Javascript/NodeJS: Array empty after pushing values in forEach loop

As others have said, your database code is asynchronous. That means that the callbacks inside your loop are called sometime later, long after your loop has already finishes. There are a variety of ways to program for an async loop. In your case, it's probably best to move to the promise interface for your database and then start using promises to coordinate your multiple database calls. You can do that like this:

Promise.all(riders.map(rider => {
    return Rider.findOne({_id: rider}).exec();
})).then(foundRiders => {
    // all found riders here
}).catch(err => {
    // error here
});

This uses the .exec() interface to the mongoose database to run your query and return a promise. Then, riders.map() builds and returns an array of these promises. Then,Promise.all()monitors all the promises in the array and calls.then()when they are all done or.catch()` when there's an error.

If you want to ignore any riders that aren't found in the database, rather than abort with an error, then you can do this:

Promise.all(riders.map(rider => {
    return Rider.findOne({_id: rider}).exec().catch(err => {
        // convert error to null result in resolved array
        return null;
    });
})).then(foundRiders => {
    foundRiders = foundRiders.filter(rider => rider !== null);
    console.log(founderRiders);
}).catch(err => {
    // handle error here
});

To help illustrate what's going on here, this is a more old fashioned way of monitoring when all the database callbacks are done (with a manual counter):

riders.forEach(function(rider){
    let cntr = 0;
    Rider.findOne({_id: rider}, function(err, foundRider){
        ++cntr;
        if(err){
            console.log("program tried to look up rider for the forEach loop finalizing the results, but could not find");
        } else {
            foundRiders.push(foundRider);
        }
        // if all DB requests are done here
        if (cntr === riders.length) {
            // put code here that wants to process the finished foundRiders
            console.log(foundRiders);
        }
    });
});

The business of maintaining a counter to track multiple async requests is what Promise.all() has built in.

The code above assumes that you want to parallelize your code and to run the queries together to save time. If you want to serialize your queries, then you could use await in ES6 with a for loop to make the loop "wait" for each result (this will probably slow things down). Here's how you would do that:

async function lookForRiders(riders) {
    let foundRiders = [];
    for (let rider of riders) {
        try {
            let found = await Rider.findOne({_id: rider}).exec();
            foundRiders.push(found);
        } catch(e) {
            console.log(`did not find rider ${rider} in database`);
        }
    }
    console.log(foundRiders);
    return foundRiders;
}

lookForRiders(riders).then(foundRiders => {
    // process results here
}).catch(err => {
    // process error here
});

Note, that while this looks like it's more synchronous code like you may be used to in other languages, it's still using asynchronous concepts and the lookForRiders() function is still returning a promise who's result you access with .then(). This is a newer feature in Javascript which makes some types of async code easier to write.