Handling UnknownAction in Middleware

Demonstrate how to create a middleware that listens for UnknownAction to perform logging or error reporting.
const express = require('express');
const app = express();

// Middleware function to handle unknown actions
const unknownActionHandler = (req, res, next) => {
  // Log the unknown action or report an error
  console.error(`Unknown action: ${req.method} ${req.originalUrl}`);
  // Send response with 404 status
  res.status(404).send('Action not found.');

// Apply the middleware

app.listen(3000, () => {
  console.log('Server listening on port 3000');
This example captures any requests that do not match defined routes (unknown actions) using an Express.js middleware. It logs the unknown action and sends a 404 response.
/* CSS for custom 404 page */
body {
  display: flex;
  justify-content: center;
  align-items: center;
  height: 100vh;
  font-family: Arial, sans-serif;

.not-found {
  text-align: center;

.not-found h1 {
  font-size: 48px;
  color: #333;

.not-found p {
  color: #666;
This CSS provides styling for a custom 404 page. It centers text both horizontally and vertically and styles the heading and paragraph elements.
<!DOCTYPE html>
<html lang="en">
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>404 Not Found</title>
  <link rel="stylesheet" href="styles.css">
  <div class="not-found">
    <p>The action you requested could not be found.</p>
This HTML snippet creates a custom 404 page that displays a message indicating the requested action could not be found. It includes a link to the CSS file that contains the styling.