... 
      newArticle.save((err,newArticle)=>{
      if(err){
        req.flash('error','Sorry , an error has occured. Thamk you try again later')
        //when i use console.log(req.flash('error')) it shows normally in the terminal
        return res.redirect('/AddArticle')
      }else{
        req.flash('success','Thank you , your article has been added')
        //when i use console.log(req.flash('success')) it shows normally in the terminal
        return res.redirect('/AddArticle')
      }
    })
}

  

this is the route fonction:

  router.post('/AddFormArticle',control.FormAddArticle)

this is control FormAddArticle function that i used to render the flash message and the page:

  exports.FormAddArticle = (req,res)=>{
  res.render('FormArticle',{title : "Ajouter un Article" , success : req.flash('success') , 
  error : req.flash('error') })