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JavaScript Middleware in Context

Andrés Muñoz
Andrés Muñoz
Courses Plus Student 4,031 Points

Why the catch error 404 on line 24 is not fired ?

I dont get why the middleware that catch the error is not fired like in the previous videos ?

1 Answer

When Express sends a response back to the client, all other middleware or routes AFTER the response wont be reached.

app.use((req, res, next) => {
    const {body: {number}} = req;
    const num = parseFloat(number);
    const result = num * 2;
    req.doubled = result;

app.get('/', (req, res, next) => {

app.post('/', ({doubled}, res, next) => {
  res.render('index', {doubled});

app.use((req, res, next) => {
  const err = new Error('Not Found');
  err.status = 404;

In the video we only reach the GET / and POST / routes which successfully sends a response back to the client with res.render method. Therefore, we never reach number 4.

But if we were to make a request to say GET /bro we would enter number 1 call its next method, skip number 2 and 3 (no response is sent back to client) and finally reach number 4 to send an error because a response wasn't found.