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