req.flash() returns empty array

[MazeAzad]

I don't know why req.flash() return an empty array
please help me

exports.logout=(req,res,next)=>{
req.logout(function(err){
if(err){ return next(err)}
});
req.flash("success","logout was successful");
res.redirect("/login");
}
exports.getLoginPage = (req, res) => {
res.render("login",{
message:req.flash("success"),
error:req.flash("error")
});

it happens when i logout an I want have message that the logout was successful