JavaScript 代码未按所需顺序运行（Node.js、MongoDB）

Question

我知道 Node.js 的非阻塞 I/O 以及什么是异步函数，但我很难指出为什么这段代码运行起来像它运行一样。

我正在连接到 MongoDB 集合，搜索重复项，将第一个重复的值放入数组对象 (dupIndex) 中。 当数组打印时我看到 2 个值 (console.log(dupIndex);)，但是当我稍后使用 .length 属性时看到 0 (console.log(dupIndex.length); em>) -- 当我真正期待 2 时。

我想继续并使用我在 dupIndex 中的数据来操作集合（例如使用 deleteMany 方法），但是如果它显示为 0，我不能，至少不是这样。有人可以解释一下并帮助我吗？

谢谢！

//connect us to a running MongoDB server
const {MongoClient, ObjectID} = require('mongodb');

var dataBase = "TodoApp";
//connecting to a database
//connects to the database /TodoApp, if no such db exists it auto creates it
MongoClient.connect(`mongodb://localhost:27017/${dataBase}`, (err, db)=>{
    if(err){
        //return or 'else' - so if an error happens, it won't continue
        return console.log(`Unable to connect. Error: ${err}`);
    }

    console.log(`Connected to MongoDB server. Database: ${dataBase}.`);
        var dupIndex = [];
    //find all
    db.collection('Todos')
    .find()
    .toArray().then((docs) => {
        for ( var i =0; i< docs.length -1; i++){
            for(var j =i+1; j< docs.length; j++){
                    if(docs[i].text === docs[j].text)
                    {   
                        console.log(`in`);
                        dupIndex.push(docs[i].text);
                        i++;
                    }
            }
        }
        console.log(dupIndex);

    }, (err)=> {
        console.log(`unable t o fetch`);
    });

    console.log(dupIndex.length);
    // for(var i = 0; dupIndex)
    //close the connection to the db
    db.close();
});

Answer 1

因为 console.log(dupIndex.length);在嵌套循环之前运行。

db.collection('Todos')
.find()
.toArray()

这是一个异步调用，控制权传递给 console.log(dupIndex.length); 尝试编写 console.log(dupIndex.length);在 console.log(dupIndex) 旁边；

例如：

 db.collection('Todos')
.find()
.toArray().then((docs) => {
    for ( var i =0; i< docs.length -1; i++){
        for(var j =i+1; j< docs.length; j++){
                if(docs[i].text === docs[j].text)
                {   
                    dupIndex.push(docs[i].text);
                    i++;
                }
        }
    }
    return dupIndex;
}, (dupIndexRecieved)=> {
    console.log(dupIndexRecieved.length); data recieved here
}, (err)=> {
    console.log(`unable t o fetch`);
});

Answer 2

//将我们连接到正在运行的 MongoDB 服务器

const {MongoClient, ObjectID} = require('mongodb');

var dataBase = "TodoApp";
//connecting to a database
//connects to the database /TodoApp, if no such db exists it auto creates it
MongoClient.connect(`mongodb://localhost:27017/${dataBase}`, (err, db)=>{
if(err){
    //return or 'else' - so if an error happens, it won't continue
    return console.log(`Unable to connect. Error: ${err}`);
}

console.log(`Connected to MongoDB server. Database: ${dataBase}.`);
    var dupIndex = [];
//find all
db.collection('Todos')
.find()
.toArray().then((docs) => {
    for ( var i =0; i< docs.length -1; i++){
        for(var j =i+1; j< docs.length; j++){
                if(docs[i].text === docs[j].text)
                {   
                    console.log(`in`);
                    dupIndex.push(docs[i].text);
                    i++;
                }
        }
    }

    //Mongo db find returns a promise. The value you are accessing with the then function. So whatever you want to do with the db query return value you have to do it here inside the then function.

    console.log(dupIndex.length);

    console.log(dupIndex);
    // for(var i = 0; dupIndex)
    //close the connection to the db
    db.close();

}, (err)=> {
    console.log(`unable t o fetch`);
});

//when you make the call here its get called before the then function. Thats why the length is zero. 

});