find longest substring in string

str-finder.js

function find2(str) {
  let _helper = [];
  return str
    .split('')
    .reduce((acum, item, i, originalArr) => {
      if (_helper.some(char => char === item)) {
        acum.push(_helper);
        _helper = [];
      }
      _helper.push(item);
      if (i === originalArr.length - 1) {
        acum.push(_helper);
      }
      return acum;
    }, [])
    .map(x => x.join(''))
    .reduce(
      (a, b) => (a.length > b.length ? a : b),
      '',
    );
}

console.log(find2('abcdcfepoi')); // cfepoi
console.log(find2('dertyuioplkjhgfdqwe')); // dertyuioplkjhgf
console.log(find2('')); // ''