try this

module.exports.addImage = (req, file, callback) => {
    const url = req.protocol + '://' + req.get('host');
    const newImage = new ImageModel({
        name: file.filename,
        path: url + '/images/' + file.filename,
    });
   newImage.save(callback);
};
----------------------------------------------------------

const saveImage = async () => {
    let images = [];
    for (let i in req.files) {
        if (req.files.hasOwnAttribute(i) {
            const file = req.files[i];
            await (new Promise((resolve, reject) => {
                ImageModel.addImage(req, file, (error, savedImage) => {
                    if (error) reject();
                    images.push(savedImage);
                    resolve();
                });
            ));
        }
    }
    return images;
};

----------------------------------------------------------------

saveImage().then(result => {
    console.log(`RESULT`, result);
    res.status(200).json({
        status: 'success',
        images: result
    });
}).catch(error => {
    res.status(502).json({
        status: 'fail',
        error
    });
});