express: Error handling middleware

Error handling middleware function takes four arguments.

Example

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

  console.error(err.stack);

  res.status(500).send('Something broken!');

});

As you see above example, first argument to error handling middleware is an error object, second one is request, third one is response object and last one is next object.

index.js

//Load express module
const express = require('express')

//Put new Express application inside app variable
const app = express()

const port = 3000

//When user hits the home page, then the message prints in browser.
app.get('/', (req, res) => res.send('Welcome to Node.js Programming'))

app.use('/welcome', (req, res, next) => {
 next(new Error("Unknown error while executing welcome request"))
})

app.get('/welcome', (err, req, res, next) => {
  console.error(err.stack);
  res.status(500).send('Something broken!');
});

// Start the express application on port 8080 and print server start message to console.
app.listen(port, () => console.log(`Application started listening on port ${port}!`));

Run index.js.

Open browser and hit the url ‘http://localhost:3000/welcome’, you can see below messages in console.

Application started listening on port 3000!

Error: Unknown error while executing welcome request

    at app.use (C:\Users\Public\nodeExamples\index.js:13:7)

    at Layer.handle [as handle_request] (C:\node_modules\express\lib\router\layer.js:95:5)

    at trim_prefix (C:\node_modules\express\lib\router\index.js:317:13)

    at C:\node_modules\express\lib\router\index.js:284:7

    at Function.process_params (C:\node_modules\express\lib\router\index.js:335:12)

    at next (C:\node_modules\express\lib\router\index.js:275:10)

    at expressInit (C:\node_modules\express\lib\middleware\init.js:40:5)

    at Layer.handle [as handle_request] (C:\node_modules\express\lib\router\layer.js:95:5)

    at trim_prefix (C:\node_modules\express\lib\router\index.js:317:13)

    at C:\node_modules\express\lib\router\index.js:284:7

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