If you are working on a migration from classical web sites to Single Page Applications (SPA) you will find yourself dealing with a domain where all the code lives, mixed technologies for which you are forced to run the backend server and bunch of inconveniences like applying database migrations or redeploying backend code.
You should be able to develop the SPA locally though and consume the APIs remotely but you probably do not want to allow cross domain requests or even separate the application in two different domains.
A reverse proxy should help you big time. With a reverse proxy you can concentrate on just developing your SPA bits locally while hitting the existing API remotely and yet being able to remotely deploy the app when ready. All you need to do is detect where the SPA is running and route through the local proxy the API requests.
Node http-proxy can be used to create an https-to-https proxy as presented below: