ewa.js

import R from 'ramda';

function ewa (decay, data, arr=[]) {
  const beta = Math.E**(-1/decay);
  const checkArr = (b) => {
    return arr.length === 0 
    ? R.prop('value', data[1])
    : R.prop('value', arr[R.length(arr)-1]) * b + R.prop('value', data[1]) * (1-b)
  };
  
  if (R.length(data) === 0) return arr;
  
  return ewa(
    decay,
    R.tail(data),
    R.append(
      R.pipe(
        R.assoc('value', checkArr(beta)),
        R.assoc('label', `${decay} Day`),
      )(data[1]),
      arr
    ),
  );
}