¿Why can’t I concatenate two arrays in this query handler?

I am trying to merge arrays from a request to already existing arrays in a MongoDB database. When I printed the output of the request, the arrays did not merge. What seems to be the issue here?

router.post('/add-publication-data', async (req, res) => {
    try {
        const publication = await Publications.findOne({ _id: req.body._id });
        publication.toObject();
        publication.additionalauthors.concat(req.body.additionalauthors)
        publication.students.concat(req.body.students)
        console.log(publication.students)
        publication.institutions.concat(req.body.institutions)
        publication.keywords.concat(req.body.keywords)
        publication.highlights.concat(req.body.highlights)
        publication.save()
            .then(
                data => {
                    res.json(data);
                })
            .catch(e => {
                res.json({
                    message: e
                });
            });
    } catch (err) { console.log(err); res.json({ message: err }) };
});

Answer

concat() method

Your result is the expected behavior of concat method. From MDN documentation:

The concat() method is used to merge two or more arrays. This method does not change the existing arrays, but instead returns a new array.

You will need to assign the result back to have the merged array, change from:

publication.additionalauthors.concat(req.body.additionalauthors)

to:

publication.additionalauthors = publication.additionalauthors.concat(req.body.additionalauthors)

push() method

Another solution is to use push method

The push() method adds one or more elements to the end of an array and returns the new length of the array.

publication.additionalauthors.push(...req.body.additionalauthors)