[рони]

Владислав Star,

function fn(a) {
  var n = 0;
  while (a > 1000) {
    a /= 1000;
    n++;
  }
  return +a.toFixed(1) + (["", "k", "m", "b", "t"][n] || "");
}

alert(fn(145));
alert(fn(145800));
alert(fn(6458300));