList.toggle('red', val < 10 );
});