expressjs.com/_includes/api/es/4x/app-render.md

app.render(view, [locals], callback)

Returns the rendered HTML of a view via the callback function. It accepts an optional parameter that is an object containing local variables for the view. It is like res.render(), except it cannot send the rendered view to the client on its own.

Think of `app.render()` as a utility function for generating rendered view strings. Internally `res.render()` uses `app.render()` to render views.

The local variable `cache` is reserved for enabling view cache. Set it to `true`, if you want to cache view during development; view caching is enabled in production by default.

app.render('email', function(err, html){
  // ...
});

app.render('email', { name: 'Tobi' }, function(err, html){
  // ...
});