Bpatcher resizing

Hi !
I’m trying to resize a Bpatcher with "this.patcher.box.rect", but I can’t find how to add/substract a value to one of the existing coordinates (here to the vertical right one).
I have a toggle going in a message [size $1] going in js object.
Here is the code (please, forgive my javascript newbieness) :

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function size(x)
{
r = this.patcher.box.rect;
if (x > 0)
r[3] = (r[3] + 100);
else
r[3] = (r[3] – 100);
}
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Thanx for your advices.