back.js

var overshoot = 1.70158;

export var backIn = (function custom(s) {
  s = +s;

  function backIn(t) {
    return t * t * ((s + 1) * t - s);
  }

  backIn.overshoot = custom;

  return backIn;
})(overshoot);

export var backOut = (function custom(s) {
  s = +s;

  function backOut(t) {
    return --t * t * ((s + 1) * t + s) + 1;
  }

  backOut.overshoot = custom;

  return backOut;
})(overshoot);

export var backInOut = (function custom(s) {
  s = +s;

  function backInOut(t) {
    return ((t *= 2) < 1 ? t * t * ((s + 1) * t - s) : (t -= 2) * t * ((s + 1) * t + s) + 2) / 2;
  }

  backInOut.overshoot = custom;

  return backInOut;
})(overshoot);