let ar8 = [];

function f8() {
  let out = document.querySelector(".out-8");
  let out2 = document.querySelector(".out-8-1");
  let str = "";

  ar8[3] = 3.14;
  ar8[4] = 17;
  ar8[6] = 5;

  for (i = 0; i < ar8.length; i++) {
    str += `${ar8[i]}-`;
  }
  out.innerHTML = str;
  out2.innerHTML = ar8.length;
}

document.querySelector(".b-8").onclick = f8;