Google Sheets WEIGHTED_AVERAGE custom function

/**
 * Calculates weighted average with latter values weighing more.
 *
 * Weights for each value are calculated using https://en.wikipedia.org/wiki/Weighted_arithmetic_mean#Exponentially_decreasing_weights
 * 
 * The average itself is calculated by summing up each value multiplied by its weight: https://en.wikipedia.org/wiki/Weighted_arithmetic_mean#Convex_combination_example
 *
 * @param {number|Array<number>} values Values, of which to calculate average
 * @param {number} delta Delta, 0 < delta < 1, basically, how much weight the latest value will have when calculating average
 * @return The input multiplied by 2.
 * @customfunction
 */
function WEIGHTED_AVERAGE(values, delta) {
  if (!Array.isArray(values)) {
    return values;
  }
  
  const w = 1 - delta;
  const m = values.length;
  
  return values.reverse().reduce(function(result, value, i) { 
    return result + weight(i) * value;
  }, 0);
  
  function V1() {
    return (1 - Math.pow(w, m)) / (1 - w);
  }
  
  function weight(i) { 
    return Math.pow(w, i) / V1(w, m);
  }
}