Trying to understand rövarspråket translator

I'm doing a beginner exercise, create a rövarspråket translator.

Write a function translate() that will translate a text into “rövarspråket”. That is, double every consonant and place an occurrence of "o" in between. For example, translate("this is fun") should return the string "tothohisos isos fofunon".

The solutions I was coming up with were bad, so I found one here and am trying to understand it.

var translate = function(text) {
  var string = text.toLowerCase();
  var vowels = ["a", "e", "i", "o", "u", " "];
  var y = "";
  for (i = 0; i < string.length; i++) {
     var current = string.charAt(i); 
    if (vowels.indexOf(current) != -1) {
        y = (y + (current));
    } else {
        y = (y + (current + "o" + current));
    }
  }
  return y;
}

console.log(translate("this is fun"));

• Why does if (vowels.indexOf(current) != -1) need -1 specifically? I tried -2, -1000, 1000, but they all break the function.

• If I change y = ""; to y = "XYZ", translate("this is fun") returns "XYZtothohisos isos fofunon". Why is it only before the first consonant and not all of them?