What's the best way to get from Chengdu to Zhangjiajie?

As I know, there is no direct train or flight between these two places, let alone an overnight one.

To travel by train, I would suggest you take a train to Changsha first. Then near Changsha Train Station, there are direct buses to your destination.

Or you can fly to Changsha first. Then take shuttle bus to Changsha West Bus Station, where u can take a direct bus to the scenic area.