Find occurrence of character in string JS

findOccurence.js

var theString = "This is a string.";
console.log(theString.split("i").length - 1);

//Example: Count amount of vowels in string
function amoutVowel(wort){
  var vowels = ["a","e","i","o","u","ä","ö","ü","E","O","A","I","U"];

  var countVowel = 0;
  for(var i = 0; i < vowels.length; i++) {
    if (wort.indexOf(vowels[i]) !== -1) {
      countVowel += wort.split(vowels[i]).length - 1;
    }
  }
  console.log(countVowel);
}