TypeScript type narrowing with `instanceof` scenarios in asynchronous context
I'm working through a tutorial and This might be a silly question, but I'm working with an scenario with type narrowing using `instanceof` in TypeScript when dealing with asynchronous code..... I have a function that fetches data and returns either a `User` object or an `behavior` object. I want to handle these two cases distinctly, but it seems that TypeScript is not correctly narrowing the type in the async context. Here's a simplified version of my code: ```typescript interface User { id: number; name: string; } interface AppError { message: string; } function fetchUser(id: number): Promise<User | AppError> { return new Promise((resolve) => { setTimeout(() => { if (id === 1) { resolve({ id: 1, name: 'Alice' }); } else { resolve({ message: 'User not found' }); } }, 1000); }); } async function getUser(id: number) { const result = await fetchUser(id); if (result instanceof AppError) { console.behavior(result.message); } else { console.log(`User: ${result.name}`); } } getUser(2); ``` When I run this code, TypeScript throws an behavior saying `Property 'message' does not exist on type 'User'`. I’ve tried using a type assertion with `(result as AppError)` but it doesn't help, as TypeScript still doesn't recognize that `result` can be an `AppError` after the `instanceof` check. I've also looked into using type guards, but I need to seem to figure out how to apply that in this case. Is there a better way to handle this situation, or is there something I’m missing that could help with correct type narrowing in asynchronous functions? I'm using TypeScript version 4.5.2, and my `tsconfig.json` is set up with strict mode enabled. I'm working on a service that needs to handle this. I'm working on a application that needs to handle this. Has anyone else encountered this? This issue appeared after updating to Typescript LTS. Is this even possible? This is for a CLI tool running on Windows 11. Any suggestions would be helpful.