Node.js / Express.js - How to override/intercept res.render function?

Question

I'm building a Node.js app with Connect/Express.js and I want to intercept the res.render(view, option) function to run some code before forwarding it on to the original render function.

app.get('/someUrl', function(req, res) {      res.render = function(view, options, callback) {         view = 'testViews/' + view;         res.prototype.render(view, options, callback);     };      res.render('index', { title: 'Hello world' }); }); 

It looks like a contrived example, but it does fit in an overall framework I'm building.

My knowledge of OOP and Prototypal inheritance on JavaScript is a bit weak. How would I do something like this?

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Update: After some experimentation I came up with the following:

app.get('/someUrl', function(req, res) {      var response = {};      response.prototype = res;      response.render = function(view, opts, fn, parent, sub){         view = 'testViews/' + view;         this.prototype.render(view, opts, fn, parent, sub);     };      response.render('index', { title: 'Hello world' }); }); 

It seems to work. Not sure if it's the best solution as I'm creating a new response wrapper object for each request, would that be a problem?

Answer 1

Old question, but found myself asking the same thing. How to intercept res render? Using express 4.0x something now.

You can use/write middleware. The concept was a bit daunting to me at first, but after some reading it made a little more sense. And just for some context for anyone else reading this, the motivation for overriding res.render was to provide global view variables. I want session to be available in all my templates without me having to type it in every res object.

The basic middleware format is.

app.use( function( req, res, next ) {     //....     next(); } ); 

The next param and function call are crucial to execution. next is the callback function, to allow multiple middleware to do their thing without blocking. For a better explanation read here

This can then be used to override render logic

app.use( function( req, res, next ) {     // grab reference of render     var _render = res.render;     // override logic     res.render = function( view, options, fn ) {         // do some custom logic         _.extend( options, {session: true} );         // continue with original render         _render.call( this, view, options, fn );     }     next(); } ); 

I've tested this code, using express 3.0.6. It should work with 4.x without issue. You can also override specific URLs combos with

app.use( '/myspcificurl', function( req, res, next ) {...} );