猫鼬 findOne 函数返回未定义

问题描述：

我知道我们在这里有很多这样的问题，但似乎没有一个答案适用于我的代码，请看:

I know we have lots of questions like this in here, but none of the answers seems to work on my code, take a look:

establishment = mongoose.model('Establishment' , new Schema({
    //_id : Schema.Types.ObjectId,
    name : String,
    cnpj : String,
    description : String,
    address : String,
    products : [],
    isActive : {type: Boolean, default: true}
}));




app.get('/home',(req, res)=>{
    var a = establishment.findOne({_id : "57d83a867d3ba20fcb657dc7" } , (err, estab)=>{
        if(err){
            return err;
        }
        console.log("inside the function: "+estab.name);
        return estab
    });
    console.log("outside the function: "+a.name)
})

当 findOne() 函数内给出任何输出时，一切都很好，但是当我返回它时，只说它未定义"

when any output is given inside the findOne() function, it all work out just fine, but when i return it, just says it's "undefined"

答

发生这种情况是由于 node js 的非阻塞、异步性质.这意味着任何需要很长时间才能完成的活动，例如文件访问、网络通信和数据库操作，都会被请求并放在一边，直到结果准备好并通过回调函数返回.

It's happening due to the non-blocking,asynchronous nature of node js. This means any activity taking a long time to finish, such as file access, network communication, and database operations, are requested and put aside until the results are ready and returned via a callback function.

这就是您未定义的原因，因为当 db 操作发生时，由于非阻塞性质，外部控制台会被执行.

That's why you getting undefined, because while the db operation is happing the outer console get executed due to non-blocking nature.

要得到结果，你可以这样做

To get the result you can do this

app.get('/home',(req, res)=>{
    establishment.findOne({_id : "57d83a867d3ba20fcb657dc7" } , (err, estab)=>{
        if(err){
            return res.send(err);
        }
        console.log("inside the function: "+estab.name);
        // what ever proccing you need to do with result do here and finally return res
        res.json(estab)
    });
})

猫鼬 findOne 函数返回未定义

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