const text = "Hello World";
let message = "";
let previous = "";

for (let f = 0; f < text.length; f++) {
  if (message != "") {
    previous = text[f - 1];
    if (previous != " " && text[f] != " ") {
      message += ".";
    }
  }
  message += text[f];
}
alert(message);  // "H.e.l.l.o W.o.r.l.d"