Javascript Prevent Draggable div outside parent div

I have draggable div. I want this draggable div to be restricted within parent using javascript. I know how to prevent draggable div using jquery UI draggable plugin. But I am not sure how to restrict draggable using native javascript. Please find below my code:

<!DOCTYPE HTML>
<html>
<head>
<style>
.draggabletarget {
  float: left; 
  width: 100px; 
  height: 35px;
  margin: 15px;
  padding: 10px;
  border: 1px solid #aaaaaa;
}
</style>
</head>
<body>

<div class="parent" style="border:1px solid red;height:600px;width:600px;">

<div class="draggabletarget" ondragstart="dragStart(event)" ondrag="dragging(event)" draggable="true" id="dragtarget">
  <p >Drag me!</p>
</div>

</div>

<script>
function dragStart(event) {
  event.dataTransfer.setData("Text", event.target.id);
}

function dragging(event) {
  document.getElementById("demo").innerHTML = "The p element is being dragged";
}

function allowDrop(event) {
  event.preventDefault();
}

</script>

</body>
</html>

In this code i have a div with class parent. I want my draggable div whose class is draggabletarget shouldn't drag outside the parent div. How can i do this in native javascript. Please suggest

Solution

function $(el){
                return document.getElementById(el);
            }
            var isDragging = false;
            
            var tzdragg = function(){
                return {
                    move : function(divid,xpos,ypos){
                        var a = $(divid);
                        $(divid).style.left = xpos + 'px';
                        $(divid).style.top = ypos + 'px';
                    },
                    setDragging : function() {
                        isDragging = true;
                    },
                    resetDragging : function() {
                        isDragging = false;
                    },
                    startMoving : function(evt){
                       
                        evt = evt || window.event;
                        var posX = evt.clientX,
                            posY = evt.clientY,
                            a = $('elem'),
                        divTop = a.style.top,
                        divLeft = a.style.left;
                        
                        divTop = divTop.replace('px','');
                        divLeft = divLeft.replace('px','');
                        var diffX = posX - divLeft,
                            diffY = posY - divTop;
                        document.onmousemove = function(evt){
                            evt = evt || window.event;
                            var posX = evt.clientX,
                                posY = evt.clientY,
                                aX = posX - diffX,
                                aY = posY - diffY;
                       var boun=document.getElementById("parent").offsetWidth-document.getElementById("elem").offsetWidth;
                           
                            if((aX>0)&&(aX<boun)&&(aY>0)&&(aY<boun))
                            tzdragg.move('elem',aX,aY);
                        }
                    },
                    stopMoving : function(){
                        var a = document.createElement('script');
                        document.onmousemove = function(){}
                        
                        if(!isDragging) {
                            console.log('clicked');
                        }
                    },
                }
            }();

#elem
        {
            position: absolute;
            width: 100px;
            height: 100px;
            background-color: black;
            -webkit-user-select: none;
            -moz-user-select: none;
            -o-user-select: none;
            -ms-user-select: none;
            -khtml-user-select: none;
            user-select: none;
            cursor: default;
        }

<div style="height:400px; width:400px; border:solid 1px black;position: absolute;top: 0;
left: 0;" id='parent'>
    <div id='elem' onmousedown='tzdragg.resetDragging();tzdragg.startMoving(event);' onmouseup='tzdragg.stopMoving();' onmousemove='tzdragg.setDragging();'
        >
    </div>
    </div>