// 成绩等级分为A、B和C三级
function getGrade(score) {
  return score < 60 ? 'C' :
    score < 80 ? 'B' : 'A';
};
// 学生及其成绩
let students = [
  { name: '张三', score: 84 },
  { name: '李四', score: 58 },
  { name: '王五', score: 99 },
  { name: '赵六', score: 69 }
];

function groupBy(students) {
  return [{}].concat(students).reduce((s, c) => {
    const grade = getGrade(c.score);
    !s[grade] && (s[grade] = []);
    s[grade].push(c);
    return s;
  })
}

// console.log(groupBy(students));