在对象数组中，按特定键对分组的值进行计数

function countBy(arr, key) {
  let counts = {};
  
  for (let obj of arr) {
    let group = obj[key];
    counts[group] = (counts[group] || 0) + 1;
  }
  
  return counts;
}

// 示例用法
let data = [
  { name: 'Alice', group: 'A' },
  { name: 'Bob', group: 'B' },
  { name: 'Charlie', group: 'A' },
  { name: 'David', group: 'C' },
  { name: 'Eve', group: 'B' },
  { name: 'Frank', group: 'A' }
];

let result = countBy(data, 'group');
console.log(result);

{
  A: 3,
  B: 2,
  C: 1
}