Docs for app.render:
Render a view with a callback responding with the rendered string. This is the app-level variant of
res.render(), and otherwise behaves the same way.
Docs for res.render:
Render a view with a callback responding with the rendered string. When an error occurs
next(err)is invoked internally. When a callback is provided both the possible error and rendered string are passed, and no automated response is performed.
How can I figure out when to use which one?
Here are some differences:
You can call app.render on root level and res.render only inside a route/middleware.
app.render always returns the html in the callback function, whereas res.render does so only when you've specified the callback function as your third parameter. If you call res.render without the third parameter/callback function the rendered html is sent to the client with a status code of 200.
Take a look at the following examples.
app.render
app.render('index', {title: 'res vs app render'}, function(err, html) {
console.log(html)
});
// logs the following string (from default index.jade)
<!DOCTYPE html><html><head><title>res vs app render</title><link rel="stylesheet" href="/stylesheets/style.css"></head><body><h1>res vs app render</h1><p>Welcome to res vs app render</p></body></html>
res.render without third parameter
app.get('/render', function(req, res) {
res.render('index', {title: 'res vs app render'})
})
// also renders index.jade but sends it to the client
// with status 200 and content-type text/html on GET /render
res.render with third parameter
app.get('/render', function(req, res) {
res.render('index', {title: 'res vs app render'}, function(err, html) {
console.log(html);
res.send('done');
})
})
// logs the same as app.render and sends "done" to the client instead
// of the content of index.jade
res.render uses app.render internally to render template files.
You can use the render functions to create html emails. Depending on your structure of your app, you might not always have acces to the app object.
For example inside an external route:
app.js
var routes = require('routes');
app.get('/mail', function(req, res) {
// app object is available -> app.render
})
app.get('/sendmail', routes.sendmail);
routes.js
exports.sendmail = function(req, res) {
// can't use app.render -> therefore res.render
}
use app.render in scenarios where you need to render a view but not send it to a client via http. html emails springs to mind.
along with these two variants, there is also jade.renderFile which generates html that need not be passed to the client.
usage-
var jade = require('jade');
exports.getJson = getJson;
function getJson(req, res) {
var html = jade.renderFile('views/test.jade', {some:'json'});
res.send({message: 'i sent json'});
}
getJson() is available as a route in app.js.
Source: Stackoverflow.com