Access "app" variable inside of ExpressJS/ConnectJS middleware?

Question

This may not me be the right approach, but I want to conditionally add an object/parameter to the app variable inside of an expressJS/connectjS middleware call.

Since this function is a callback, what's the standard/best way to access app from inside a middleware call?

  //app.js
  var myMiddleware = require('./lib/mymiddleware.js');
  ...
  app.configure( function(){
    app.use( myMiddleware.func() );
    ...
  }

  if( 'object' !== typeof app.myObject ){
    cry( 'about it' );
  } 


  //mymiddleware.js
  module.exports.func = function( ){
    return function( req, res, next ){
       //append app object
       //app.myObject = {}
       next();
    }
  };

Note, this is not something for locals or settings to later be rendered, but something that will be used in routes and sockets later down the execution chain.

Nitzan Shaked · Accepted Answer · 2018-03-29T11:40:47.993

115

Request objects have an app field. Simply use req.app to access the app variable.

edited Mar 29 '18 at 11:40

answered Sep 18 '13 at 19:00

Nitzan Shaked

13,460
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score 10 · Answer 2 · answered Sep 18 '13 at 19:03

10

Generally I do the following.

var myMiddleware = require('./lib/mymiddleware.js')(app);
...
app.configure( function(){
  app.use( myMiddleware );
  ...
}

And the middleware would look like this...

module.exports = function(app) {
  app.doStuff.blah()

  return function(req, res, next) {
    // actual middleware
  }
}

answered Sep 18 '13 at 19:03

Morgan ARR Allen

10,556
3
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1

I do it the same as you, I like the `req.app` solution though. wasn't aware that was possible. – Plato Sep 19 '13 at 01:56
2

@Plato the problem with this that I just realized is that this only updates the app object on a page request, I need the app object to be updated as part of the startup. – qodeninja Sep 23 '13 at 23:10
Is there another way to get app into the function without passing it in the require function? – qodeninja Sep 23 '13 at 23:11
@qodeninja: if that is your new requirement, you should edit the question. Regardless, I am unclear that "at app startup" means: there is no `req` outside of a request (that is: at startup), and you specifically said "middleware", which implies a request, so my original answer should be ok. Or am I missing something? – Nitzan Shaked Sep 24 '13 at 03:28
@NitzanShaked I didnt really get how middleware worked before, now I understand that middleware only runs on page loads, rather than startup. I need to update the app object in the startup phase, to set global properties. Though, I was hoping there was a more standard way of doing that. When I actually tested your code I realized it was not what I had intended, my apologies. Though I think its pretty clear from the code I wrote what I was trying to do `typeof app.myObject` for example. The requirement didnt change, my understand of the problem did. – qodeninja Sep 24 '13 at 18:08

Andrew Faulkner · Answer 3 · 2018-03-19T06:13:52.417

You can also attach a variable to the Node global object, like so:

//some-module.js
global.someVariable = "some value!";

//another-module.js
console.log(global.someVariable); // => "some value!"

Note that Nitzan Shaked's answer (using req.app) is a much better approach.

In fact, I don't advise using this solution at all. I'm leaving it solely for completeness, because it works, but it's bad practice in almost every situation (excepting adding polyfills).

Access "app" variable inside of ExpressJS/ConnectJS middleware?

3 Answers3