Implementing Vue's defineAsyncComponent in TypeScript

Vue's defineAsyncComponent is a powerful tool for code splitting, allowing you to lazily load components only when they are needed. This can significantly improve initial load performance by reducing the size of the main JavaScript bundle. Your challenge is to implement a simplified, custom version of this functionality in TypeScript.

Problem Description

Your task is to create a function, let's call it customDefineAsyncComponent, that mimics the core behavior of Vue's defineAsyncComponent. This function should accept a loader function, which returns a Promise that resolves with the component definition. The customDefineAsyncComponent should return a Vue component that, when rendered, will execute the loader and display the resolved component.

Key Requirements:

• Lazy Loading: The component loader function should only be executed when the async component is actually rendered for the first time.
• Promise Resolution: The function must correctly handle the Promise returned by the loader.
• Error Handling: Implement basic error handling. If the Promise rejects, display a fallback component or a simple error message.
• Loading State: Provide a way to display a loading component while the async component is being fetched.
• TypeScript Support: The implementation should be written in TypeScript and properly typed.

Expected Behavior:

1. When customDefineAsyncComponent is called, it receives a loader function and optionally loading/error component configurations.
2. The returned component, when mounted, will display a "Loading..." state (or a specified loading component).
3. It will then execute the loader function.
4. If the loader Promise resolves successfully, the resolved component will be rendered.
5. If the loader Promise rejects, an error state will be displayed (or a specified error component).

Edge Cases:

• What if the loader function throws an error synchronously?
• What if the loader Promise resolves with undefined or null? (Treat as an error).
• What if the component is already loaded and requested again? (Should reuse the loaded component).

Examples

Let's assume we have some basic Vue components defined:

// Mock Component definitions
import { defineComponent } from 'vue';

const LoadingComponent = defineComponent({
  template: '<div>Loading...</div>'
});

const ErrorComponent = defineComponent({
  template: '<div>Error loading component!</div>'
});

const RealComponent = defineComponent({
  template: '<div>This is the real component!</div>'
});

// --- Example Usage ---

// Assume `customDefineAsyncComponent` is implemented as described.

// Example 1: Basic Async Component
const AsyncRealComponent = customDefineAsyncComponent({
  loader: () => new Promise(resolve => setTimeout(() => resolve(RealComponent), 1000)),
  loadingComponent: LoadingComponent,
  errorComponent: ErrorComponent
});

// In a Vue template:
// <AsyncRealComponent />
// Expected initial output: <div>Loading...</div>
// After 1 second: <div>This is the real component!</div>

Example 2: Async Component with immediate resolution

// Assume `customDefineAsyncComponent` is implemented as described.

const ImmediatelyAvailableComponent = customDefineAsyncComponent({
  loader: () => Promise.resolve(RealComponent),
  loadingComponent: LoadingComponent,
  errorComponent: ErrorComponent
});

// In a Vue template:
// <ImmediatelyAvailableComponent />
// Expected output: <div>This is the real component!</div>
// (Loading state will be very brief, potentially not even visible)

Example 3: Async Component that rejects

// Assume `customDefineAsyncComponent` is implemented as described.

const FailingAsyncComponent = customDefineAsyncComponent({
  loader: () => new Promise((_, reject) => setTimeout(() => reject(new Error('Failed to load')), 500)),
  loadingComponent: LoadingComponent,
  errorComponent: ErrorComponent
});

// In a Vue template:
// <FailingAsyncComponent />
// Expected initial output: <div>Loading...</div>
// After 0.5 seconds: <div>Error loading component!</div>

Constraints

• The loader function must return a Promise that resolves to a valid Vue component definition (or undefined/null, which should be treated as an error).
• The loadingComponent and errorComponent options are optional. If not provided, a default "Loading..." and "Error loading component!" message should be displayed.
• The solution should be implemented as a single function customDefineAsyncComponent.
• You will need to leverage Vue 3's Composition API (or Options API if you prefer, but Composition API is more idiomatic for this type of helper).
• No external libraries beyond Vue 3 itself are allowed.

Notes

• Consider how you will manage the state of the async component (loading, loaded, error).
• Think about how to memoize the loaded component to avoid refetching.
• The loader function might receive arguments. For simplicity in this challenge, assume it does not.
• You'll need to simulate Vue's rendering process conceptually when describing your logic. You don't need to build a full Vue renderer, but your implementation should be compatible with how a Vue component would behave.