admin.js

router.get("/", (req, res) => {
  const msg = req.flash("success");
  console.log("success: "+ msg );
  res.render("admin", {
    title: "admin pages",
    success: msg
  });
});

app.js

const express = require('express');
const path = require("path");
const mongoose = require("mongoose");
const cookieParser = require("cookie-parser");
const session = require('express-session');
const flash = require("connect-flash");
/* init app */
const app = express();

/* init env config */
require('dotenv').config();
const port = process.env.PORT || 3000;

// getting-started.js
mongoose.connect(process.env.DB, {
  useNewUrlParser: true,
  useUnifiedTopology: true
});

/* middleware */
app.use(express.json());
app.use(express.urlencoded({ extended: false }));

/* express session */
app.use(cookieParser("keyboard cat"));
app.use(
  session({
    secret: "keyboard cat", 
    resave: false,
    saveUninitialized: true,
    cookie: { secure: true }
  })
);

/* flash message */
app.use(flash());
app.use(function(req, res, next) {
  res.locals.messages = require("express-messages")(req, res);
  next();
});
/* route */

const adminPages = require("./route/admin_pages");
app.use("/admin/pages/", adminPages);


app.listen(port, () => {
    console.log(`Server started on ${port}`);
});

login.js

router.post(
  "/login",
  (req, res) => {
    req.flash('success','Login successfully');
    res.redirect("/admin/pages");
  }
);

message.pug

if errors
        each error in errors
            div.alert.alert-danger #{error.msg}
if message
    div.alert.alert-warning #{message}
if success && success.length > 0
    div.alert.alert-success #{success}