In Next.js 13, the introduction of the new app directory has brought about a significant shift in how components are rendered. By default, components in the app directory are treated as server components, which are rendered on the server. This default behavior is optimized for performance and data fetching but comes with limitations when it comes to client-side interactivity. To address this, Next.js 13 introduced the "use client" directive, which explicitly designates components or files as client-side JavaScript.

Why We Use "use client"

Client-Side Interactivity

If a component needs to interact with the browser (e.g., handling user events like clicks, or accessing local storage), it must be marked with "use client". This is because server components cannot access browser APIs, event listeners, or other client-side features.

Hooks and State Management

Hooks like useState, useEffect, useRef, etc., can only be used in client components. Therefore, marking a file or component with "use client" is necessary when using these hooks.

Event Handling

React event handlers (like onClick, onChange) require the component to run in the browser. Consequently, the component should be marked as a client component.

Where It Is Used

You place "use client" at the top of the file for any component that needs to be rendered on the client side:

"use client";

import { useState } from "react";

export default function MyComponent() {
  const [count, setCount] = useState(0);

  return (
    <div>
      <p>{count}</p>
      <button onClick={() => setCount(count + 1)}>Increment</button>
    </div>
  );
}

Key Points

Top-Level Directive

It must be placed at the top of the file, before any imports or other code.

Scope

When you add "use client" to a file, all components inside that file become client components, meaning they are bundled and run on the browser.

Selective Use

It’s recommended to use it only where necessary because server components are more efficient in terms of performance and data fetching.

Summary

In summary, the "use client" directive is necessary when you want to build client-side interactive components in a framework that, by default, favors server-side rendering and optimizations. By explicitly marking components as client-side, you can leverage the full power of React's client-side features, such as hooks and event handling, while still benefiting from the performance advantages of server-side rendering where it makes sense.

This balance allows developers to create highly interactive and performant web applications, taking advantage of both server-side and client-side rendering as needed.