Increment or decrement var in resize window

Create a variable that stores this value and joins an event sink for when the window changes size. You also need a way to fetch information from the current window size. I do not quite understand your question if you want the width (x) or the height (y). Next an example that does this, calculating the width but with the height code too:

var janela = 0;
function medeJanela(){
    var w = window,
        d = document,
        e = d.documentElement,
        g = d.getElementsByTagName('body')[0],
        x = w.innerWidth || e.clientWidth || g.clientWidth,
        y = w.innerHeight|| e.clientHeight|| g.clientHeight;
    janela = x - 1000;
    console.log(janela); // podes tirar isto depois, só para verificares
}
window.onresize = medeJanela;

In this way the variable is updated when the window is resized.

You just decrement 1000 of the window value every time you set the variable. For example:

var tamanhoJanela = getTamanhoJanela() - 1000;
// 1000 - 1000 =  0
// 997  - 1000 = -3
// 1003 - 1000 =  3

As I understand it would be like this:

var tamanho = 1000;
$(window).resize(function() {
var h = $(window).height();
tamanho += h > tamanho ? 1 : -1;
console.log(tamanho);
});