const seq = [
    0, 0, 0,
    1, 1, 1,
    1, 1, 1, 1, 1,
    0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
    0, 0, 0, 0, 0, 0, 0, 0, 0,
    1, 1, 1, 1, 1,
    0, 0, 0, 0, 0, 0,
    0, 0, 0, 0, 0, 0, 0, 0, 0,
    1, 1, 1, 1, 1,
  ];

const longestSeq = (seq) => seq
    .reduce(
        ({count, max}, item) => item === 0
        ? { count: ++count, max: Math.max(count, max) }
        : { count: 0, max: max },
      { count: 0, max: 0} )
    .max;

console.log(longestSeq(seq));

const seq = [
    0, 0, 0,
    1, 1, 1,
    1, 1, 1, 1, 1,
    0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
    0, 0, 0, 0, 0, 0, 0, 0, 0,
    1, 1, 1, 1, 1,
    0, 0, 0, 0, 0, 0,
    0, 0, 0, 0, 0, 0, 0, 0, 0,
    1, 1, 1, 1, 1,
  ];

const longestSeq = (seq) => {
    let maxCount = 0,
        curCount = 0,
        curItem, prevItem,
        l = seq.length+2, // +1+1 to finish last sequence and compare 'undefined' with previous
        i = 0;
    for (; i < l; ++i) {
      curItem = seq[i];
      if (curItem === prevItem) ++curCount;
      else {
        if (curCount > maxCount) maxCount = curCount;
        curCount = 1;
        prevItem = curItem;
      }
    }
    return maxCount;
}

console.log(longestSeq(seq));