Next.js 16: Solving Undefined Public Envs With CacheComponents

Hey there, tech enthusiasts and fellow developers! Welcome to an exciting deep dive into a peculiar challenge many of us might encounter while playing around with the latest and greatest features in Next.js 16. We're talking about the fantastic cacheComponents: true setting, a true game-changer designed to supercharge your app's performance by caching components, leading to incredibly snappy user experiences. It sounds awesome, right? Well, it absolutely is! However, like any powerful new feature, sometimes it can introduce unexpected behavior when combined with existing tools or patterns. Specifically, we're going to tackle a head-scratcher: the perplexing issue of undefined variables when cacheComponents is enabled, especially when you're relying on handy packages like next-public-env to manage your client-side environment variables. Imagine building out your app, meticulously setting up NEXT_PUBLIC_ variables for API keys, feature flags, or app names, only to find them mysteriously vanishing or appearing as empty objects when you expect crucial data. This can be super frustrating, leading to broken UIs, non-functional features, and a whole lot of head-scratching. Understanding why this happens and how to fix it is key to leveraging Next.js 16's full power without getting tripped up. So, buckle up, because we're about to demystify this problem and arm you with the knowledge to keep your applications running smoothly and your public environment variables always where they should be. We'll explore the core conflict, reproduce the issue, dive into the potential technical reasons, and most importantly, equip you with actionable solutions and workarounds. Let's get these undefined variables defined once and for all!

Understanding the Core Problem: Next.js 16, CacheComponents, and Environment Variables

Alright, guys, let's get down to brass tacks and really understand what's going on here with Next.js 16's cacheComponents: true and our missing NEXT_PUBLIC_ environment variables. At its heart, cacheComponents: true is an incredibly powerful optimization introduced in Next.js. What it essentially does is serialize your React components into a cache after their initial render. This means that when a user navigates away and then returns to a page, or if a component is revisited, Next.js can potentially rehydrate it much faster from this cached, pre-rendered state rather than re-rendering it from scratch. Think of it like having a super-efficient memory for your UI elements, drastically cutting down on render times and improving perceived performance. This is fantastic for static content or components whose props don't change frequently.

Now, let's talk about environment variables, specifically those prefixed with NEXT_PUBLIC_. These are variables that Next.js makes available both on the server (during build/SSR) and, crucially, bundles into the client-side JavaScript. This allows your client-side code (like React components) to access configuration values without exposing sensitive information (which should never be NEXT_PUBLIC_). Packages like next-public-env are designed to further streamline this process, often by providing a convenient API to access these variables consistently. The crux of our problem arises when these two powerful features — component caching and environment variable injection — collide. When components are serialized and cached, the mechanism for re-injecting or re-hydrating the dynamic environment variables might be bypassed, or the cached component might be revived without the up-to-date or complete environment context it expects. This leads to the frustrating scenario where your getPublicEnv() call, which usually provides a rich object of NEXT_PUBLIC_ variables, returns a dreaded Object {} instead. This isn't just an inconvenience; it can cause critical parts of your application to fail silently or crash spectacularly, as your components are expecting specific configuration values that simply aren't there. It's like your app forgot its own identity!

Digging a bit deeper, we need to consider the lifecycle of a Next.js application, especially how data flows from the server to the client. Normally, NEXT_PUBLIC_ variables are either inlined during the build process (for static optimization) or dynamically injected with the HTML response during Server-Side Rendering (SSR). When a component is cacheComponents: true, Next.js takes a snapshot of it. If this snapshot doesn't include the mechanism to re-evaluate or re-fetch the environment variables upon rehydration, then any subsequent access to those variables within the cached component will yield an empty or undefined result. The next-public-env package likely leverages process.env or a similar global mechanism to expose these variables. If cacheComponents operates at a layer that either bypasses the re-initialization of process.env for cached components, or if the way next-public-env accesses process.env is not compatible with the serialization/deserialization model of cacheComponents, then boom – our variables disappear. The failing tests clearly illustrate this: Object {} instead of the expected Object { "NEXT_PUBLIC_APP_NAME": "next-public-env", "NEXT_PUBLIC_APP_VERSION": "0.1.0", "NEXT_PUBLIC_HELLO": "world" }. This isn't a minor glitch; it's a fundamental breakdown in how the cached component perceives its runtime environment, affecting everything from error pages to loading states and regular home pages.

Reproducing the Issue: A Step-by-Step Guide

Alright, let's walk through how to see this beast in action and confirm the problem firsthand. If you're experiencing undefined variables with next-public-env and cacheComponents: true in your Next.js 16 app, you're not alone, and it's surprisingly easy to replicate. The first thing you'll need is a Next.js 16 project – either a fresh one or an existing application you're experimenting with. The key step here, the one that flips the switch on this issue, is enabling cacheComponents: true in your next.config.ts file. This is crucial for triggering the caching behavior that leads to our disappearing variables. Without it, next-public-env will likely work as expected, proving that the problem lies squarely at the intersection of these two features.

To make this concrete, navigate to your next.config.ts (or next.config.js if you're using JavaScript) file, typically located at the root of your Next.js project. Inside, you'll want to add or modify the cacheComponents flag within your configuration object. Specifically, the reported issue points to apps/next-16-standalone/next.config.ts in a monorepo setup, but the principle applies universally. Your next.config.ts should look something like this:

// next.config.ts or next.config.js

/** @type {import('next').NextConfig} */
const nextConfig = {
  reactStrictMode: true,
  // This is the line that causes our issue!
  cacheComponents: true,
  // ...other configurations
};

module.exports = nextConfig;

Once cacheComponents: true is set, the next step is to ensure you're using next-public-env to retrieve your public environment variables on the client-side. You'd typically import getPublicEnv from the package and call it within a React component. For example, in a pages/index.tsx or any client component:

// components/MyComponent.tsx

import { getPublicEnv } from 'next-public-env';
import { useEffect, useState } from 'react';

export default function MyComponent() {
  const [envData, setEnvData] = useState({});

  useEffect(() => {
    // This is where we expect our env variables!
    const publicEnv = getPublicEnv();
    console.log('Public Environment Variables:', publicEnv);
    setEnvData(publicEnv);
  }, []);

  return (
    <div>
      <h1>My App</h1>
      <p>App Name: {envData.NEXT_PUBLIC_APP_NAME || 'Undefined'}</p>
      <p>App Version: {envData.NEXT_PUBLIC_APP_VERSION || 'Undefined'}</p>
      <p>Hello: {envData.NEXT_PUBLIC_HELLO || 'Undefined'}</p>
      <pre>{JSON.stringify(envData, null, 2)}</pre>
    </div>
  );
);

And then, of course, you need to define some NEXT_PUBLIC_ variables in your .env.local file at the root of your project, like so:

env.local
NEXT_PUBLIC_APP_NAME=next-public-env
NEXT_PUBLIC_APP_VERSION=0.1.0
NEXT_PUBLIC_HELLO=world

After setting this up, run your Next.js application (npm run dev or yarn dev). When you navigate to the page containing MyComponent (or run your e2e tests as described in the original issue), you'll likely see something disheartening in your console or on your page where you expect your environment variables: Public Environment Variables: {}. Instead of the rich object populated with NEXT_PUBLIC_APP_NAME, NEXT_PUBLIC_APP_VERSION, and NEXT_PUBLIC_HELLO, you're faced with a frustratingly empty object. This is exactly what the original test failures demonstrated, impacting various pages like Error Page, Loading Page, Not Found Page, and Home Page. It's like a ghost in the machine, guys, making our public environment variables vanish! This confirms that cacheComponents: true is indeed interfering with next-public-env's ability to retrieve these crucial variables on the client-side.

Why is This Happening? The Technical Deep Dive (Speculation & Understanding)

So, what's the deal, right? Why are our precious environment variables disappearing when cacheComponents: true is enabled? This is where we put on our detective hats and dive into some informed speculation about the underlying technical mechanisms. While we don't have direct access to the exact implementation details of Next.js 16's cacheComponents or next-public-env, we can piece together a highly plausible scenario based on how React and Next.js typically handle component lifecycle, state, and environment variables. One strong possibility, and perhaps the most likely culprit, is that cacheComponents operates by serializing components without properly capturing or, more importantly, re-hydrating the dynamic runtime context, including environment variables that packages like next-public-env rely on. When a component is initially rendered, Next.js (and by extension, next-public-env) correctly populates process.env with NEXT_PUBLIC_ variables. However, when cacheComponents kicks in, it takes a snapshot – a serialized version – of that component. This snapshot might only include the component's static props and internal state, but it might not include a mechanism to re-evaluate or re-read global process.env values upon deserialization and re-use. If next-public-env is designed to read from process.env each time it's called, and the cached component is re-used without process.env being properly re-initialized or updated in its new context, then it's going to find an empty or stale object. This would lead to the Object {} we're seeing.

Another angle to consider is client-side hydration and the environment cacheComponents operates within. When a Next.js application first loads, the server-rendered HTML contains the initial state and often some embedded JavaScript with NEXT_PUBLIC_ variables. During hydration, React