calculate the z score by group. in javascript

index.tsx
const data = [
  { group: 'A', value: 10 },
  { group: 'A', value: 20 },
  { group: 'B', value: 30 },
  { group: 'B', value: 40 },
];

const groupedData = data.reduce((acc, curr) => {
  acc[curr.group] = acc[curr.group] || [];
  acc[curr.group].push(curr.value);
  return acc;
}, {});

console.log(groupedData);
// output: { A: [ 10, 20 ], B: [ 30, 40 ] }
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index.tsx
const groupZScores = {};
for (const group in groupedData) {
  const groupValues = groupedData[group];
  const sum = groupValues.reduce((acc, v) => acc + v, 0);
  const mean = sum / groupValues.length;
  const variance = groupValues.reduce((acc, v) => acc + Math.pow(v - mean, 2), 0) / (groupValues.length - 1);
  const stdDev = Math.sqrt(variance);

  groupZScores[group] = groupValues.map((v) => (v - mean) / stdDev);
}

console.log(groupZScores);
// output: { A: [ -1, 1 ], B: [ -1, 1 ] }
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