[FIXED] How to cast a plain object as a instance of another type in JavaScript

Issue

I'm trying to store a session on disk for local development, and I'm running into a problem where the application expects me to return an instance of a Session and not an Object

function storeCallback(session) {
  console.log("storeCallback ", session);
  fs.writeFileSync("./session.json", JSON.stringify(session));
  return true;
}

function loadCallback(id) {
  console.log("loadCallback ", id);
  const session = fs.readFileSync("./session.json", "utf8");
  return JSON.parse(session);
}

The session look like this:

Session {
   id: '0984e7db-ad80-4276-966d-9db54fac11c2',
   shop: 'swiftnextstep.myshopify.com',
   state: '113617456401679',
   isOnline: true
 }

Once I save and read back, it looks like this

 {
   id: '0984e7db-ad80-4276-966d-9db54fac11c2',
   shop: 'swiftnextstep.myshopify.com',
   state: '113617456401679',
   isOnline: true
 }

And I get an error InternalServerError: Expected return to be instance of Session, but received instance of Object.

How can I cast/transform the object I'm parsing from storage into the expected Session type?

Solution

Session looks to be exported only from session.ts. The constructor is empty, so that's not something you have to worry about, it's just an object with properties. To turn a non-Session into a Session, you should be able to just assign to properties of a Session.

import { Session } from "@shopify/shopify-api/dist/auth/session/session";

function loadCallback(id) {
  console.log("loadCallback ", id);
  return Object.assign(
    new Session(),
    JSON.parse(fs.readFileSync("./session.json", "utf8")))
  );
}

Answered By - CertainPerformance

Answer Checked By - David Marino (PHPFixing Volunteer)