first_repeating_character.js

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);
    }
  }

  return undefined;
}

console.log(first_repeating_character("BACDEFGHIJBQWERUAAAA"));