Drag’n’Drop algorithm

The basic Drag’n’Drop algorithm looks like this:

1. On mousedown – prepare the element for moving, if needed (maybe create a clone of it, add a class to it or whatever).

2. Then on mousemove move it by changing left/top with position:absolute.

3. On mouseup – perform all actions related to finishing the drag’n’drop.

Here’s the implementation of dragging a ball:

ball.onmousedown = function(event) {

let shiftX = event.clientX - ball.getBoundingClientRect().left;
  let shiftY = event.clientY - ball.getBoundingClientRect().top;

  ball.style.position = 'absolute';
  ball.style.zIndex = 1000;
  document.body.append(ball);

  moveAt(event.pageX, event.pageY);

  // moves the ball at (pageX, pageY) coordinates
  // taking initial shifts into account
  function moveAt(pageX, pageY) {
    ball.style.left = pageX - shiftX + 'px';
    ball.style.top = pageY - shiftY + 'px';
  }

  function onMouseMove(event) {
    moveAt(event.pageX, event.pageY);
  }

  // move the ball on mousemove
  document.addEventListener('mousemove', onMouseMove);

  // drop the ball, remove unneeded handlers
  ball.onmouseup = function() {
    document.removeEventListener('mousemove', onMouseMove);
    ball.onmouseup = null;
  };

};

ball.ondragstart = function() {
  return false;
};