How to extend Locals interface in SvelteKit

In SvelteKit, extending the Locals interface is primarily to enhance type support and ensure data type safety in middleware. When using TypeScript in SvelteKit, you can extend the Locals interface in the src/app.d.ts file, enabling safe usage of these extended types throughout the application.

The following steps and examples demonstrate how to extend the Locals interface in a SvelteKit project:

Step 1: Set up TypeScript

Ensure your SvelteKit project is configured with TypeScript. If not configured, initialize the TypeScript configuration using the following command:

bash
npx svelte-add@latest typescript

Step 2: Define the Extended Locals Interface

In the src/app.d.ts file, extend the Locals interface to include additional properties. For example, to add user authentication information in the application's middleware, extend Locals as follows:

typescript
// src/app.d.ts
/// <reference types="@sveltejs/kit" />

declare namespace App {
  interface Locals {
    userId?: number;
    isAuthenticated: boolean;
  }
}

In this code, we add two properties to the Locals interface: userId (optional) and isAuthenticated (required).

Step 3: Use the Extended Locals Interface

Once defined, safely use these properties in middleware or endpoint handling functions. For instance, create a middleware to verify user authentication:

typescript
// src/hooks.ts

import type { Handle } from '@sveltejs/kit';

export const handle: Handle = async ({ request, resolve }) => {
  // Simulate authentication logic (e.g., based on cookies or headers)
  const isAuthenticated = true; // Replace with actual logic

  if (isAuthenticated) {
    request.locals.isAuthenticated = true;
    request.locals.userId = 123; // Example user ID
  } else {
    request.locals.isAuthenticated = false;
  }

  const response = await resolve(request);
  return response;
};

Here, the middleware checks authentication status and sets the Locals properties accordingly, enabling subsequent logic to rely on these values.

Step 4: Use this information in the application

Reference these properties in any Svelte component or endpoint, as shown below:

svelte
<script lang="ts">
  export let locals: App.Locals;

  if (locals.isAuthenticated) {
    console.log(`User ID is ${locals.userId}`);
  }
</script>

In this component, we check authentication status and log the user ID if authenticated.

By following these steps, you can effectively extend and use the Locals interface safely in your SvelteKit project to enhance functionality and security.