return value from asynchronous function in Nodejs

Question

I am using nodejs to query data from Mongodb throught Mongoose. After get the data, I want do something on that data before responding it to client. But I can not get the return-value. After looking on Google, I have learned Node.js functions is asynchronous javascript function (non I/O blocking). I try this tut (http://www.youtube.com/watch?v=xDW9bK-9pNY) but it is not work. Below is my code. The myObject is valued inside "find()" function and undefined outside "find()" function. So what should I do to get the data? Thanks!

var Person = mongoose.model('Person', PersonSchema);
var Product = mongoose.model('Product', ProductSchema);
var myObject = new Object();

Person.find().exec(function (err, docs) {
    for (var i=0;i<docs.length;i++)
    { 
    Product.find({ user: docs[i]._id},function (err, pers) {
    myObject[i] = pers;
    console.log(myObject[i]); //return the value is ok
    });
    console.log(myObject[i]); //return undefined value
    }
    console.log(myObject); //return undefined value
});
    console.log(myObject); //return undefined value

app.listen(3000);
console.log('Listening on port 3000');

Answer 1

The reason you're getting undefined values is because the find function is asynchronous, and can finish at any time. In your case, it is finishing after you're using console.log(), so the values are undefined when you're accessing them.

To fix this problem, you can only use the values inside the find function's callback. It would look something like this:

var Person = mongoose.model('Person', PersonSchema);
var Product = mongoose.model('Product', ProductSchema);
var myObject = new Object();

function getData(docs, callback) {
  function loop(i) {
    Product.find({ user: docs[i]._id}, function (err, pers) {
      myObject[i] = pers;

      if (i < docs.length) {
        loop(i + 1);
      } else {
        callback();
      }
    });
  };
  loop(0);
};

Person.find().exec(function(err, docs) {
  getData(docs, function() {
    // myObject has been populated at this point
  });
});

The data processing has been moved to a loop that waits for the previous iteration to complete. This way, we can determine when the last callback has fired in order to fire the callback in the wrapper function.

Answer 2

Keep in mind that by the time the console.log functions are executed, the query has not yet finished, thus will display "undefined". That's the essence of nodeJS's asynchronicity.

For example,

Person.find().exec(function (err, docs) {
    for (var i=0;i<docs.length;i++)
    { 
    Product.find({ user: docs[i]._id},function (err, pers) {
    myObject[i] = pers;
    console.log(myObject[i]); //return the value is ok
    });
    console.log(myObject[i]); //return undefined value
    }
    console.log(myObject); //return undefined value
});

console.log(myObject); // <-- Initially, this value will be undefined. After some miliseconds (Or the duration of the .exec function, myObject will contain the results.

If you want to actually wait until the query is finished so you can use the values, I would recommend moving the app.listen(3000); and console.log('Listening on port 3000'); into the function's final callback.

I'd also recommend you to check out this node module. It will help you build asynchronous / synchronous functions more easily, and allow you to execute a callback when all the asynchronous functions are finished.