[Dilettante_Pro]

var items = [
  {
  blue: false,
  red: true
  },
  {
  blue: false,
  red: false
  },
  {
  blue: false,
  red: true
  },
  {
  blue: false,
  red: false
  },
  {
  blue: false,
  red: true
  }
];

var result = 100 * items.filter(item => item.red).length / items.length;

alert(result + "%");