在阅读 Ant Design BackTop
代码时候看到 tween 算法，以前一直没有深入看看，这里稍微研究下

const easeInOutCubic = (t: number, b: number, c: number, d: number) => {
  const cc = c - b;
  t /= d / 2;
  if (t < 1) {return (cc / 2) * t * t * t + b;
  } else {return (cc / 2) * ((t -= 2) * t * t + 2) + b;
  }
};

Linear：无缓动效果；
Quadratic：二次方的缓动（t^2）；
Cubic：三次方的缓动（t^3）；
Quartic：四次方的缓动（t^4）；
Quintic：五次方的缓动（t^5）；
Sinusoidal：正弦曲线的缓动（sin(t)）；
Exponential：指数曲线的缓动（2^t）；
Circular：圆形曲线的缓动（sqrt(1-t^2)）；
Elastic：指数衰减的正弦曲线缓动；
Back：超过范围的三次方缓动（(s+1)t^3 – st^2）；
Bounce：指数衰减的反弹缓动。

每个效果都分三个缓动方式（方法），分别是：
easeIn：从 0 开始加速的缓动；
easeOut：减速到 0 的缓动；
easeInOut：前半段从 0 开始加速，后半段减速到 0 的缓动。

var Tween = {Linear: function(t,b,c,d){return c*t/d + b;},
    Quad: {easeIn: function(t,b,c,d){return c*(t/=d)*t + b;
        },
        easeOut: function(t,b,c,d){return -c *(t/=d)*(t-2) + b;
        },
        easeInOut: function(t,b,c,d){if ((t/=d/2) < 1) return c/2*t*t + b;
            return -c/2 * ((t)*(t-2) - 1) + b;
        }
    },
    Cubic: {easeIn: function(t,b,c,d){return c*(t/=d)*t*t + b;
        },
        easeOut: function(t,b,c,d){return c*((t=t/d-1)*t*t + 1) + b;
        },
        easeInOut: function(t,b,c,d){if ((t/=d/2) < 1) return c/2*t*t*t + b;
            return c/2*((t-=2)*t*t + 2) + b;
        }
    },
    Quart: {easeIn: function(t,b,c,d){return c*(t/=d)*t*t*t + b;
        },
        easeOut: function(t,b,c,d){return -c * ((t=t/d-1)*t*t*t - 1) + b;
        },
        easeInOut: function(t,b,c,d){if ((t/=d/2) < 1) return c/2*t*t*t*t + b;
            return -c/2 * ((t-=2)*t*t*t - 2) + b;
        }
    },
    Quint: {easeIn: function(t,b,c,d){return c*(t/=d)*t*t*t*t + b;
        },
        easeOut: function(t,b,c,d){return c*((t=t/d-1)*t*t*t*t + 1) + b;
        },
        easeInOut: function(t,b,c,d){if ((t/=d/2) < 1) return c/2*t*t*t*t*t + b;
            return c/2*((t-=2)*t*t*t*t + 2) + b;
        }
    },
    Sine: {easeIn: function(t,b,c,d){return -c * Math.cos(t/d * (Math.PI/2)) + c + b;
        },
        easeOut: function(t,b,c,d){return c * Math.sin(t/d * (Math.PI/2)) + b;
        },
        easeInOut: function(t,b,c,d){return -c/2 * (Math.cos(Math.PI*t/d) - 1) + b;
        }
    },
    Expo: {easeIn: function(t,b,c,d){return (t==0) ? b : c * Math.pow(2, 10 * (t/d - 1)) + b;
        },
        easeOut: function(t,b,c,d){return (t==d) ? b+c : c * (-Math.pow(2, -10 * t/d) + 1) + b;
        },
        easeInOut: function(t,b,c,d){if (t==0) return b;
            if (t==d) return b+c;
            if ((t/=d/2) < 1) return c/2 * Math.pow(2, 10 * (t - 1)) + b;
            return c/2 * (-Math.pow(2, -10 *  t) + 2) + b;
        }
    },
    Circ: {easeIn: function(t,b,c,d){return -c * (Math.sqrt(1 - (t/=d)*t) - 1) + b;
        },
        easeOut: function(t,b,c,d){return c * Math.sqrt(1 - (t=t/d-1)*t) + b;
        },
        easeInOut: function(t,b,c,d){if ((t/=d/2) < 1) return -c/2 * (Math.sqrt(1 - t*t) - 1) + b;
            return c/2 * (Math.sqrt(1 - (t-=2)*t) + 1) + b;
        }
    },
    Elastic: {easeIn: function(t,b,c,d,a,p){if (t==0) return b;  if ((t/=d)==1) return b+c;  if (!p) p=d*.3;
            if (!a || a < Math.abs(c)) {a=c; var s=p/4;}
            else var s = p/(2*Math.PI) * Math.asin (c/a);
            return -(a*Math.pow(2,10*(t-=1)) * Math.sin((t*d-s)*(2*Math.PI)/p )) + b;
        },
        easeOut: function(t,b,c,d,a,p){if (t==0) return b;  if ((t/=d)==1) return b+c;  if (!p) p=d*.3;
            if (!a || a < Math.abs(c)) {a=c; var s=p/4;}
            else var s = p/(2*Math.PI) * Math.asin (c/a);
            return (a*Math.pow(2,-10*t) * Math.sin((t*d-s)*(2*Math.PI)/p ) + c + b);
        },
        easeInOut: function(t,b,c,d,a,p){if (t==0) return b;  if ((t/=d/2)==2) return b+c;  if (!p) p=d*(.3*1.5);
            if (!a || a < Math.abs(c)) {a=c; var s=p/4;}
            else var s = p/(2*Math.PI) * Math.asin (c/a);
            if (t < 1) return -.5*(a*Math.pow(2,10*(t-=1)) * Math.sin((t*d-s)*(2*Math.PI)/p )) + b;
            return a*Math.pow(2,-10*(t-=1)) * Math.sin((t*d-s)*(2*Math.PI)/p )*.5 + c + b;
        }
    },
    Back: {easeIn: function(t,b,c,d,s){if (s == undefined) s = 1.70158;
            return c*(t/=d)*t*((s+1)*t - s) + b;
        },
        easeOut: function(t,b,c,d,s){if (s == undefined) s = 1.70158;
            return c*((t=t/d-1)*t*((s+1)*t + s) + 1) + b;
        },
        easeInOut: function(t,b,c,d,s){if (s == undefined) s = 1.70158; 
            if ((t/=d/2) < 1) return c/2*(t*t*(((s*=(1.525))+1)*t - s)) + b;
            return c/2*((t-=2)*t*(((s*=(1.525))+1)*t + s) + 2) + b;
        }
    },
    Bounce: {easeIn: function(t,b,c,d){return c - Tween.Bounce.easeOut(d-t, 0, c, d) + b;
        },
        easeOut: function(t,b,c,d){if ((t/=d) < (1/2.75)) {return c*(7.5625*t*t) + b;
            } else if (t < (2/2.75)) {return c*(7.5625*(t-=(1.5/2.75))*t + .75) + b;
            } else if (t < (2.5/2.75)) {return c*(7.5625*(t-=(2.25/2.75))*t + .9375) + b;
            } else {return c*(7.5625*(t-=(2.625/2.75))*t + .984375) + b;
            }
        },
        easeInOut: function(t,b,c,d){if (t < d/2) return Tween.Bounce.easeIn(t*2, 0, c, d) * .5 + b;
            else return Tween.Bounce.easeOut(t*2-d, 0, c, d) * .5 + c*.5 + b;
        }
    }
}

参考

Tween 效果
http://www.cnblogs.com/cloudg…
https://github.com/zhangxinxu…
https://www.zhangxinxu.com/st…
如何使用 Tween.js 各类原生动画运动缓动算法