JavaScript: Collision detection

The first thing to have is the actual function that will detect whether you have a collision between the ball and the object.

For the sake of performance it will be great to implement some crude collision detecting technique, e.g., bounding rectangles, and a more accurate one if needed in case you have collision detected, so that your function will run a little bit quicker but using exactly the same loop.

Another option that can help to increase performance is to do some pre-processing with the objects you have. For example you can break the whole area into cells like a generic table and store the appropriate object that are contained within the particular cells. Therefore to detect the collision you are detecting the cells occupied by the ball, get the objects from those cells and use your collision-detecting function.

To speed it up even more you can implement 2d-tree, quadtree or R-tree.

An answer without jQuery, with HTML elements as parameters:

This is a better approach that checks the real position of the elements as they are being shown on the viewport, even if they're absolute, relative or have been manipulated via transformations:

function isCollide(a, b) {
    var aRect = a.getBoundingClientRect();
    var bRect = b.getBoundingClientRect();

    return !(
        ((aRect.top + aRect.height) < (bRect.top)) ||
        (aRect.top > (bRect.top + bRect.height)) ||
        ((aRect.left + aRect.width) < bRect.left) ||
        (aRect.left > (bRect.left + bRect.width))
    );
}

You can try jquery-collision. Full disclosure: I just wrote this and released it. I didn't find a solution, so I wrote it myself.

It allows you to do:

var hit_list = $("#ball").collision("#someobject0");

which will return all the "#someobject0"'s that overlap with "#ball".

Here's a very simple bounding rectangle routine. It expects both a and b to be objects with x, y, width and height properties:

function isCollide(a, b) {
    return !(
        ((a.y + a.height) < (b.y)) ||
        (a.y > (b.y + b.height)) ||
        ((a.x + a.width) < b.x) ||
        (a.x > (b.x + b.width))
    );
}

To see this function in action, here's a codepen graciously made by @MixerOID.