Function articleReturner is executed asynchronously and return cetarticle;
doesn't make a lot of sense. You need to use callbacks or promises. Here is the code that uses callback to return result from articleReturner:
function articleReturner(celien, callback) {
Article.findOne({ lien: celien}, function (err, cetarticle){
console.log(cetarticle);
callback(err,cetarticle);
});
}
app.get('/selection/oui/', function(req, res) {
var celien = req.param("value"); //
console.log(celien); // it works
articleReturner(celien, function(err, cetarticle){
res.render('selection_form.ejs', cetarticle);
});
});