Tribonacci - it works basically like a Fibonacci, but summing the last 3 (instead of 2) numbers of the sequence to generate the next.

tribonacci.js

function tribonacci(n) {
  let seq = [1,1,1];
  let index;

  for (let i = 3; i < n; i++) {
    seq.push(seq[seq.length - 1] + seq[seq.length - 2] + seq[seq.length - 3])
  }
  console.log(seq)
  return seq.slice(0, n);
}

tribonacci(12)