The Mean (30-period SMA) in JavaScript

JavaScript

/**
 * Simple Moving Average over a trailing window of n values.
 *
 * Returns an array the same length as `values`. The first n-1 positions are
 * null (warmup); a position emits only when its trailing window of n entries
 * is fully populated with finite numbers, and the value is then sum / n.
 * n <= 0 or fewer than n inputs yields an all-null result. null/NaN inputs
 * are skipped from the running sum and count.
 *
 * @param {(number|null)[]} values - Input series
 * @param {number} n - Window size (default 30)
 * @returns {(number|null)[]} Same-length array of SMA values (or null)
 */
export function sma(values, n = 30) {
  const len = values.length;
  const result = new Array(len).fill(null);
  if (n <= 0 || len < n) return result;

  let sum = 0;
  let count = 0;

  for (let i = 0; i < len; i++) {
    const v = values[i];
    if (v != null && !Number.isNaN(v)) {
      sum += v;
      count++;
    }
    // drop the element that just left the trailing window
    if (i >= n) {
      const old = values[i - n];
      if (old != null && !Number.isNaN(old)) {
        sum -= old;
        count--;
      }
    }
    // emit only once a full window of finite values is present
    if (i >= n - 1 && count === n) {
      result[i] = sum / n;
    }
  }

  return result;
}