code: function getVelocity(deltaTime, x, y) {
      return {
          x: x / deltaTime || 0,
          y: y / deltaTime || 0
      };
  }
docs: calculate the velocity between two points. unit is in px per ms.
  @param {Number} deltaTime
  @param {Number} x
  @param {Number} y
  @return {Object} velocity `x` and `y`