how to make multiple movable elements

I have this code for creating movable windows (elements), and I call this function when I create a new window:

function dragWindow(elmnt) {
    var pos1 = 0, pos2 = 0, pos3 = 0, pos4 = 0;
    elmnt.querySelector(".window-caption").onmousedown = dragMouseDown;
    function dragMouseDown(e) {
        e.preventDefault();
        pos3 = e.clientX;
        pos4 = e.clientY;
        document.onmouseup = closeDragElement;
        document.onmousemove = elementDrag;
    }
    function elementDrag(e) {
        e.preventDefault();
        pos1 = pos3 - e.clientX;
        pos2 = pos4 - e.clientY;
        pos3 = e.clientX;
        pos4 = e.clientY;
        elmnt.style.top = (elmnt.offsetTop - pos2) + "px";
        elmnt.style.left = (elmnt.offsetLeft - pos1) + "px";
    }
    function closeDragElement() {
        // alert(elmnt.id);
        document.onmouseup = null;
        document.onmousemove = null;
    }
}

The problem is:

if I create a new window, I can't move windows they created before.

I called the function again on each of the windows (in developer console); And it showed me the right answer:

So, when I create a new window, I should call dragWindow again for each window.