Middleware

what is middleware

• a function that takes a request object and either terminates the request/response cycle or passes control to another middleware function

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

app.use((req, res, next) => {
  console.log('Time:', Date.now());
  next();
})

expresss middlewares

app.use(express.json());                      //express middleware to parse json object such as in req.body
app.use(express.urlencoded({extended:true})); //express middleware to parse incoming request with URL payloads ?key=value
app.use(express.static('public'));            //express middleware to serve static files (e.g. served at localhost:3001/readme.txt)

third party middlewares

app.use(helmet());                            //third party middleware to secure response header
app.use(morgan('tiny'));                      //third party middleware to log requests 

Error Handling Middleware

• Inside middleware folder, create a file called errorHandler.js

const errorHandler = (err,req,res,next) =>{
  console.log(err.stack);
  res.status(500).json({
    success:false,
    error:err.message
  }); 
}
module.exports = errorHandler;

//then in the app.js, we need to use this middleware
const errorHandler = require('./middleware/error');
//use this middleware after all the routes and controllers
app.use(errorHandler);

TryCatch Handler middleware

• This avoid repeating the try/catch code in each route
• Inside the middleware folder create a file called trycatchHandler.js

const trycatchHandler = fn => (req,res,next) =>
	Promise
	.resolve(fn(req,res,next))
	.catch(next)
module.exports = asyncHandler;

//then we can use it in each route by wrapping async inside the middleware
const trycatchHandler = require('../middleware/trycatchHandler'); 

router.get('/', trycatchHandler( async (req,res,next)=>{
    const result = await Product.find();
    res.status(200).json({success:true,count:result.length,data:result});
}))

Replace TryCatchHandler with express-async-errors

$ npm i express-async-errors
//then we can remove trycatchHandler in all routes
router.get('/', async (req,res,next)=>{
    const result = await Product.find();
    res.status(200).json({success:true,count:result.length,data:result});
});