db.collection('cloths').aggregate()
  .lookup({
    from: 'recommoned',
    localField: 'sn',
    foreignField: 'coverProductList',
    as: 'coverImgUrls',
  })
  .project({
    _id: false, 
    coverImgUrls: true
  })
  .match({
    coverImgUrls: _.neq([])
  })
  .replaceRoot({
    newRoot: '$coverImgUrls'
  })
  .end({
    success: function (res) {
      console.log(res.list)
    },
    fail: function (err) {
      console.error(err)
    }
  })

上面是会报错的，改成下面的写法就不报错了

.replaceRoot({
  newRoot: {
    root: '$coverImgUrls'
  }
})