Vue实现一个返回顶部backToTop组件

export function scrollIt(
 destination = 0,
 duration = 200,
 easing = "linear",
 callback
) {
 // define timing functions -- 过渡动效
 let easings = {
  // no easing, no acceleration
  linear(t) {
   return t;
  },
  // accelerating from zero velocity
  easeInQuad(t) {
   return t * t;
  },
  // decelerating to zero velocity
  easeOutQuad(t) {
   return t * (2 - t);
  },
  // acceleration until halfway, then deceleration
  easeInOutQuad(t) {
   return t <0.5 &＃63; 2 * t * t : -1 + (4 - 2 * t) * t;
  },
  // accelerating from zero velocity
  easeInCubic(t) {
   return t * t * t;
  },
  // decelerating to zero velocity
  easeOutCubic(t) {
   return --t * t * t + 1;
  },
  // acceleration until halfway, then deceleration
  easeInOutCubic(t) {
   return t <0.5 &＃63; 4 * t * t * t : (t - 1) * (2 * t - 2) * (2 * t - 2) + 1;
  },
  // accelerating from zero velocity
  easeInQuart(t) {
   return t * t * t * t;
  },
  // decelerating to zero velocity
  easeOutQuart(t) {
   return 1 - --t * t * t * t;
  },
  // acceleration until halfway, then deceleration
  easeInOutQuart(t) {
   return t <0.5 &＃63; 8 * t * t * t * t : 1 - 8 * --t * t * t * t;
  },
  // accelerating from zero velocity
  easeInQuint(t) {
   return t * t * t * t * t;
  },
  // decelerating to zero velocity
  easeOutQuint(t) {
   return 1 + --t * t * t * t * t;
  },
  // acceleration until halfway, then deceleration
  easeInOutQuint(t) {
   return t <0.5 &＃63; 16 * t * t * t * t * t : 1 + 16 * --t * t * t * t * t;
  }
 };
 // requestAnimationFrame()的兼容性封装：先判断是否原生支持各种带前缀的
 //不行的话就采用延时的方案
 (function() {
  var lastTime = 0;
  var vendors = ["ms", "moz", "webkit", "o"];
  for (var x = 0; x