[SOLVED] What does it do to pass a callback returning an expression involving `Math.random()` to the `sort` method?

What does it do to pass a callback returning an expression involving `Math.random()` to the `sort` method?

I have this simple array:

var RedirUrl = new Array(4);
RedirUrl[0] = 'http://mafi.se/mf_redir/new_install_'+this_version+'.html';
RedirUrl[1] = 'http://ifurls.com/mf_redir/new_install_'+this_version+'.html';
RedirUrl[2] = 'http://ez.se/xml-update/mf_redir/new_install_'+this_version+'.html';
RedirUrl[3] = 'http://ilovre.net/mf_redir/new_install_'+this_version+'.html';
RedirUrl[4] = 'http://rihel.com/mf_redir/new_install_'+this_version+'.html';

and then

RedirUrl.sort(function() {return 0.5 - Math.random()})

The last bit is what is confusing me. I understand the "sort" and I understand the Math.random but the return 0.5 confuses me... what exactly is that?

(Needless to say I downloaded it off the net as it does what i want it to do... but i just dont understand it.)

Solution

It's sorting the list of URLs with a sorting method that randomly returns values that are greater than or less than 0 in about half the cases each.

Math.random() returns a number betweeen 0 and 1. Therefore 0.5 - Math.random() is a randomly decided value between -0.5 and 0.5. About half of those values are greater than zero and half of those are less than zero.

So about half of the time the comparison function will say that the first object is greater than the second and half of the time it will say the opposite.

It's a simple way to randomly shuffle the array. As has been pointed out in the comments, it's not a good way to shuffle an array (because the distribution is not even).

This question has a working implementation of the known-good Fisher-Yates shuffle.