https://developers.weixin.qq.com/miniprogram/dev/wxcloud/reference-sdk-api/database/command/aggregate/AggregateCommand.arrayElemAt.html

const $ = db.command.aggregate
db.collection('exams').aggregate()
  .project({
    first: $.arraElemAt(['$scores', 0]),
    last: $.arraElemAt(['$scores', -1]),
  })
  .end()

示例代码错误。应为：$.arrayElemAt(['$scores', 0])