how can i check if the text is an Isogram ?

THIS IS WHAT I HAVE SO FAR :

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function isIsogram(text) {
  // add each char to a set
  // https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Set
  // note: a set drops dup values
  // thus, to see if all the chars were unique,
  // check length of text and the size of the set
 
 
  var splittedText = text.split("");
  text.forEach(function(element){
      if(element.repeat(1) === true || text.length === 0)
      return true;
      else
      return false;
  });
 
}


console.log(isIsogram("abc"))