Backbone js .listenTo vs .on

Question

What are the advantages and disadvantages of the following 2 lines of code? I don't understand why there are 2 different ways to do the same thing.

this.listenTo(app.Todos, 'change:completed', this.filterOne); app.Todos.on('change:completed', this.filterOne); 

Also when using .on, how do I determine is the default context?

Answer 1

listenTo is the newer and better option because these listeners will be automatically removed for you during stopListening which is called when a view gets removed (via remove()). Prior to listenTo there was a really insidious problem with phantom views hanging around forever (leaking memory and causing misbehavior) because view methods were referenced as event listeners on models even though the view instances themselves were long gone and no longer in the DOM.

If you want to read the back story for listenTo, search the backbone github repository for listenTo and read through some of the longer issue discussions.

As to the default context, several things can end up bound to this:

• if you do the binding via this.listenTo, it will always be the view instance (pointed out by Wim Leers in the comments)
• without this.listenTo, the story gets complicated
  • For misc events, it will be the global object (best to avoid this)
  • for DOM events, it will be the source element just like in regular DOM event binding
  • If you provide an explicit context (the 3rd argument to foo.on), backbone will use that (thus this is a more robust approach)
  • If you use the ECMA standard function () {//your event handler}.bind(this), you can also manually control the context (also recommended)
  • As @mu pointed out, _.bind or $.proxy are available alternatives to ECMA function.bind
  • For backbone views, doing this.bindAll('onClick', ...) will ensure the view instance is the this context when any view methods are used as event handlers
• any events wired up by using the view's standard events property will get bound for you automatically to the view instance by backbone (this is belt & suspenders with bindAll)

So to summarize into some guidelines:

• use the events property whenever possible as it is concise and correct
• use this.listenTo for all bindings to models and collections
• any additional bindings remember to bind the context reliably using your preferred method. I usually use ECMA Function.bind because hey, standards, but there are several good options here.