What is the below express router doing?

Question

The tutorial that I am following has the following 2 router methods for getting the articles from db.

router.param('article', function(req, res, next, slug) {
  Article.findOne({ slug: slug})
    .populate('author')
    .then(function (article) {
      if (!article) { return res.sendStatus(404); }

      req.article = article;

      return next();
    }).catch(next);
});

router.get('/:article', auth.optional, function(req, res, next) {
  Promise.all([
    req.payload ? User.findById(req.payload.id) : null,
    req.article.populate('author').execPopulate()
  ]).then(function(results){
    var user = results[0];

    return res.json({article: req.article.toJSONFor(user)});
  }).catch(next);
});

I have two questions regarding the methods above,

1. How does Promise.all() work ?
2. Why do we need to repopulate author field in router.get() method when already did that in the router.param() method?

Answer 1

1. Promise.all(iterables) is used when you have multiple promises that you need to be done before executing other task. In the given case

   [ req.payload ? User.findById(req.payload.id) : null, [ req.payload ? User.findById(req.payload.id) : null, req.article.populate('author').execPopulate() ]

2. Populate will not be executed until a callback is passed or execPopulate is called . So your router param will not actually populate('author'). the .then just attaches it to the req.article. When you call populate again with same path('author') then it resets any query options that was set by the previous populate call.

this is mentioned in the Mongoose docs also here at the end as a note