function findFirstRepeat(test) {
  const seen = {};
  let solution = -1;

  test.split('').some((c, i) => {
    if (!seen.hasOwnProperty(c)) {
      seen[c] = i;
    } else if (solution < 0 || seen[c] < solution) {
      solution = seen[c];
    }
    
    return solution === 0;
  });
  
  return solution >= 0 ? test[solution] : false;
}

console.log(`earliest repeat: ${findFirstRepeat('abadfdasdfasdfasfasf')}`);

function findFirstRepeat(input) {
  const charHash = {};
  const charArray = input.split('');
  const matchArray = new Array(charArray.length);
  
  charArray.forEach((char, index) => {
    if (charHash[char] !== undefined) matchArray[charHash[char]] = 'MATCH';
    else charHash[char] = index;
  });
  
  return charArray[matchArray.indexOf('MATCH')] || 'none found';
}

document.write(findFirstRepeat("abcdebcd") + "<br/>") // should return "b"
document.write(findFirstRepeat("abba")) // should return "a"