inGroupsOf function from prototype.js implementation for underscore.js

in_groups_of.js

_.mixin({
    inGroupsOf: function(array, number, fillWith) {
        fillWith = fillWith || null;
        var index = -number, slices = [];

        if (number < 1) return array;

        while ((index += number) < array.length) {
            var s = array.slice(index, index + number);
            while(s.length < number)
                s.push(fillWith);
            slices.push(s);
        }
        return slices;
    }
});

/*
_.inGroupsOf(['a', 'b', 'c', 'd', 'e', 'f', 'g', 'h', 'i'], 3);
//-> [["a", "b", "c"], ["d", "e", "f"], ["g", "h", "i"]]

_.inGroupsOf(['a', 'b', 'c', 'd', 'e', 'f', 'g', 'h', 'i'], 4);
//-> [["a", "b", "c", "d"], ["e", "f", "g", "h"], ["i", null, null, null]]

_.inGroupsOf(['a', 'b', 'c', 'd', 'e', 'f', 'g', 'h', 'i'], 4, 'foo');
//-> [["a", "b", "c", "d"], ["e", "f", "g", "h"], ["i", "foo", "foo", "foo"]]
*/