<input class='singleInfo__price' value='0'>
<button class='plus'>+</button>
$(".plus").click(function() {
  var $price = $(".singleInfo__price");
  $price.val(parseInt($price.val()) + 1);
  $price.change();
});

External CSS

This Pen doesn't use any external CSS resources.

External JavaScript

  1. https://cdnjs.cloudflare.com/ajax/libs/jquery/3.3.1/jquery.min.js