Exponential easing

const easeInOut = (t, p = 5) =>
  t <= 0.5
    ? Math.pow(t * 2, p) / 2
    : 1 - Math.pow(2 - t * 2, p) / 2

Rather than separate declarations for cubicInOut, quartInOut and quintInOut easing functions, this method allows for easing to any power by passing a second argument p. t is assumed to be normalised between 0 and 1. Values of p below 1 result in inverse easing, where the object enters at full speed, slows down and then accelerates out.