error handler expressjs middleware not catching errors

Question

I'm not understanding why the error handler does not catch the error as described here :

const express = require("express");
​
const app = express();
​
app.use(function (err, req, res, next) {
  res.status(400).send("error");
});
​
app.get("*", (req, res) => {
  throw new Error("there is an error. this message will not be seen.");
});
​
app.listen(8001, () => console.log("listening 8001"));

I'm expecting to receive a payload string of "error" here but am seeing "there is an error. this message will not be seen." Thanks!

Answer 1

I had this problem before.

The error handler must be located on last of router set.

You should change location like this

const express = require("express");
​
const app = express();
​
app.get("*", (req, res) => {
  throw new Error("there is an error. this message will not be seen.");
});

app.use(function (err, req, res, next) {
  res.status(400).send("error");
});
​
app.listen(8001, () => console.log("listening 8001"));

express and koa read router like linear, so if error handler locate first of router set, it will not work.