负值（负值）为什么与索引一起使用？

Question

I am new to jquery and basically , i decided to put Jquery to some practical use and so i build the following script :

$(function () {
    $(document).on('keydown', checkey);

    var index = 0,
        $items = $('ul li a');


    function checkey(e) {

        // console.log($items);    

        if (e.which == 38) {
            // key up
            index -= 1;
        };
        if (e.which == 40) {
            //key down
            index += 1;
        }

        console.log(index);

        $items.eq(index).trigger('focus');
    }


}); 

useing this script you can navigate your menu useing the up and down arrow keys .

the fiddle is here . now i have one question about the code that i have used , in particular the below line of code:

$items.eq(index).trigger('focus');

now, the index variable is incremented and decremented in my script, but when i keep pressing keyup, and the index keep decrementing and goes below 0 IE -1 , -2 , -3 , -4 ... etc , $items.eq(index).trigger('focus'); , this line still works , i was't expecting it to work with negative values , i was expecting it only to work from 0-4 .

so how and why do negative values work with the below statement :

eg. $items.eq(-2).trigger('focus'); ??

NOTE : in the fiddle please check the console and experiment with arrow up and arrow down, to understand what i am talking about .

Answer 1

From the jQuery API documentation :

Providing a negative number indicates a position starting from the end of the set, rather than the beginning.