let array = [];
let $ = function(){
return document.querySelector(...arguments);
};
for(let i = 0; i<=15; i++){
let tmp = `.rad-${i}{border-radius: var(--x${i})!important}`;
array.push(tmp);
}
$('#output').value = array.join('\n');
<textarea id='output'></textarea>
#output {position: relative;width: 100%;height: 60vh;}