Conditional rendering with Server Components

Tommy Lunde Barvåg

@tommybarvaag

Published on December 1, 20235 months ago

It's not often that I find myself wanting to render different content based on the platform that the user is on. But when I have done so in the past, I've found it to be a bit of a pain. I've written several different approaches to this problem, and none of them have been particularly satisfying always leaving me with some headaches like

How do I make sure that the client and server render the same content?
How do I only serve the client the content that it needs?
How do I make sure that the client doesn't re-render the content that the server already rendered?

Enter React Server Components. RSC are a new way of building React applications that allows you to render components on the server. RSC never re-render. They run once on the server to generate the UI. The rendered value is sent to the client and locked in place. As far as React is concerned, this output is immutable, and will never change.

If you're interested in learning more about RSC, I recommend you check out this great article by Josh Comeau.

Hi there! You're seeing this because you're on desktop right now. If you load this page on mobile, you'll see a different callout.

The problem

Let's say that we have a component that we want to render differently based on the platform that the user is on. For example, we want to render a traditional dialog for desktop users and a iOS drawer style dialog for mobile users.

Fortunately for us, I allready have this setup on this site. If you're on desktop and click a link to connect you'll see a traditional dialog. If you're on mobile and click the same link, you'll see a iOS drawer style dialog.

A previous solution

In the past, I've solved this problem by using a custom useWindowSize hook that returns the window size and a boolean indicating whether the user is on mobile or not.

import { useEffect, useMemo, useState } from "react";

function useWindowSize(): {
  width: number;
  height: number;
  isMobile: boolean;
  isDesktop: boolean;
} {
  const [windowSize, setWindowSize] = useState<{
    width: number;
    height: number;
  }>({
    width: 0,
    height: 0
  });
  useEffect(() => {
    // Handler to call on window resize
    function handleResize() {
      // Set window width/height to state
      setWindowSize({
        width: window.innerWidth,
        height: window.innerHeight
      });
    }
    // Add event listener
    window.addEventListener("resize", handleResize);
    // Call handler right away so state gets updated with initial window size
    handleResize();
    // Remove event listener on cleanup
    return () => window.removeEventListener("resize", handleResize);
  }, []); // Empty array ensures that effect is only run on mount

  // Memoize the result of the calculations using useMemo
  const memoizedWindowSize = useMemo(() => {
    return {
      ...windowSize,
      isMobile: typeof windowSize?.width === "number" && windowSize.width < 768,
      isDesktop: typeof windowSize?.width === "number" && windowSize.width >= 768
    };
  }, [windowSize]);

  return memoizedWindowSize;
}

export { useWindowSize };

And then I would use the useWindowSize hook to render the content conditionally.

const { isMobile } = useWindowSize();
const Component = isMobile ? MobileDialog : DesktopDialog;

This approach works, but it has some problems

The client bundle contains both the mobile and desktop components. This means that the client has to download more code than it needs.
In a scenario where we have a server rendered page, the client will re-render the content that the server already rendered, on other words hydration.
The client has to determine the device type by screen width. This is not always reliable as the user might resize the browser window.

A better solution

With React Server Components, we can render different content based on the platform that the user is on. And because the server renders the content, we don't have to worry about extra code being downloaded or the client re-rendering the content that the server already rendered.

With Next.js headers we can easily access the user-agent header and use it to determine the platform that the user is on. The user-agent header contains information about the user's browser, device, and operating system.

import { headers as getHeaders } from "next/headers";

function getUserAgent() {
  const headers = getHeaders();
  return headers.get("user-agent");
}

We can then use the user-agent header to determine the platform that the user is on.

Composing our solution

We want to compose our solution by utilizing the user-agent header to determine the platform that the user is on and then serve the user with the appropriate content.

Starting with the complete solution, here's what the code looks like.

import { PlatformReturnType } from "@/lib/actions/bowser-actions";
import { ShowPlatformProps } from "@/types";

function ShowPlatformContent({
  platform,
  platforms: { mobile, tablet, desktop, touch, bot, fallback }
}: ShowPlatformProps & {
  platform: PlatformReturnType;
}) {
  if (platform.isError && fallback) {
    return <>{fallback}</>;
  }

  if (platform.isBot) {
    // find any available react node and prioritize the following order:
    // 1. bot
    // 2. fallback
    // 3. desktop
    // 4. touch
    // 5. mobile
    // 6. tablet
    const node= bot || fallback || desktop || touch || mobile || tablet;
    return node ? <>{node}</> : null;
  }

  if (platform.isMobile && mobile) {
    return <>{mobile}</>;
  }

  if (platform.isTablet && tablet) {
    return <>{tablet}</>;
  }

  if (platform.isDesktop && desktop) {
    return <>{desktop}</>;
  }

  if (platform.isTouch && touch) {
    return <>{touch}</>;
  }

  return null;
}

export { ShowPlatformContent };

Before I go into explaining how this works, let's take a look at how I use this component in my connect page.

export default function ConnectDialogPage() {
  return (
    <ShowPlatform
      platforms={{
        desktop: <ConnectDesktop />,
        touch: <ConnectDialogTouch />,
        fallback: <ConnectDialog />
      }}
    />
  );
}

Neat right?

You could also use the server action directly in your page component, but I prefer to use a component for this as it makes it easier to reuse the logic.

import { getPlatform } from "@/lib/actions/bowser-actions";

export default function ConnectDialogPage() {
  const platform = getPlatform();

  if (platform.isError) {
    return <ConnectDialog />;
  }

  if (platform.isDesktop) {
    return <ConnectDesktop />;
  }

  if (platform.isTouch) {
    return <ConnectDialogTouch />;
  }

  return null;
}

How it works

The <ShowPlatform /> component takes a platforms prop that is an object with the following shape

{
  mobile?: ReactNode;
  tablet?: ReactNode;
  desktop?: ReactNode;
  touch?: ReactNode;
  bot?: ReactNode;
  fallback?: ReactNode;
}

The <ShowPlatform /> component then uses the getPlatform() action to determine the current platform. The getPlatform() action feeds the user-agent to the Bowser library and returns an object that can be used to identify the current platform.

The Bowser library is a browser detector library that parses the user-agent header and returns an object with information about the current platform.

"use server";

import Bowser from "bowser";
import { headers as getHeaders } from "next/headers";

function getPlatform() {
  const headers = getHeaders();

  try {
    const browser = Bowser.parse(headers.get("user-agent"));

    const { isMobile, isTablet, isDesktop, isTouch, isBot, isError } = {
      isMobile: browser.platform.type === "mobile",
      isTablet: browser.platform.type === "tablet",
      isDesktop: browser.platform.type === "desktop",
      isTouch: browser.platform.type === "mobile" || browser.platform.type === "tablet",
      isBot: browser.platform.type === "bot",
      isError: !browser.platform.type
    };

    return { isMobile, isTablet, isDesktop, isTouch, isBot, isError };
  } catch (error) {
    return {
      isMobile: false,
      isTablet: false,
      isDesktop: false,
      isTouch: false,
      isBot: false,
      isError: true
    };
  }
}

As the <ShowPlatform /> component is rendered on the server, and the action is declared with the "use server" directive, the client bundle never needs to handle anything related to the Bowser library or the user-agent header.

If the component determines that the current platform is mobile, the client bundle will only contain the mobile component. And likewise, if the component determines that the current platform is desktop, the client bundle will only contain the desktop component.

Graceful Degradation

In scenarios where platform detection fails, a fallback component ensures the user still receives content, demonstrating a graceful degradation strategy. This is particularly crucial for unrecognized browsers.

SEO considerations

For bots, we have made available a bot property that can be used to render content specifically for bots. This is useful for SEO purposes. For example, you might want to ensure all bots see the same content as desktop users. If you don't provide a bot property, the component will render the different content based on a prioritized order.

Summary

Lately, I've been giving a lot of thought to the various ways I can use React Server Components. These components seem like a great tool, especially for solving some of the challenges I've encountered in the past.

It's pretty exciting to think about the different problems we can tackle with them. I'm keen to explore more about their capabilities and see how they can make our projects better and more efficient.

Resources

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