将类添加到数组的最后一项

var points = new Array(870);
for (var i = 0; i < 870; i++){points[i] = i + 1;}
points[points.length-1] = '<span class="custom-class">'+points[points.length-1]+'</span>'
$(".numbers").html(points.join(', '))

.custom-class{
  color: red;
}

<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script>
<div class=numbers></div>

var points = new Array(870);
for (var i = 0; i < 870; i++) {
  points[i] = `<span>${i + 1}</span>`
}
$(".numbers").html(points.join(' '))
  .find(':last-child')
  .addClass('myClass')

.myClass {
  color: red;
}

<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script>
<div class="numbers"></div>