Alex_mos

<select id="border_style" >
        <option value="dotted">Dotted</option>
        <option value="dashed">Dashed</option>
        <option value="solid">Solid</option>
        <option value="double">Double</option>
        <option value="groove">Groove</option>
        <option value="ridge">Ridge</option>
        <option value="inset">Inset</option>
        <option value="outset">Outset</option>
    </select></section>
<center><div style="background-color: antiquewhite; width: 50px; height: 50px;" id="div"></div></center>

let div = document.getElementById('div');//Сам блок
var select = document.getElementById("border_style");

select.addEventListener('change', ()=>{
  div.style.border = `1px ${select.value} red`
})

console.log(n.slice(n.length - 3).split('').map(Number))