Can you tell if one element is touching another using JavaScript?

Question

I want to use JavaScript to check if one div element (that can be dragged) is touching another div element.

Here is some code:

Answer 1

Update: I realize now that this only accounts for the intersection of the top left corner of the target element and therefore, doesn't provide a complete solution. But I'll leave it up for posterity in case someone finds it useful for other purposes.

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Use element.getBoundingClientRect() and document.elementFromPoint()

1. You can use element.getClientBoundingRect() (src) to get the position of the target (clicked, dragged, etc.) element.

2. Temporarily hide the target element, then use document.elementFromPoint(x, y) (src) to get the top-most element at that position, and then check it's class name for comparison (you could compare any attribute or property instead, if you prefer).

   To achieve cross-browser compatible behavior from this method read: document.elementFromPoint – a jQuery solution (You don't have to use jQuery to achieve this result. The method can be replicated in pure JS.)

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Addendum:

I am only showing the function for detecting overlap instead of showing drag-and-drop or drag-to-move functionality because it isn't clear which of those, if either, you are trying to implement and there are other answers showing how to accomplish various drag patterns.

In any case, you can use the detectCollision() function below in combination with any drag solution.

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var box2 = document.getElementById('box2'),
    box3 = document.getElementById('box3');
box2.onclick = detectCollision;
box3.onclick = detectCollision;

function detectCollision(e) {
    var elem        = e.target,
        elemOffset  = elem.getBoundingClientRect(),
        elemDisplay = elem.style.display;
    
    // Temporarily hide element
    elem.style.display = 'none';
    
    // Check for top-most element at position
    var topElem = document.elementFromPoint(elemOffset.left, elemOffset.top);

    // Reset element's initial display value.
    elem.style.display = elemDisplay;

    // If a top-most element is another box
    if (topElem.className.match(/box/)) {
        alert(elem.id + " is touching " + topElem.id);
    } else {
        alert(elem.id + " isn't touching another box.");
    };
}

#box1 {
    background-color: LightSeaGreen;
}
#box2 {
    top: 25px;
    left: -25px;
    background-color: SandyBrown;
}
#box3 {
    background-color: SkyBlue;
}
.box {
    position: relative;
    display: inline-block;
    width: 100px;
    height: 100px;    
}
.clickable {
    cursor: pointer;
}

<div class="box" id="box1"></div>
<div class="box clickable" id="box2"></div>
<div class="box clickable" id="box3"></div>

Answer 2

A more complete solution

Below is a "plain old JavaScript" rewrite of the overlap detection function found in this answer to the question titled "jQuery/Javascript collision detection".

The only real difference between the two is the replacement of the use of jQuery to get element position and width for calculating the bounding box.

Surprisingly, native JavaScript makes this task easier with the well-supported (IE4+) Element.getBoundingClientRect() method, which returns the four values needed to create the position matrix returned by the getPositions function.

I added a click handler for the boxes as a simple demonstration of how you might use the function to compare a target (clicked, dragged, etc.) element to a set of selected elements. I will concede, however, that jQuery makes DOM selection and event binding much easier than native JS.

var boxes = document.querySelectorAll('.box');

boxes.forEach(function (el) {
  if (el.addEventListener) {
      el.addEventListener('click', clickHandler);
  } else {
      el.attachEvent('onclick', clickHandler);
  }
})

var detectOverlap = (function () {
    function getPositions(elem) {
        var pos = elem.getBoundingClientRect();
        return [[pos.left, pos.right], [pos.top, pos.bottom]];
    }

    function comparePositions(p1, p2) {
        var r1, r2;
        if (p1[0] < p2[0]) {
          r1 = p1;
          r2 = p2;
        } else {
          r1 = p2;
          r2 = p1;
        }
        return r1[1] > r2[0] || r1[0] === r2[0];
    }

    return function (a, b) {
        var pos1 = getPositions(a),
            pos2 = getPositions(b);
        return comparePositions(pos1[0], pos2[0]) && comparePositions(pos1[1], pos2[1]);
    };
})();

function clickHandler(e) {
    
    var elem     = e.target,
        elems    = document.querySelectorAll('.box'),
        elemList = Array.prototype.slice.call(elems),
        within   = elemList.indexOf(elem),
        touching = [];
    if (within !== -1) {
        elemList.splice(within, 1);
    }
    for (var i = 0; i < elemList.length; i++) {
        if (detectOverlap(elem, elemList[i])) {
            touching.push(elemList[i].id);
        }
    }
    if (touching.length) {
        console.log(elem.id + ' touches ' + touching.join(' and ') + '.');
        alert(elem.id + ' touches ' + touching.join(' and ') + '.');
    } else {
        console.log(elem.id + ' touches nothing.');
        alert(elem.id + ' touches nothing.');
    }

}

#box1 {
    background-color: LightSeaGreen;
}
#box2 {
    top: 25px;
    left: -25px;
    background-color: SandyBrown;
}
#box3 {
    left: -50px;
    background-color: SkyBlue;
}
#box4 {
    background-color: SlateGray;
}
.box {
    position: relative;
    display: inline-block;
    width: 100px;
    height: 100px;
    color: White;
    font: bold 72px sans-serif;
    line-height: 100px;
    text-align: center;
    cursor: pointer;
}
.box:hover {
    color: Black;
}

<p>Click a box to see which other boxes are detected as touching it.<br />
<em>If no alert dialog box appears, open your console to view messages.</em></p>
<div class="box" id="box1">1</div>
<div class="box" id="box2">2</div>
<div class="box" id="box3">3</div>
<div class="box" id="box4">4</div>