Question

If i want to find all elements that are inside a box region, what is the best way to do it as a Firefox extension? If i check all leave elements and call getBoundingClientRect(), it’d be too slow given that there can easily be more than 500 leaves on a page.

Any help will be appreciated. Thanks.

Answer 1

You can use document.elementFromPoint and visit each fifth pixel (every fifth is much faster than visiting every single pixel), adding each found element to an array:

function getElementsInRegion(x, y, width, height) {

    var elements = [],
        expando = +new Date,
        cx = x,
        cy = y,
        curEl;

    height = y + height;
    width = x + width;

    while ((cy += 5) < height) {
        cx = x;
        while (cx < width) {
            curEl = document.elementFromPoint(cx, cy);
            if ( curEl && !curEl[expando] ) {
                curEl[expando] = new Number(0);
                elements.push(curEl);
                cx += curEl.offsetWidth;
            } else {
                cx += 5;
            }
        }
    }

    return elements;

}