SOURCE

var h1 = document.getElementsByClassName("h1")[0]

h1.onmousedown = function(e){
    var x = e.clientX - h1.offsetLeft
    var y = e.clientY - h1.offsetTop
    document.onmousemove = function(e){
   h1.style.left =e.clientX -x + "px";
    h1.style.top = e.clientY -y + "px";
}
}
document.onmouseup = function(){
document.onmousemove = null;
}

<div class="h1">
    
</div>

.h1{
    width: 100px;
    height: 100px;
    border-radius: 50%;
    background: red;
    position: absolute;
}