How does PortalJS relate to CKAN?

[rufuspollock]

PortalJS is described as:

a framework for rapidly building rich data portal frontends using a modern frontend approach.

Datopian are also stewards of CKAN which is also a platform or framework for building data portals.

So how do the two relate? Is PortalJS a replacement for CKAN? Or is it a complement? Or what?

Overall, how does PortalJS relate to CKAN?