Playlist length

[Zazama]

Hello,
I have a problem with the length of the playlist prototype.
In the spotify.js file it says:

* Gets the metadata from a Spotify "playlist" URI.
[...]
@param {Number} length (optional) number of tracks to get. defaults to 100.

But what if I want to get the complete playlist, without knowing how big it is? Is there a way to get the playlist length?

Edit: If I enter -1, I get all songs of the playlist (more than 100), but now the get prototype isn't working anymore, if the array is bigger than 100.