Why You Should Always Explicitly Mark Client Components in Next.js

In Next.js, a common mistake developers make is assuming that a component without the use client directive is a server component. This misunderstanding often arises when developers import components that use client-side functionality (like state or event handlers) into other client components. While this may work in the short term, it can lead to confusion and issues as the codebase grows.

In this article, we’ll dive into why it’s essential to explicitly mark components that rely on client-side features with the use client directive — rather than relying on imports — and how failing to do so can create maintainability issues.

Understanding the use client Directive

The use client directive tells Next.js that a component should be treated as a client component, meaning it will be rendered on the client-side. Client components can use features like useState, useEffect, and event handlers that rely on the browser environment.

By default, components in Next.js are treated as server components, which are rendered on the server. These components do not have access to client-side features like state or DOM manipulation.

The Problem: Assuming a Component Is a Server Component Because It Lacks use client

It’s easy to assume that if a component doesn’t explicitly have the use client directive at the top, it must be a server component. But this isn’t always true. Let’s break down a common scenario:

Example Scenario: Importing Client Functionality Without use client

Consider you have two components: a Sidebar and a FavoriteButton. The FavoriteButton component is interactive — it manages state with useState and listens to events like onClick — meaning it needs to be a client component.

However, if FavoriteButton is imported into Sidebar, and Sidebar is marked with the use client directive, FavoriteButton will automatically become a client component, even if it doesn’t have the use client directive at the top.

Here’s an example:

Sidebar.js (Client Component)

// components/Sidebar.js
'use client'; // Sidebar is explicitly a client component

import FavoriteButton from './FavoriteButton';

export default function Sidebar() {
  return (
    <div>
      <h2>Sidebar</h2>
      <FavoriteButton />
    </div>
  );
}

FavoriteButton.js (Missing use client directive but Uses Client-Side Features)

// components/FavoriteButton.js

import { useState } from 'react';

export default function FavoriteButton() {
  const [liked, setLiked] = useState(false);

  const toggleLike = () => {
    setLiked((prevLiked) => !prevLiked);
  };

  return (
    <button onClick={toggleLike}>
      {liked ? 'Unlike' : 'Like'}
    </button>
  );
}

In this case, even though FavoriteButton.js doesn’t have the use client directive, it will function as a client component because it’s being used inside the Sidebar component, which is explicitly marked as a client component.

Why This Is a Problem

At first glance, this may not seem like an issue, especially if the code works. However, as your application grows, this reliance on implicit behavior can lead to several problems:

1. Lack of Clarity: Future developers (or even you after some time) might not immediately understand why FavoriteButton is functioning as a client component. Without the use client directive, it can be unclear that this component relies on client-side functionality.

2. Maintainability: As your codebase grows, it becomes harder to track which components are client components and which are server components. This can lead to confusion, especially if you’re working in a large team or revisiting the code after a long time.

3. Potential Bugs: If you move FavoriteButton to another part of your app that isn’t a client component, you could introduce bugs or break functionality because it’s no longer running on the client as expected.

Best Practice: Always Mark Client Components Explicitly

Even though FavoriteButton works as a client component in the example above, the better practice is to explicitly mark it as a client component because it uses client-side features like useState and event handlers.

Good Example: Explicitly Marking Client Components

FavoriteButton.js (Explicitly Marked as a Client Component)

// components/FavoriteButton.js
'use client'; // Explicitly mark as a client component

import { useState } from 'react';

export default function FavoriteButton() {
  const [liked, setLiked] = useState(false);

  const toggleLike = () => {
    setLiked((prevLiked) => !prevLiked);
  };

  return (
    <button onClick={toggleLike}>
      {liked ? 'Unlike' : 'Like'}
    </button>
  );
}

By marking FavoriteButton as a client component explicitly, we make the code clearer and more maintainable. Anyone reading the code can immediately understand that this component relies on client-side functionality, without needing to trace how it’s being used elsewhere in the app.

Benefits of Explicitly Marking Client Components

1. Clarity: There’s no ambiguity about whether a component is a client or server component. It’s immediately clear from the presence of the use client directive.

2. Consistency: By following this approach consistently throughout your codebase, you make the intent of your components clear and easy to understand for future developers (or your future self).

3. Avoiding Implicit Behavior: Relying on implicit behavior (like importing a component into a client component) can lead to subtle bugs or confusing code. Being explicit removes these potential issues.

4. Improved Codebase Hygiene: Clean, clear, and maintainable code is the hallmark of a well-structured application, and marking client components explicitly contributes to this.

Conclusion

Relying on imports to dictate whether a component functions as a client component is a common mistake in Next.js development. Even if a component works fine when imported into a client component, it’s a good practice to explicitly mark any component that uses client-side functionality (like state or event handlers) with the use client directive. This approach ensures that your code is clear, maintainable, and free from potential confusion as your application grows.

By explicitly declaring client components, you reduce the chance of bugs, make your codebase more maintainable, and improve overall code clarity for yourself and future collaborators. So, the next time you write a component that relies on client-side features, remember to mark it as a client component — don't rely on implicit behavior!