Calculate the simple moving average (SMA) from an object array

sma.js

function sma(data, steps = 7, key = 'value') {
  return data.map((obj, index) => {
    const offset = index - Math.floor(steps / 2);
    const window = data.slice(offset, steps + offset);
    return Object.assign({}, obj, {
      average: window.reduce((sum, curr) => {
        return curr[key] ? sum + curr[key] : null;
      }, 0) / window.length
    });
  }).filter(d => d.average);
}