This example shows how to create multiple error-validator-middlewares for your Express app.

error-validator-middleware

/* Say you want to add multiple validation middlewares to Express app, this is an example of how to do it. 
First create 2 middlewares: first one(e.g. validateRequest1) to validate and the 2nd one to handle errors (e.g validateRequest1Handler). 
If you want to add a 2nd validation middleware, create two more middlewares (validateRequest2 & validateRequest2Handler).

Note - Error-handling-middleware v/s non-error-handling-middleware
Error handler middleware must take 4 parameters(err, req, res, next) including "err" as first parameter. Where as are normal middlewares only take 3(req, res, next).
*/


//This is how it would look like..
var express = require('express');
var app = express();

app.use(validateRequest1);
app.use(validateRequest1Handler); //only called if validateRequest1 throws error
app.use(validateRequest2);
app.use(validateRequest2Handler); // only called if validateRequest2 throws error

/* Control-flow
Case 1: validateRequest1 throws error and validateRequest1Handler handles & returns response:

Now, if validateRequest1 throws an error, validateRequest1Handler is called. But if validateRequest1 doesn't throw error, control jumps to the next non-error-handling-middleware (validateRequest2 in the above case).
- validateRequest1Handler should act like the last handler.. so you should return response here itself.

Case 2:validateRequest1 throws error and validateRequest1Handler also throws error.
 -  If validateRequest1Handler also throws error, control will directly jump to next error-error-handler "validateRequest2Handler" ( & not to next middleware "validateRequest2").

Case 3: validateRequest1 doesn't throw error but validateRequest2 throws error.
Similar to case 1, control will go from validateRequest2 to next error-handler-middleware "validateRequest2Handler"
- validateRequest2Handler should act like the last handler.. so you should return response here itself.
*/

function validateRequest1(req, res, next) {
        
        // throw err; //control goes to validateRequest1Handler
         
       // next(); //control goes to validateRequest2
}
 function validateRequest1Handler(err, req, res, next) {

        //res.send("error"); // response will be sent back to browser and ends request.
 	
        //throw err; //control goes to validateRequest2Handler
 	
 }

 function validateRequest2(req, res, next) {
 	
       // throw err; //goes to validateRequest2Handler
         
       // next(); // goes to your app
 }

 function validateRequest2Handler(err, req, res, next) {

 	 // res.send("error"); // response will be sent backt to browser and ends.

 	 //throw err; // If this is the last error handler, your app will crash.
 }