Programming challenge to find an element within a sequence.

WIN-sequence-challenge.js

/*
 * TASK:
 * Calculate the 15th element for this sequence: 2, 2, 4, 6, 10, 16, ...
 */

const sequence = (n) => {
  if (n < 0) {
    throw new Error('n must be >= 0');
  }
  return _sequence(n, 0, 0, 0);
}

const _sequence = (n, i, prev1, prev2) => {
  const thisElem = (i <= 1)
      ? 2
      : prev1 + prev2;
  return i == n
      ? thisElem
      : _sequence(n, i + 1, prev2, thisElem);
}

for (let i = 0; i < 15; i++) {
  console.log(i + ' -> ' + sequence(i));
}

/*
 * QUESTIONS:
 *
 * 1) What is the 15th element of the sequence?
 *
 *      1220
 *
 * 2) What is your solution time and space complexity, for the n(th) element
 *    of the sequence?

 *      Time complexity = O(n)
 *      Space complexity = O(n), due to the recursive call stack
 * 
 *      This could become O(1) if a loop was used instead.
 *
 * 3) What clean code principles have been used, and why?
 *
 *      - Line breaks to enhance readability
 *      - Sensible variable names for readability
 *      - Clean public interface (sequence), with the internal implementation
 *          details kept private (_sequence), for ease of use
*/