How do you return a custom 404 response for unknown routes in Express?

At a railway enquiry desk, if none of the specific counters can help you, the last counter says, "No such train." The 404 handler is that last counter. It works only because it sits at the end: any request that no earlier route answered falls through to it.

const express = require("express");
const app = express();

app.get("/api/notices", (req, res) => res.json([]));

// 404 handler: no path, registered after every route
app.use((req, res) => {
  res.status(404).json({
    error: "Not found",
    path: req.originalUrl,
  });
});

// optional: error handler after it
app.use((err, req, res, next) => {
  console.error(err);
  res.status(500).json({ error: "Internal server error" });
});

app.listen(3000);

Points to know:

• Express has a default 404 page, but it is plain HTML, which is awkward for an API client expecting JSON.
• A 404 is not an error to Express: nothing throws. The error handler runs only when next(err) is called.
• For a missing resource inside a route, such as a student ID that is not in the database, reply with 404 right there.
• A single-page app often sends index.html for unknown non-API paths instead.

• Putting the catch-all above the routes, which makes every request return 404.
• Returning 200 with an error message for a missing resource.