使函数在mongodb中运行

Question

I need to make a function like this.

function getDataFromCollection(collectionName) {
    let data;

    MongoClient.connect(url, function(err, db) {
        if (err) throw err;
        db.collection(collectionName).find({}).toArray(function(err, result) {
            data = result;
            console.log(result);
            db.close();
        });
    });

    return data;
}

The problem is that data is undefined, but when I execute console.log(result), it works. Any idea to help me. Thanks in advance.

Answer 1

There's a very simple explanation. The function(err, result) is essentially asynchronous and is not called immediately but after some time when the data is fetched from mongo. The function(err, result) is thus a callback. So, data is not set to result immediately but after a while. Now, you return data immediately and don't wait for it to be populated (inside the function(err, result) callback) so undefined is obviously returned.

The solution would be to use JavaScript's Promises which lets you use asynchronous code and callbacks. We return a Promise from getDataFromCollection and chain a .then when we call it. The function or callback that is passed to the then is executed when the promise is resolved inside the getDataFromCollection function with the data that was passed to the resolve function. Thus, the callback inside then would be called when you receive the result .

All the code -

function getDataFromCollection(collectionName) {
        return new Promise(function(resolve, reject) {
            MongoClient.connect(url, function(err, db) {
                if (err) {
                  reject(err);
                  return; 
                }
                db.collection(collectionName).find({}).toArray(function(err, result) {
                    if (err) {
                       reject(err);
                       return;
                    }
                    console.log(result);
                    db.close();
                    resolve(result);
               });
           });
        });
    }

Consume the function like so.

getDataFromCollection("collection")
   .then(function(result) {
      // use result
   })
   .catch(function(err) {
     console.log(err);
   });

Read up about Promises from here .