What is the difference between setting a property on app.locals and calling app.set()?

Question

I\'m in the process of learning Express - and thinking of the best place to save config style data. Options available are either in app.locals or app.set (settings)... so:

Answer 1

The express api reference says :

By default Express exposes only a single app-level local variable, settings.

and either way is ok:

app.locals.title = 'My App';

app.set('title', 'My App');
// use settings.title in a view

Answer 2

Wow, all of the answers are wrong, so let me give it a try. Despite what others say assinging to the app.local argument is different from using app.set(). Watch,

app.js
  app.locals.foo = 'bar';
  app.set('baz', 'quz');  


index.jade
  block content                            
    dl                                     
      dt app.locals.foo = 'bar';            
      dd \#{settings.foo} = #{settings.foo}
      dd \#{foo} = #{foo}                  

      dt app.set('baz', 'quz')             
      dd \#{settings.baz} = #{settings.baz}
      dd \#{baz} = #{baz}

If you ran this code, what you would see is,

app.locals.foo = 'bar';
  #{settings.foo} =
  #{foo} = bar
app.set('baz', 'quz')
  #{settings.baz} = quz
  #{baz} =

The reason for this is setting app.locals sets attributes of the object that the view uses as its environment; what the view will read from without qualification. Conversely, app.set sets attributes on app.locals.settings. You can verify this if you clobber app.locals.settings in the above with app.locals.settings = {}, which will make #{settings.baz} undefined.

So which do you use? If it's not an app setting based on the response (res.set) or global configuration (app.set), use the direct write to app.locals.

Answer 3

Lots of people indeed use locals instead of app.set so my advice is to use that.

Answer 4

All properties of app.locals are available in templates. Using app.set assigns properties to app.locals.settings, which is used for global application settings and is inherited by mounted applications. For example:

var app1 = express(),
    app2 = express();

app1.set('inheritable', 'foo');
app1.locals.notInheritable = 'bar';

app1.use('/mount', app2);

app2.get('inheritable') === 'foo'; // true
app2.locals.notInheritable === 'bar'; // false

So it's really a question of preference and whether or not you're mounting applications.