How to detect when an element over another element in JavaScript?

Question

I wrote most of the code... And when the element is "fully" over the other element it works. The problem is that I don't just want it to be true when the element is "fully" over the element, I also want it to be true when the element is partly over the other element.

Here is my code:

    element = this.element.getStyles('left', 'top', 'width', 'height');
    elementLeftX = element.left.toInt();
    elementLeftY = element.top.toInt();
    elementRightX = (element.width.toInt() + element.left.toInt());
    elementRightY = (element.top.toInt() + element.height.toInt());

    el = this.positions ? this.positions[i] : this.getDroppableCoordinates(el); // Element drop area
    elLeftX = el.left.toInt();
    elLeftY = el.top.toInt();
    elRightX = (el.width.toInt() + el.left.toInt());
    elRightY = (el.height.toInt() + el.top.toInt());

   if (((elLeftY <= elementLeftY) && (elementLeftY <= elRightY)) && ((elLeftY <= elementRightY) && (elementRightY <= elRightY))) {
        if (((elLeftX <= elementLeftX) && (elementLeftX <= elRightX)) && ((elLeftX <= elementRightX) && (elementRightX <= elRightX))) {
            return true;
        } else {
            return false;
        }
    } else {
        return false;
    }

I am very confused and I have been playing around for a while and I just can't get it to work.

Answer 1

Standard video game method:

doElsCollide = function(el1, el2) {
    el1.offsetBottom = el1.offsetTop + el1.offsetHeight;
    el1.offsetRight = el1.offsetLeft + el1.offsetWidth;
    el2.offsetBottom = el2.offsetTop + el2.offsetHeight;
    el2.offsetRight = el2.offsetLeft + el2.offsetWidth;

    return !((el1.offsetBottom < el2.offsetTop) ||
             (el1.offsetTop > el2.offsetBottom) ||
             (el1.offsetRight < el2.offsetLeft) ||
             (el1.offsetLeft > el2.offsetRight))
};

See demo

Answer 2

If you are interested in using jQuery (or if you already are) there are some great collision detection plugins available. Here is a previous answer detailing one https://stackoverflow.com/a/6052254/374866

Answer 3

When you are attempting is called "collision detection". You need to get the element's COMPUTED offsets and dimensions, not just the element's style declarations.

This posting may help explain:

jQuery/JavaScript collision detection