What's the difference between “app.render” and “res.render” in express.js?

Question

Docs for app.render:

Render a view with a callback responding with the rendered string. This is the app-level variant of res.render(), an

Answer 1

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.