useLoaderData undefined error with Sanity query in Remix loader

This is a classic Remix loader issue where useLoaderData() returns undefined. Since you mentioned the same code works on another page, here are the most likely causes:

1. Loader Not Being Called (Most Common)

The most common reason is that your route file isn't properly exporting the loader or the file naming doesn't match Remix's routing conventions. Check:

• Is your file in the app/routes/ directory?
• Is the loader export actually being recognized? Try adding a console.log at the very top of your loader to verify it's running:

export async function loader() {
  console.log("LOADER EXECUTING"); // Check server logs
  const posts = await getClient().fetch(/* ... */);
  console.log("Posts count:", posts?.length);
  return { posts };
}

If you don't see "LOADER EXECUTING" in your server console when you visit the page, the loader isn't running at all.

2. Client-Side Navigation Issue

If you're navigating to this page from another page, Remix might be trying to call the loader but getting an error that's swallowed. Add error handling:

export async function loader() {
  try {
    const posts = await getClient().fetch(
      `*[_type == "post"]{ 
        _id, 
        title, 
        slug, 
        author,
        "authorName": author->name,
        "authorImage": author->image,
        mainImage, 
        publishedAt, 
        categories,
        excerpt, 
        myTags, 
        featuredPost,
        "categoryName": *[_type == 'category' && _id in ^.categories[]._ref]{title, slug},
      } | order(publishedAt desc)`
    );
    
    return { posts: posts || [] };
  } catch (error) {
    console.error("Sanity fetch error:", error);
    throw new Response("Failed to load posts", { status: 500 });
  }
}

3. Sanity Client Configuration

Since it works elsewhere, this is less likely, but verify your getClient() function returns a properly configured client:

import { createClient } from "@sanity/client";

export function getClient() {
  return createClient({
    projectId: process.env.SANITY_PROJECT_ID, // or your hardcoded ID
    dataset: process.env.SANITY_DATASET || "production",
    apiVersion: "2024-01-01",
    useCdn: false, // Important for loaders
  });
}

4. Defensive Component Code

As a quick fix while debugging, make your component more defensive:

export default function LatestBlogPosts() {
  const data = useLoaderData();
  const posts = data?.posts || [];

  if (!posts || posts.length === 0) {
    return <div>No posts found</div>;
  }

  return (
    <section className="">
      {/* your existing JSX */}
    </section>
  );
}

5. Check for TypeScript Errors

If you're using TypeScript, make sure there are no type errors preventing the loader from compiling. Run npm run typecheck to verify.

Debugging Steps

1. Check your server console logs (not browser console) when you load the page
2. Verify the route file is in the correct location and named correctly
3. Try hard-refreshing the page (Ctrl+Shift+R) instead of navigating to it
4. Check if you have an ErrorBoundary export in the same file that might be catching errors silently

The fact that identical code works on another page strongly suggests either a route configuration issue or the loader isn't being executed at all on this specific route. The console.log approach will quickly tell you which it is.