Before filter for all post requests in node.js

Question

I have the below code in my node.js app.js:

var express = require('express')
    ,cors = require('cors')
    ,app=express();
    var users = require('./routes/users');
    //some other codes
    .....
    app.use('/', routes);
    app.use('/users', users );

If a request is made to /users/adduser, it will go to the users.js in the routes folder.

Now I want to add a filter which will capture all the POST requests and do some validations and only if the conditions are satisfied the POST should go to its handler.

I.e if i get a /users/adduser with a POST request, before going to the method in the users.js in the routes folder, I should be able to capture that request and stop it if the condition is not met.

UPDATE 1 Now i am having this app.use function, but in the result i am getting undefined as its not waiting till the function is returning value

app.use(function (req, res, next) {
    req.db = db;
    if (req.method != "POST") {
        next();
    }
    else {
        var userData = req.body;
        var result = Check(userData);
        if(result){
        next();            
        }
    }
});

function Check(userdata) { 

    var url = "someurl"+userdata.Id;

    var request = require("request");
    request({
        url: url,
        json: true
    }, function (error, response, body) {

        if (!error && response.statusCode === 200) {
            if (response.toJSON().body.id == userId) { 
                return true;
                // i also tried next();
            }  
        }
    })
};

Answer 1

You can write a simple middleware function for that:

var validator = function(req, res, next) {
  // If the request wasn't a POST request, pass along to the next handler immediately.
  if (req.method !== 'POST') return next();

  // Perform your validations.
  Check(req.body, function(err) {
    // Validation failed, or an error occurred during the external request.
    if (err) return res.sendStatus(400);
    // Validation passed.
    return next();
  });
};

function Check(userdata, callback) { 
  var url     = "someurl"+userdata.Id;
  var request = require("request");
  request({ url: url, json: true }, function(error, response, body) {
    if (!error && response.statusCode === 200) {
      if (response.toJSON().body.id === userId) { 
        return callback(null, true);
      }
    }
    return callback(new Error());
  })
};

You have various points at which you can insert this middleware, which kind of depend on how exactly your app is structured.

One option:

app.use('/users', validator, users);

Or, if you have a separate router for /users (in ./routes/users.js):

router.use(validator);

Or, if you have a separate POST route for /users/adduser:

router.post('/adduser', validator, function(req, res) { ... });

In the last case you don't have to check req.method in the validator middleware because it's limited to the POST handler anyway.