drag.js

const drag = (() => {
  return {
    move: function(elem, xpos, ypos) {
      elem.style.left = xpos + 'px';
      elem.style.top = ypos + 'px';
    },
    start: function(elem, container, event) {
      let posX = event.clientX,
          posY = event.clientY,
          elemTop = (elem.style.top).replace('px', ''),
          elemLeft = (elem.style.left).replace('px', ''),
          elemWidth = parseInt(elem.style.width),
          elemHeight = parseInt(elem.style.height),
          containerWidth = parseInt(container.style.width),
          containerHeight = parseInt(container.style.height);

      container.style.cursor = 'move';

      let diffX = posX - elemLeft,
          diffY = posY - elemTop;

      document.onmousemove = function(event) {
        let posX = event.clientX,
            posY = event.clientY,
            aX = posX - diffX,
            aY = posY - diffY;

        if(aX < 0)
          aX = 0;
        if(aY < 0)
          aY = 0;
        if(aX + elemWidth > containerWidth)
          aX = containerWidth - elemWidth;
        if(aY + elemTop > containerHeight)
          aY = containerHeight - elemHeight;
        drag.move(elem, aX, aY);
      }
    },
    stop: function(container) {
      container.style.cursor = 'default';

      document.onmousemove = function() {};
    }
  }
})();