My codility solution to find the longest chain in an array where A[i] -> A[A[i]]:

longest-chain.js

const solution = A => {
  let longest = 0
  let next = 0
  let count = 1
  while (count < A.length) {
    let len = 0
    while (A[next] !== -1) {
      len++
      const prev = next
      next = A[next]
      A[prev] = -1
    }
    if (len > A.length / 2) return len // early exit since we already have longest
    if (len > longest) longest = len
    next++
    count++
  }
  return longest
}