elastic.js

var tau = 2 * Math.PI,
    amplitude = 1,
    period = 0.3;

export var elasticIn = (function custom(a, p) {
  var s = Math.asin(1 / (a = Math.max(1, a))) * (p /= tau);

  function elasticIn(t) {
    return a * Math.pow(2, 10 * --t) * Math.sin((s - t) / p);
  }

  elasticIn.amplitude = function(a) { return custom(a, p * tau); };
  elasticIn.period = function(p) { return custom(a, p); };

  return elasticIn;
})(amplitude, period);

export var elasticOut = (function custom(a, p) {
  var s = Math.asin(1 / (a = Math.max(1, a))) * (p /= tau);

  function elasticOut(t) {
    return 1 - a * Math.pow(2, -10 * (t = +t)) * Math.sin((t + s) / p);
  }

  elasticOut.amplitude = function(a) { return custom(a, p * tau); };
  elasticOut.period = function(p) { return custom(a, p); };

  return elasticOut;
})(amplitude, period);

export var elasticInOut = (function custom(a, p) {
  var s = Math.asin(1 / (a = Math.max(1, a))) * (p /= tau);

  function elasticInOut(t) {
    return ((t = t * 2 - 1) < 0
        ? a * Math.pow(2, 10 * t) * Math.sin((s - t) / p)
        : 2 - a * Math.pow(2, -10 * t) * Math.sin((s + t) / p)) / 2;
  }

  elasticInOut.amplitude = function(a) { return custom(a, p * tau); };
  elasticInOut.period = function(p) { return custom(a, p); };

  return elasticInOut;
})(amplitude, period);