renderNextPage.js

// Load the NextJS page
const nextPage = require("./nextPage");
const createRequestObject = require("./createRequestObject");
const createResponseObject = require("./createResponseObject");

// Render the Next.js page
const renderNextPage = (event) => {
  // The Next.js page is rendered inside a promise that is resolved when the
  // Next.js page ends the response via `res.end()`
  const promise = new Promise((resolve) => {
    // Create a Next.js-compatible request and response object
    // These mock the ClientRequest and ServerResponse classes from node http
    // See: https://nodejs.org/api/http.html
    const req = createRequestObject({ event });
    const res = createResponseObject({
      onResEnd: (response) => resolve(response),
    });

    // Check if page is a Next.js page or an API route
    const isNextPage = nextPage.render instanceof Function;
    const isApiRoute = !isNextPage;

    // Perform the render: render() for Next.js page or default() for API route
    if (isNextPage) return nextPage.render(req, res);
    if (isApiRoute) return nextPage.default(req, res);
  });

  // Return the promise
  return promise;
};

module.exports = renderNextPage;