为什么我的异步mongodb查询函数挂起？

Question

First of all, please forgive me if this is a duplicate, I am new to coding and Javascript in general.

I have an async function that queries mongodb based on an objects passed in the function call. The function executes, and returns the results to a callback function which logs the results to the console, and then hangs. Ultimately, I want to take the results of the async query and then do something with them outside the original async function. I am not understanding why it hangs after it logs to the console.

const MongoClient = require('mongodb').MongoClient;
let fObj = {
    field : {},
    limit : 100
}

let cObj = {
    dbName : 'myNewDatabase',
    colName : 'newCollection'
}

async function findDoc(cObj,fObj) {
    const url = 'mongodb://localhost:27017';
    const client = new MongoClient(url, { useNewUrlParser: true });
    try {
        await client.connect();
        const db = client.db(cObj.dbName);
        const col = db.collection(cObj.colName);
        console.log(`Connection Made to ${db.databaseName} database.`);
        return await col.find(fObj.field).limit(fObj.limit).toArray();
        client.close();
    } catch (err) {
        console.log(err.stack);
    }
};

findDoc(cObj,fObj).then(function(result) {
    console.log(result);
});

The code executes, and logs the results to the console, but then hangs. I have to ctrl-c out to get it to end. What am I missing?

Answer 1

I suppouse you're running your code with NodeJs. This implies that you have a promise hanging up, which keeps the server running. I assume this is because your connection to the DB is still open after you have found the document.

You need to move your client.close(); statement above the return statement, because it is never reached otherwise and your server will hang up forever.

Your code will look like this in the end:

const MongoClient = require('mongodb').MongoClient;
let fObj = {
    field : {},
    limit : 100
}

let cObj = {
    dbName : 'myNewDatabase',
    colName : 'newCollection'
}

async function findDoc(cObj,fObj) {
    const url = 'mongodb://localhost:27017';
    const client = new MongoClient(url, { useNewUrlParser: true });
    try {
        await client.connect();
        const db = client.db(cObj.dbName);
        const col = db.collection(cObj.colName);
        console.log(`Connection Made to ${db.databaseName} database.`);
        const result = await col.find(fObj.field).limit(fObj.limit).toArray();
        client.close();
        return result;
    } catch (err) {
        console.log(err.stack);
    }
};

findDoc(cObj,fObj).then(function(result) {
    console.log(result);
});

Also, I advise you to enclose your whole async function's body into the try clause. This way you will be able to effectively intercept any error. Imagine your new MongoClient failed to instantiate - you would end up with an uncaught error inside a promise, which isn't very nice.