js realizes the special effect of product parabola added to shopping cart

/*!
 * by zhangxinxu(.com) 2012-12-27
 * you can visit http://www.zhangxinxu.com/wordpress/?p=3855 to get more infomation
 * under MIT license
*/
var funParabola = function(element, target, options) {
  /*
   * 网页模拟现实需要一个比例尺
   * 如果按照1像素就是1米来算，显然不合适，因为页面动不动就几百像素
   * 页面上，我们放两个物体，200~800像素之间，我们可以映射为现实世界的2米到8米，也就是100:1
   * 不过，本方法没有对此有所体现，因此不必在意
  */
   
  var defaults = {
    speed: 166.67, // 每帧移动的像素大小，每帧（对于大部分显示屏）大约16~17毫秒
    curvature: 0.001, // 实际指焦点到准线的距离，你可以抽象成曲率，这里模拟扔物体的抛物线，因此是开口向下的
    progress: function() {},
    complete: function() {}
  };
   
  var params = {}; options = options || {};
   
  for (var key in defaults) {
    params[key] = options[key] || defaults[key];
  }
   
  var exports = {
    mark: function() { return this; },
    position: function() { return this; },
    move: function() { return this; },
    init: function() { return this; }
  };
   
  /* 确定移动的方式 
   * IE6-IE8 是margin位移
   * IE9+使用transform
  */
  var moveStyle = "margin", testDiv = document.createElement("div");
  if ("oninput" in testDiv) {
    ["", "ms", "webkit"].forEach(function(prefix) {
      var transform = prefix + (prefix? "T": "t") + "ransform";
      if (transform in testDiv.style) {
        moveStyle = transform;
      }
    });    
  }
   
  // 根据两点坐标以及曲率确定运动曲线函数（也就是确定a, b的值）
  /* 公式： y = a*x*x + b*x + c;
  */
  var a = params.curvature, b = 0, c = 0;
   
  // 是否执行运动的标志量
  var flagMove = true;
   
  if (element && target && element.nodeType == 1 && target.nodeType == 1) {
    var rectElement = {}, rectTarget = {};
     
    // 移动元素的中心点位置，目标元素的中心点位置
    var centerElement = {}, centerTarget = {};
     
    // 目标元素的坐标位置
    var coordElement = {}, coordTarget = {};
     
    // 标注当前元素的坐标
    exports.mark = function() {
      if (flagMove == false) return this;
      if (typeof coordElement.x == "undefined") this.position();
      element.setAttribute("data-center", [coordElement.x, coordElement.y].join());
      target.setAttribute("data-center", [coordTarget.x, coordTarget.y].join());
      return this;
    }
     
    exports.position = function() {
      if (flagMove == false) return this;
       
      var scrollLeft = document.documentElement.scrollLeft || document.body.scrollLeft,
        scrollTop = document.documentElement.scrollTop || document.body.scrollTop;
       
      // 初始位置
      if (moveStyle == "margin") {
        element.style.marginLeft = element.style.marginTop = "0px";
      } else {
        element.style[moveStyle] = "translate(0, 0)";
      }
       
      // 四边缘的坐标
      rectElement = element.getBoundingClientRect();
      rectTarget = target.getBoundingClientRect();
       
      // 移动元素的中心点坐标
      centerElement = {
        x: rectElement.left + (rectElement.right - rectElement.left) / 2 + scrollLeft,
        y: rectElement.top + (rectElement.bottom - rectElement.top) / 2  + scrollTop
      };
       
      // 目标元素的中心点位置
      centerTarget = {
        x: rectTarget.left + (rectTarget.right - rectTarget.left) / 2 + scrollLeft,
        y: rectTarget.top + (rectTarget.bottom - rectTarget.top) / 2 + scrollTop    
      };
       
      // 转换成相对坐标位置
      coordElement = {
        x: 0,
        y: 0  
      };
      coordTarget = {
        x: -1 * (centerElement.x - centerTarget.x),
        y: -1 * (centerElement.y - centerTarget.y)  
      };
       
      /*
       * 因为经过(0, 0), 因此c = 0
       * 于是：
       * y = a * x*x + b*x;
       * y1 = a * x1*x1 + b*x1;
       * y2 = a * x2*x2 + b*x2;
       * 利用第二个坐标：
       * b = (y2+ a*x2*x2) / x2
      */
      // 于是
      b = (coordTarget.y - a * coordTarget.x * coordTarget.x) / coordTarget.x;  
       
      return this;
    };    
     
    // 按照这个曲线运动
    exports.move = function() {
      // 如果曲线运动还没有结束，不再执行新的运动
      if (flagMove == false) return this;
       
      var startx = 0, rate = coordTarget.x > 0? 1: -1;
 
      var step = function() {
        // 切线 y'=2ax+b
        var tangent = 2 * a * startx + b; // = y / x
        // y*y + x*x = speed
        // (tangent * x)^2 + x*x = speed
        // x = Math.sqr(speed / (tangent * tangent + 1));
        startx = startx + rate * Math.sqrt(params.speed / (tangent * tangent + 1));
         
        // 防止过界
        if ((rate == 1 && startx > coordTarget.x) || (rate == -1 && startx < coordTarget.x)) {
          startx = coordTarget.x;
        }
        var x = startx, y = a * x * x + b * x;
         
        // 标记当前位置，这里有测试使用的嫌疑，实际使用可以将这一行注释
        element.setAttribute("data-center", [Math.round(x), Math.round(y)].join());
         
        // x, y目前是坐标，需要转换成定位的像素值
        if (moveStyle == "margin") {
          element.style.marginLeft = x + "px";
          element.style.marginTop = y + "px";
        } else {
          element.style[moveStyle] = "translate("+ [x + "px", y + "px"].join() +")";
        }
         
        if (startx !== coordTarget.x) {
          params.progress(x, y);
          window.requestAnimationFrame(step);  
        } else {
          // 运动结束，回调执行
          params.complete();
          flagMove = true;  
        }
      };
      window.requestAnimationFrame(step);
      flagMove = false;
       
      return this;
    };
     
    // 初始化方法
    exports.init = function() {
      this.position().mark().move();
    };
  }
   
  return exports;
};
 
/*! requestAnimationFrame.js
 * by zhangxinxu 2013-09-30
*/
(function() {
  var lastTime = 0;
  var vendors = [&#39;webkit&#39;, &#39;moz&#39;];
  for(var x = 0; x < vendors.length && !window.requestAnimationFrame; ++x) {
    window.requestAnimationFrame = window[vendors[x] + &#39;RequestAnimationFrame&#39;];
    window.cancelAnimationFrame = window[vendors[x] + &#39;CancelAnimationFrame&#39;] ||  // name has changed in Webkit
                   window[vendors[x] + &#39;CancelRequestAnimationFrame&#39;];
  }
 
  if (!window.requestAnimationFrame) {
    window.requestAnimationFrame = function(callback, element) {
      var currTime = new Date().getTime();
      var timeToCall = Math.max(0, 16.7 - (currTime - lastTime));
      var id = window.setTimeout(function() {
        callback(currTime + timeToCall);
      }, timeToCall);
      lastTime = currTime + timeToCall;
      return id;
    };
  }
  if (!window.cancelAnimationFrame) {
    window.cancelAnimationFrame = function(id) {
      clearTimeout(id);
    };
  }
}());

/* 元素 */
var element = document.getElementById("element"), 
  target = document.getElementById("target");
// 抛物线元素的的位置标记
var parabola = funParabola(element, target).mark();
// 抛物线运动的触发
document.body.onclick = function() {
  element.style.marginLeft = "0px";
  element.style.marginTop = "0px";
  parabola.init();
};

/* 本demo演示脚本基于ieBetter.js, 项目地址：https://github.com/zhangxinxu/ieBetter.js */
 
// 元素以及其他一些变量
var eleFlyElement = document.querySelector("#flyItem"), eleShopCart = document.querySelector("#shopCart");
var numberItem = 0;
// 抛物线运动
var myParabola = funParabola(eleFlyElement, eleShopCart, {
  speed: 400,
  curvature: 0.002,  
  complete: function() {
    eleFlyElement.style.visibility = "hidden";
    eleShopCart.querySelector("span").innerHTML = ++numberItem;
  }
});
// 绑定点击事件
if (eleFlyElement && eleShopCart) {
  [].slice.call(document.getElementsByClassName("btnCart")).forEach(function(button) {
    button.addEventListener("click", function() {
      // 滚动大小
      var scrollLeft = document.documentElement.scrollLeft || document.body.scrollLeft || 0,
        scrollTop = document.documentElement.scrollTop || document.body.scrollTop || 0;
 
      eleFlyElement.style.left = event.clientX + scrollLeft + "px";
      eleFlyElement.style.top = event.clientY + scrollTop + "px";
      eleFlyElement.style.visibility = "visible";
       
      // 需要重定位
      myParabola.position().move(); 
    });
  });
}