对数组中对象的嵌套属性使用下划线countBy

function countBy(arr, property) {
  return arr.reduce((count, obj) => {
    const value = obj[property];
    count[value] = (count[value] || 0) + 1;
    return count;
  }, {});
}

// 示例用法
const users = [
  { name: 'Alice', age: 25, gender: 'female' },
  { name: 'Bob', age: 30, gender: 'male' },
  { name: 'Charlie', age: 25, gender: 'male' },
  { name: 'David', age: 35, gender: 'male' },
  { name: 'Eve', age: 30, gender: 'female' },
];

const countByAge = countBy(users, 'age');
console.log(countByAge);
// 输出：{ '25': 2, '30': 2, '35': 1 }