 <p>The reason for this </p> <p><code>this.without.apply(this, this.done())</code></p> <p>is that "_.without" does not accept as argument the array of items to be excluded as a single array ([]) argument</p> <p>See here <a href="https://github.com/documentcloud/underscore/issues/104" rel="nofollow">_.without to accept array as second argument</a></p> <p><em>apply</em>, which is part of the JS Function.prototype, here is a workaround to inject the excluding items in a single array argument</p>