draggable quadratic bezier curve SVG example (MIT License)

<!DOCTYPE html>
<html>
<head>
<title>draggable quadratic bezier curve</title>
<style>
#figure1 {
  border: 1px solid black;
}

.draggable {
  cursor: move;
}

.control-point {
  opacity: 0.5;
  stroke: none;
  fill: red;
}
</style>
</head>
<body>
<svg id="figure1" width="300" height="300">
  <path d="M70 250 20 110 250 50" stroke="black" fill="none" id="controlPath"/>
  <path d="M70 250Q20 110 250 50" stroke="green" fill="none" id="curve"/>
  <circle cx="70" cy="250" r="8" id="p1" class="control-point draggable"/>
  <circle cx="20" cy="110" r="8" id="p2" class="control-point draggable"/>
  <circle cx="250" cy="50" r="8" id="p3" class="control-point draggable"/>
</svg>
<script>
var controlPath = document.getElementById('controlPath');
var curve = document.getElementById('curve');
var p1 = document.getElementById('p1');
var p2 = document.getElementById('p2');
var p3 = document.getElementById('p3');
var dragElem = null;

var svg = document.getElementById('figure1');
svg.addEventListener('mousemove', onMouseMove);
svg.addEventListener('mouseup', onMouseUp);

var elements = [p1, p2, p3];
for (i = 0; i < elements.length; i++) {
  elements[i].addEventListener('mousedown', onMouseDown);
}

var mouseOffsetX, mouseOffsetY;

function getClientPointInSVG(ev) {
  var p, m;

p = svg.createSVGPoint();
  p.x = ev.clientX;
  p.y = ev.clientY;

m = dragElem.getScreenCTM();
  return p.matrixTransform(m.inverse());
}

function onMouseDown(ev) {
  var p;

dragElem = ev.target;
  p = getClientPointInSVG(ev);
  mouseOffsetX = p.x - dragElem.getAttribute('cx');
  mouseOffsetY = p.y - dragElem.getAttribute('cy');
}

function onMouseMove(ev) {
  var p;

if (!dragElem) {
    return;
  }

p = getClientPointInSVG(ev);
  dragElem.setAttribute('cx', p.x - mouseOffsetX);
  dragElem.setAttribute('cy', p.y - mouseOffsetY);

controlPath.setAttribute('d',
    'M' + p1.getAttribute('cx') + ' ' + p1.getAttribute('cy') +
    ' ' + p2.getAttribute('cx') + ' ' + p2.getAttribute('cy') +
    ' ' + p3.getAttribute('cx') + ' ' + p3.getAttribute('cy'));
  curve.setAttribute('d',
    'M' + p1.getAttribute('cx') + ' ' + p1.getAttribute('cy') +
    'Q' + p2.getAttribute('cx') + ' ' + p2.getAttribute('cy') +
    ' ' + p3.getAttribute('cx') + ' ' + p3.getAttribute('cy'));
}

function onMouseUp(ev) {
  dragElem = null;
}
</script>
</body>
</html>