A simple way to get the right host to query your API in serverless next.js

getHost.ts

const getHost = (context: NextPageContext): { host: string; proto: string } => {
   const { req } = context;
   if (req !== undefined) {
     const {
       "x-forwarded-host": host,
       "x-forwarded-proto": proto
     } = req.headers;
     if (
       typeof host === "string" &&
       typeof proto === "string" &&
       (proto === "http" || proto === "https")
     ) {
       // Proto doesn't have the colon on server
       return { host, proto: proto + ":" };
     }
   } else if (window && window.location) {
     // Parse url to determine our current host
     const { href } = window.location;
     const { host, protocol: proto } = new URL(href);
     return { host, proto };
   }
   throw new Error("Could not determine host to fetch API.");
 };