Puppeteer is a Node library which provides a high-level API to control headless Chrome over the DevTools Protocol. It can also be configured to use full non-headless Chrome.

Its demo is worth thousand words. API Documentation covers all the aspects of browser emulation/handling I could think of.

Whether it will be a replacement of NightwatchJS or a sub-component in the current end-to-end stack – fun times of trials & errors will tell.