无法返回从 mongodb 获取的数据

Question

I can successfully fetch all data stored in a collection called "topics" by using find() with cursor and foreach. However, when I try to set the fetched information to variable call "data" and send it back to the page, it always display an empty array.

How can I fix it?

app.get('/api/get_topics', (req, res)=>{
    let data = [];
    MongoClient.connect(url, (err, db)=>{
        const dbo = db.db('adoption');
        let cursor = dbo.collection('topics').find();
        cursor.forEach((el)=>{
            const obj = {
                img     : el.img,
                title   : el.title,
                content : el.content
            }
            data.push(obj);
        });
        db.close();
    });
    res.json(data); // [] ---> always empty
    res.end();
});

Answer 1

try this

app.get('/api/get_topics', (req, res)=>{
    let data = [];
    MongoClient.connect(url, (err, db)=>{
        const dbo = db.db('adoption');
        let cursor = dbo.collection('topics').find();
        cursor.forEach((el)=>{
            const obj = {
                img     : el.img,
                title   : el.title,
                content : el.content
            }
            data.push(obj);
        });
        db.close();
        res.json(data); // [] ---> always empty
        res.end();
    });
});

Answer 2

You need to return the data after your connection to the database is done. Before you close the connection and after your forEach : because "forEach" is syncron & blocks the event loop

 app.get('/api/get_topics', (req, res)=>{ let data = []; MongoClient.connect(url, (err, db)=>{ const dbo = db.db('adoption'); let cursor = dbo.collection('topics').find(); cursor.forEach((el)=>{ const obj = { img: el.img, title: el.title, content: el.content } data.push(obj); }); res.json(data); res.end(); db.close(); }); });

Note: You should create a global connection and not for every request. A connection can be re used.

Answer 3

The reason for the empty data is because MongoClient.connect() is an asynchronous function, meaning the code is not waiting for your callback to push the content to your data array before setting the response data.

As others have pointed out, you can set and end your response within your callback, but I am skeptical that that will work. Another approach would be to use an async/await method . You can see this usage from the official MongoDB docs .

In addition, I believe you can omit res.end() or move it to the catch.

For example:

app.get('/api/get_topics', (async (req, res) => {
    try {
        let data = [];
        const db = await MongoClient.connect(url);
        const dbo = db.db('adoption');
        const cursor = dbo.collection('topics').find();
        cursor.forEach((el)=>{
            const obj = {
                img     : el.img,
                title   : el.title,
                content : el.content
            }
            data.push(obj);
        });
        db.close();
        res.json(data);
    } catch (err) {
        // Handle an error here...
        console.log(err);
        res.end();
    }
}));

Answer 4

MongoClient.connect(url, callback) ...

• callback (function) – this will be called after executing this method. The first parameter will contain the Error object if an error occured, or null otherwise. While the second parameter will contain the initialized db object or null if an error occured. ( https://mongodb.github.io/node-mongodb-native/api-generated/mongoclient.html )

Which hints that the connect() fnc is an async function. Hence res.json(data); res.end(); will be executed before the callback of the connect function is being called where you populate your array.