Tengo dos archivos app.js y test.ejs con código:

aplicación.js

 const express = require('express') const app = express() app.use(express.urlencoded({ extended: false })); app.set('view engine', 'ejs') app.get('/', function (req, res) { res.render('pages/test'); }); app.post("/", function (request, response) { console.log(`info - ${request.body.test}`); }); app.listen(3000, console.log('port: 3000'))

prueba.ejs

 ...<script src="https://ajax.googleapis.com/ajax/libs/jquery/3.5.1/jquery.min.js"></script>... ...<input id="fn_test" name="test" onkeyup="showResult(this.value)" type="text" required /> ... ... <script> function showResult(str) { $.ajax({ url: "/", method: "POST", contentType: "application/json", success: (result) => { } }); } ...</script>

Respuesta de la consola: info - indefinido