Backbone.js - Todo Collection - What exactly is happening in this return statement?

Question

I'm looking back at the Backbone todo list and have a question about the collection.

Here is the code:

window.TodoList = Bacbone.Collection.extend({          model: Todo,          localStorage: new Store("todos"),          done: function() {             return this.filter(function(todo){return todo.get("done")})         },          remaining: function() {             return this.without.apply(this, this.done());         }        }) 

I understand everything that is going on here, except for the 'remaining' function.

The return statement: return this.without.apply(this, this.done()); is using a proxy to an underscore method - _.without

According to Underscore docs, here is what that is for:

without_.without(array, [*values]) Returns a copy of the array with all instances of the values removed. === is used for the equality test.

_.without([1, 2, 1, 0, 3, 1, 4], 0, 1); => [2, 3, 4]

So, I get that it is saying to return everything in the collection without a 'done' attribute with the value of 'true'.

What I don't understand is the 'apply' function that is being chained to it. That doesn't appear in the Backbone docs or the Underscore docs. At least not anywhere I can find it.

Can anyone explain in detail what is going on with those elements in the Return statement?

Answer 1

this is referring to the collection.

apply is a method of javascript functions that allows you to set context of a method and send an array of values to the caller.

apply expects context as the first parameter then an array or array-like (such as arguments) which will be passed in as parameters the function.

You can do the same thing with .call except the 2nd+ params are comma separated.

apply and call are native to javascript.

So...

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

the method this.done() returns an array, but uses the context of the collection and passes in a series of values to be ignored via the without method. Which in turn returns all todos that aren't done within the collection.

Example:

_.without([1,2,3,4],1,2); === _.without.call([], [1,2,3,4], 1, 2);