如果循环行为不符合预期，则循环多次[MongoDB，Nodejs]

如何解决如果循环行为不符合预期，则循环多次[MongoDB，Nodejs]

代码：-1

app.get("/",function(req,res) {

  // var whichDay = dayFormated.dayFormated();
  //
  // console.log(whichDay);
  Item.find({},function(err,itemsFound) {

    if (itemsFound.length === 0) {
      console.log(itemsFound.length);
      console.log("i Am Here if condition.");
      Item.insertMany(defaultItems,function(err) {
        console.log("I am inside insertMany");
        if (err) {
          console.log(err);
        } else {
          console.log("Successfully inserted default values to DB");
        }
      });
      **res.redirect('/');**
      console.log("exiting if bye :P");
    } else {
      console.log("back again at redirect");
      // console.log(itemsFound);
      res.render('list',{
        titleToDoList: "Today",itemslist: itemsFound
      });
    }

  });

});

控制台日志：-1

0
i Am Here if condition.
exiting if bye :P
0
i Am Here if condition.
exiting if bye :P
0
i Am Here if condition.
exiting if bye :P
0
i Am Here if condition.
exiting if bye :P
0
i Am Here if condition.
exiting if bye :P
0
i Am Here if condition.
exiting if bye :P
0
i Am Here if condition.
exiting if bye :P
0
i Am Here if condition.
exiting if bye :P
0
i Am Here if condition.
exiting if bye :P
0
i Am Here if condition.
exiting if bye :P
I am inside insertMany
Successfully inserted default values to DB
back again at redirect
I am inside insertMany
{ BulkWriteError: E11000 duplicate key error collection: todolistDB.items index: _id_ dup key: { _id: ObjectId('5f55f40136e7195ed7614bbb') }
    at OrderedBulkOperation.handleWriteError (My_Project_Directorynode_modules/mongodb/lib/bulk/common.js:1257:9)
    at resultHandler (My_Project_Directorynode_modules/mongodb/lib/bulk/common.js:521:23)
    at handler (My_Project_Directorynode_modules/mongodb/lib/core/sdam/topology.js:942:24)
    at fn (My_Project_Directorynode_modules/mongodb/lib/cmap/connection_pool.js:356:13)
    at handleOperationResult (My_Project_Directorynode_modules/mongodb/lib/core/sdam/server.js:558:5)
    at MessageStream.messageHandler (My_Project_Directorynode_modules/mongodb/lib/cmap/connection.js:277:5)
    at MessageStream.emit (events.js:198:13)
    at processIncomingData (My_Project_Directorynode_modules/mongodb/lib/cmap/message_stream.js:144:12)
    at MessageStream._write (My_Project_Directorynode_modules/mongodb/lib/cmap/message_stream.js:42:5)
    at doWrite (_stream_writable.js:415:12)
    at writeOrBuffer (_stream_writable.js:399:5)
    at MessageStream.Writable.write (_stream_writable.js:299:11)
    at Socket.ondata (_stream_readable.js:710:20)
    at Socket.emit (events.js:198:13)
    at addChunk (_stream_readable.js:288:12)
    at readableAddChunk (_stream_readable.js:269:11)
  name: 'BulkWriteError',driver: true,code: 11000,writeErrors: [ WriteError { err: [Object] } ],result:
   BulkWriteResult {
     result:
      { ok: 1,writeErrors: [Array],writeConcernErrors: [],insertedIds: [Array],nInserted: 0,nUpserted: 0,nMatched: 0,nModified: 0,nRemoved: 0,upserted: [] } },insertedDocs:
   [ { _id: 5f55f40136e7195ed7614bbc,itemName: 'Wakeup',__v: 0 },{ _id: 5f55f40136e7195ed7614bbd,itemName: 'Bath',__v: 0 } ] }
I am inside insertMany
{ BulkWriteError: E11000 duplicate key error collection: todolistDB.items index: _id_ dup key: { _id: ObjectId('5f55f40136e7195ed7614bbb') }
    at OrderedBulkOperation.handleWriteError (My_Project_Directorynode_modules/mongodb/lib/bulk/common.js:1257:9)
    at resultHandler (My_Project_Directorynode_modules/mongodb/lib/bulk/common.js:521:23)
    at handler (My_Project_Directorynode_modules/mongodb/lib/core/sdam/topology.js:942:24)
    at fn (My_Project_Directorynode_modules/mongodb/lib/cmap/connection_pool.js:356:13)
    at handleOperationResult (My_Project_Directorynode_modules/mongodb/lib/core/sdam/server.js:558:5)
    at MessageStream.messageHandler (My_Project_Directorynode_modules/mongodb/lib/cmap/connection.js:277:5)
    at MessageStream.emit (events.js:198:13)
    at processIncomingData (My_Project_Directorynode_modules/mongodb/lib/cmap/message_stream.js:144:12)
    at MessageStream._write (My_Project_Directorynode_modules/mongodb/lib/cmap/message_stream.js:42:5)
    at doWrite (_stream_writable.js:415:12)
    at writeOrBuffer (_stream_writable.js:399:5)
    at MessageStream.Writable.write (_stream_writable.js:299:11)
    at Socket.ondata (_stream_readable.js:710:20)
    at Socket.emit (events.js:198:13)
    at addChunk (_stream_readable.js:288:12)
    at readableAddChunk (_stream_readable.js:269:11)
  name: 'BulkWriteError',__v: 0 } ] }

---

代码：-2

Item.find({},function(err) {
        console.log("I am inside insertMany");
        if (err) {
          console.log(err);
        } else {
          console.log("Successfully inserted default values to DB");
          **res.redirect('/');**
        }
      });
      console.log("exiting if bye :P");
    } else {
      console.log("back again at redirect");
      // console.log(itemsFound);
      res.render('list',itemslist: itemsFound
      });
    }

  });

});

控制台日志：-2

    [nodemon] 2.0.4
    [nodemon] to restart at any time,enter `rs`
    [nodemon] watching path(s): *.*
    [nodemon] watching extensions: js,mjs,json
    [nodemon] starting `node app.js`
    server running on port 3000
    0
    i Am Here if condition.
    exiting if bye :P
    I am inside insertMany
    Successfully inserted default values to DB
    back again at redirect

---

代码2可以正常工作，但是代码1可以循环多次。有人可以向我解释为什么一个代码多次循环吗？我加粗了代码的地方以供参考。谢谢！。

请参阅我粘贴的日志，其中介绍了该情况。

解决方法

由于无法发表评论而添加为答案。

代码1循环多次，因为res.redirect('/')在Item.insertMany的回调函数之外。

这是nodejs异步行为的经典示例。当代码到达mongo查询时，它将触发回调，并进入回调队列，等待数据库的响应。

因此，nodejs移至回调之外的下一个代码块，即res.redirect('/')。这使代码再次调用'/'端点。并且在完成insert db操作（这将使长度大于0）之前，您的代码始终将长度查找为0，并不断在循环中命中/端点。