Adding value to the arguments object

Question

I'm having problems with adding values to the arguments objekt in a JavaScript method.

I'm trying to add an additional success callback to the Backbone.Model.fetch() method, my model looks like this:

models.Settings = Backbone.Model.extend({
    url: "path/to/settings.json",

    dataLoaded: function() {
        this.set({
            data: new collections.Data( this.get("data") )
        });
    },

    fetch: function() {
        var
            cb;

        if(!arguments.length) arguments[0] = {};

        if(arguments[0].hasOwnProperty('success')) {
            cb = arguments[0].success;
        }

        arguments[0].success = function() {
            this.dataLoaded();
            if(typeof cb === 'function') cb.apply(undefined, arguments);
        }.bind(this);

        models.Settings.__super__.fetch.apply(this, arguments);
    }
});

The code works well as long as a value (any value) is passed to fetch(). The problem occurs when the arguments object is initially empty. I'm able to set arguments[0] and the success value, as I'm supposed to. But the callback is never run.

What am I missing?

Answer 1

The arguments property is an object, not an array, which means that when you set arguments[0] = {};, it then looks like (http://jsfiddle.net/nikoshr/n93wfm8a/)

{
    0: {}
}

However, models.Settings.__super__.fetch.apply expects an array (or at least a length property matching the content) and does not get it.

Solutions

1. Most hackish, set a length property :

   if(!arguments.length) {
       arguments[0] = {};
       arguments.length = 1;
   }
   

   http://jsfiddle.net/nikoshr/n93wfm8a/1/

2. Slightly better, initialize arguments with an array

   if(!arguments.length) {
       arguments = [{}];
   }
   

   http://jsfiddle.net/nikoshr/n93wfm8a/2/

3. Rewrite the whole thing to take advantage of the method's signature model.fetch([options]) and use Function.prototype.call

   fetch: function(opts) {
       var cb;
       opts = opts || {};
   
       if (opts.hasOwnProperty('success')) {
           cb = opts.success;
       }
   
       opts.success = function() {
           this.dataLoaded();
           if(typeof cb === 'function') cb.apply(this, arguments);
       }.bind(this);
   
       Settings.__super__.fetch.call(this, opts);
   }
   

   http://jsfiddle.net/nikoshr/n93wfm8a/4/