error handling in express applicaiton

app.js

const express = require('express');
const http = require('http');

const app = express();

app.get('/', (req, res) => {
  res.send('Hey!!');
});

app.post('/exceptionRoute', async (req, res) => {
  const { username } = req.body;
  /* since we are not passing username, any operation on undefined will result in error,
  any unhandled error/exception in async await will result 😢😢😢
  [DEP0018] DeprecationWarning: Unhandled promise rejections are deprecated. In the future, 
  promise rejections that are not handled will terminate the Node.js process with a non-zero exit code
  */
  const lengthOfSUername = username.length;
  res.json({ status: true, length: lengthOfSUername });
});

app.post('/handleException', async (req, res) => {

  /* in above route "exceptionRoute", we have seen, since we are not handling excxeption
  that would result in nodejs process to break and that's not good thing. to avoid we will use
  try and catch  🤦‍♂️🤦‍♂️🤦‍♂️  */

  try {
    const { username } = req.body;
    const lengthOfSUername = username.length;
    res.json({ status: true, length: lengthOfSUername });
  } catch (error) {
    res.status(500).send(`something wen't wrong`);
  }
});

/* some higher order function  ❤❤ */
const handlerException = fn => (req, res, next) => {
  fn(req,res).catch((error) => next(error));
};


const someRouteFunction = async (req,res) => {
    /* using try-catch is fine but, if you writing large application, and writing 
    try catch for all routers result in lot of boiler plate code and also you can't centralize all
    errors in one place, so let's see how to do that */
    
    /* Now, we will see how to catch errors/exceptions in async await function without try-catch, and also we will 
    centralize errors while we are at it  😉😎😍 */
    const { username } = req.body;
    const lengthOfSUername = username.length;
    res.json({ status: true, length: lengthOfSUername });
}

app.post('/handleExceptionGraceFully', handlerException(someRouteFunction));


/* for catching 404 errors */
app.use((req, res) => {
  res.status(404).send('404');
})

/* for cathing all errors,
 this is default error handler provided by express,
 we will use this function to centralize all our errors
 */
app.use((err, req, res, next) => {
  /* now here if your error in development then send stack trace to display whole error,
  if it's in production then just send error message and log error with some library as winston etc.. */
  if(process.env.NODE_ENV === 'production') {
    return res.status(500).send(`something wen't wrong`);
  }
  res.status(500).send(`hey!! we caugth the error 👍👍, ${err.stack} `);
})


http.createServer(app).listen(3000, () => {
  console.log('server is up');
})