code: function forLeftButton(props) {
    const { position, scene, scenes } = props;
    const interpolate = getSceneIndicesForInterpolationInputRange(props);
  
    if (!interpolate) return { opacity: 0 };
  
    const { first, last } = interpolate;
    const index = scene.index;
  
    return {
      opacity: position.interpolate({
        inputRange: [
          first,
          first + Math.abs(index - first) / 2,
          index,
          last - Math.abs(last - index) / 2,
          last,
        ],
        outputRange: [0, 0.5, 1, 0.5, 0],
      }),
    };
  }
docs: iOS UINavigationController style interpolators