Backbone example app and javascript apply

Question

Hi can someone explain why in backbone example app (http://backbonejs.org/examples/todos/index.html) in remaining() function, is called using apply (this.without.apply(this, this.done());) and not this.without(this.done())

 // Filter down the list of all todo items that are finished.
done: function() {
  return this.where({done: true});
},

// Filter down the list to only todo items that are still not finished.
remaining: function() {
  return this.without.apply(this, this.done());
},

Thank You !

#Update

Debugger output

this.without(this.done())
[child, child, child, child]
this.without.apply(this, this.done());
[child, child, child]

Answer 1

Variable list of arguments

The key is in the way without is written:

function () {
  var args = slice.call(arguments);
  args.unshift(this.models);
  return _[method].apply(_, args);
}

It's anticipating a variable list of arguments, and one way to do that is to use apply:

...
return this.without.apply(this, ['pass', 'these', 'arguments']);

There's more about apply in the MDN documentation.

Answer 2

You asked what is the difference between these two calls:

this.without( this.done() )

vs.

this.without.apply( this, this.done() );

To clarify, let's remove the nested this.done() call. Now the first one is:

var value = this.done();
this.without( value );

That code obviously calls this.without() and passes it a single argument, whatever value was returned by this.done(). If value happens to be an array, the entire array is passed as a single argument.

The second version becomes:

var array = this.done();
this.without.apply( this, array );

That calls this.without() with a variable number of arguments, one argument for each element of array. (And I called it array instead of value this time, because for this code to make sense it has to be an array.)

.apply() also sets this in the called function, so passing this as the first argument just passes this along to that function in the same manner as a regular this.without() method call.