function getLargestNumber(array) {
  let result = array.slice();
  result.sort((a, b) => {
    if (a + '' + b < b + '' + a) {
      return 1;
    }
    return -1;
  });
  return result.join('')/1;
}

console.log('result:', getLargestNumber([50, 2, 1, 9]));