刚开始学前端，记录下学到的知识，原生js实现拖拽弹性重力运行。记录下，直接上代码：

• html和css部分

<div></div>
div {
    width: 100px;
    height: 100px;
    position: absolute;
    left: 0;
    top: 0;
    background-color: chocolate;
    border-radius: 50%;
}

• js部分

    <script>
        var oDiv = document.getElementsByTagName("div")[0];
        var lastX = 0;
        var lastY = 0;
        var speedX = 0;
        var speedY = 0;
        oDiv.onmousedown = function (e) {
            var self = this;
            clearInterval(this.timer);
            var event = e || window.event;
            var disX = event.clientX - this.offsetLeft;
            var disY = event.clientY - this.offsetTop;
            document.onmousemove = function (e) {
                self.style.left = e.clientX - disX + "px";
                self.style.top = e.clientY - disY + "px";

                speedX = self.offsetLeft - lastX;
                speedY = self.offsetTop - lastY;

                lastX = self.offsetLeft;
                lastY = self.offsetTop;
            }

            document.onmouseup = function () {
                document.onmousemove = null;
                startMovee(self,speedX,speedY);
            }
        }


        function startMovee(dom, speedX, speedY) {
            clearInterval(dom.timer);
            // 加速度和能量损失
            var g = 3, e = 0.8;
            dom.timer = setInterval(function () {
                speedY += g;

                var newTop = dom.offsetTop + speedY;
                var newLeft = dom.offsetLeft + speedX;

                var clientHeight = document.documentElement.clientHeight;
                var clientWidth = document.documentElement.clientWidth;

                if (newTop >= clientHeight - dom.clientHeight) {
                    speedY *= -1;
                    speedX *= e;
                    speedY *= e;
                    newTop = clientHeight - dom.clientHeight;
                }

                if (newTop <= 0) {
                    speedY *= -1;
                    speedX *= e;
                    speedY *= e;
                    newTop = 0;
                }

                if (newLeft >= clientWidth - dom.clientWidth) {
                    speedX *= -1;
                    speedX *= e;
                    speedY *= e;
                    newLeft = clientWidth - dom.clientWidth;
                }

                if (newLeft <= 0) {
                    speedX *= -1;
                    speedX *= e;
                    speedY *= e;
                    newLeft = 0;
                }

                //停止运动
                if (Math.abs(speedY) < 1) {
                    speedY = 0;
                }
                if (Math.abs(speedX) < 1) {
                    speedX = 0;
                }
                if (speedX == 0 && speedY == 0 && dom.offsetTop <= (clientHeight - dom.clientHeight)) {
                    clearInterval(dom.timer);
                } else {
                    dom.style.left = newLeft + "px";
                    dom.style.top = newTop + "px";
                }

            }, 30)
        }
    </script>