What Is It ​

interface Person {
  name: string
  age: number
  grade: string
}

const list: Person[] = [
  { name: '小明', age: 2, grade: '小一班' },
  { name: '小张', age: 5, grade: '大二班' },
  { name: '小夏', age: 3, grade: '中一班' },
  { name: '小王', age: 3, grade: '小一班' },
  { name: '小李', age: 4, grade: '中一班' },
]

// return groupBy(list, (single: Person) => single.grade)
console.log(groupBy(list, (single: Person) => single.grade))