`headers`

Each route can define its own HTTP headers. One of the common headers is the Cache-Control header that indicates to browser and CDN caches where and for how long a page is able to be cached.

import type { HeadersFunction } from "@remix-run/node"; // or cloudflare/deno

export const headers: HeadersFunction = ({
  actionHeaders,
  loaderHeaders,
  parentHeaders,
}) => ({
  "X-Stretchy-Pants": "its for fun",
  "Cache-Control": "max-age=300, s-maxage=3600",
});

Usually your data is a better indicator of your cache duration than your route module (data tends to be more dynamic than markup), so the action's & loader's headers are passed in to headers() too:

import type { HeadersFunction } from "@remix-run/node"; // or cloudflare/deno

export const headers: HeadersFunction = ({
  loaderHeaders,
}) => ({
  "Cache-Control": loaderHeaders.get("Cache-Control"),
});

Note: actionHeaders & loaderHeaders are an instance of the Web Fetch API Headers class.

Because Remix has nested routes, there's a battle of the headers to be won when nested routes match. In this case, the deepest route wins. Consider these files in the routes directory:

├── users.tsx
└── users
    ├── $userId.tsx
    └── $userId
        └── profile.tsx

If we are looking at /users/123/profile then three routes are rendering:

<Users>
  <UserId>
    <Profile />
  </UserId>
</Users>

If all three define headers, the deepest module wins, in this case profile.tsx.

We don't want surprise headers in your responses, so it's your job to merge them if you'd like. Remix passes in the parentHeaders to your headers function. So users.tsx headers get passed to $userId.tsx, and then $userId.tsx headers are passed to profile.tsx headers.

That is all to say that Remix has given you a very large gun with which to shoot your foot. You need to be careful not to send a Cache-Control from a child route module that is more aggressive than a parent route. Here's some code that picks the least aggressive caching in these cases:

import type { HeadersFunction } from "@remix-run/node"; // or cloudflare/deno
import parseCacheControl from "parse-cache-control";

export const headers: HeadersFunction = ({
  loaderHeaders,
  parentHeaders,
}) => {
  const loaderCache = parseCacheControl(
    loaderHeaders.get("Cache-Control")
  );
  const parentCache = parseCacheControl(
    parentHeaders.get("Cache-Control")
  );

  // take the most conservative between the parent and loader, otherwise
  // we'll be too aggressive for one of them.
  const maxAge = Math.min(
    loaderCache["max-age"],
    parentCache["max-age"]
  );

  return {
    "Cache-Control": `max-age=${maxAge}`,
  };
};

All that said, you can avoid this entire problem by not defining headers in parent routes and only in leaf routes. Every layout that can be visited directly will likely have an "index route". If you only define headers on your leaf routes, not your parent routes, you will never have to worry about merging headers.

Note that you can also add headers in your entry.server file for things that should be global, for example:

import type { EntryContext } from "@remix-run/node"; // or cloudflare/deno
import { RemixServer } from "@remix-run/react";
import { renderToString } from "react-dom/server";

export default function handleRequest(
  request: Request,
  responseStatusCode: number,
  responseHeaders: Headers,
  remixContext: EntryContext
) {
  const markup = renderToString(
    <RemixServer context={remixContext} url={request.url} />
  );

  responseHeaders.set("Content-Type", "text/html");
  responseHeaders.set("X-Powered-By", "Hugs");

  return new Response("<!DOCTYPE html>" + markup, {
    status: responseStatusCode,
    headers: responseHeaders,
  });
}

Just keep in mind that doing this will apply to all document requests, but does not apply to data requests (for client-side transitions for example). For those, use handleDataRequest.