实现一个div的拖拽效果

        class Drag {
            //构造函数
            constructor(el) {
                this.el = el;
                //鼠标摁下时的元素位置
                this.startOffset = {};
                //鼠标摁下时的鼠标位置
                this.startPoint = {};
                let move = (e) => {
                    this.move(e);
                };
                let end = (e) => {
                    document.removeEventListener("mousemove", move);
                    document.removeEventListener("mouseup", end);
                };
                el.addEventListener("mousedown", (e) => {
                    this.start(e);
                    document.addEventListener("mousemove", move);
                    document.addEventListener("mouseup", end);
                })
            }
            //摁下时的处理函数
            start(e) {
                let { el } = this;
                this.startOffset = {
                    x: el.offsetLeft,
                    y: el.offsetTop
                }
                this.startPoint = {
                    x: e.clientX,
                    y: e.clientY
                }
            }
            //鼠标移动时的处理函数
            move(e) {
                let { el, startOffset, startPoint } = this;
                let newPoint = {
                    x: e.clientX,
                    y: e.clientY
                }
                let dis = {
                    x: newPoint.x - startPoint.x,
                    y: newPoint.y - startPoint.y,
                }
                el.style.left = dis.x + startOffset.x + "px";
                el.style.top = dis.y + startOffset.y + "px";
            }
        }

        (function () {
            let box = document.querySelector("#box");
            let dragbox = new Drag(box);
        })()