Simple Easing Method.

easing.js

/**
 * Easing Function f(t) where t, f(t) <- [0, 1]
 * return an Easing Factory.
 */
function easing(f) {
  /**
   * x = f(t) where x <- [x0, x1], t <- [0, t1]
   * return an Easing Instance.
   */
  return function(x0, x1, t1) { // x = f(t), x <-[x0, x1]
    var t = 0;
    /**
     * Each call returns the next value.
     * Return null means no more value.
     */
    return function () {
      return (t <= t1)
        ? f(t++ / t1) * (x1 - x0) + x0
        : null;
    }
  }
}

var linearFunc = function (x) { return x; }
  , linearEasingFactory = easing(linearFunc)
  , myEasingInstance = linearEasingFactory(5, 10, 10)
  , x;
while((x = myEasingInstance()) !== null)
  console.log(x);