first_repeating_character.js

function first_repeating_character(ch) {
  if(!ch){
    return null;
  }
    let charSet = new Set(ch)
    for (var c of [...ch] ) {
      if(charSet.has(c)){
        return c;
      }     
      else {
        charSet.add(c)
      }  
    }
    return 'None'
}


function first_repeating_character(characters) {
  if (characters == undefined || characters.length == 0) {
    return undefined;
  }

  let dict = {};

  for (i = 0; i < characters.length; i++) {
    if (dict[characters.charAt(i)] == undefined) {
      dict[characters.charAt(i)] = true;
    } else {
      return characters.charAt(i);
    }
  }
console.log(dict);
  return undefined;
}

console.log(first_repeating_character("ACDEFGHIAJBQWERUAAAA"));
console.log(first_repeating_character('CDEFGHIJQWERUAAAA'));