How do you design a Node.js application to be highly modular and extensible, allowing for easy integration of new features and components?

Designing a highly modular and extensible Node.js application involves the following best practices:

1. **Break the application into separate modules**: Divide your application’s code into several smaller, focused modules, each responsible for a specific functionality. Create separate directory structures for controllers, models, views, routes, and other utilities. Organize the code into files that export specific functionality.

For example, if you have a user-related module, create a user directory, and inside it, create separate files for the controller, model, and route.

- user
    - user.controller.js
    - user.model.js
    - user.route.js

2. **Use the module pattern**: The module pattern allows you to create self-contained and reusable functionality pieces. In Node.js, a module is a separate file, which encapsulates the related code and exposes only the desired functionality.

To create a module, use the ‘exports‘ or ‘module.exports‘ to expose the specific parts of the module:

// math.js
exports.add = (a, b) => a + b;
exports.subtract = (a, b) => a - b;

To use the module in another file, use the ‘require‘ function:

// app.js
const math = require('./math');
console.log(math.add(1, 2)); // 3

3. **Leverage npm for package management**: Node Package Manager (npm) can be a vital tool for managing your application’s dependencies, versioning, and installation. Create a ‘package.json‘ file which contains all the required metadata about your application and its dependencies:

{
  "name": "my-application",
  "version": "1.0.0",
  "dependencies": {
    "express": "^4.17.1",
    "mongoose": "^5.12.0"
  }
}

4. **Adhere to SOLID principles**: SOLID is a set of principles that help ensure a scalable and maintainable application:

a. **Single Responsibility Principle (SRP)** - Each module and function should have only one responsibility.

b. **Open-Closed Principle (OCP)** - Modules should be open for extension but closed for modification.

c. **Liskov Substitution Principle (LSP)** - Derived classes or modules should be substitutable for their base classes/modules.

d. **Interface Segregation Principle (ISP)** - Many client-specific interfaces are better than one general-purpose interface.

e. **Dependency Inversion Principle (DIP)** - High-level modules should not depend on low-level modules, but they should depend on abstractions.

5. **Use the Event-driven and Asynchronous nature of Node.js**: One of the core strengths of Node.js lies in its event-driven and asynchronous behavior. Consider using EventEmitter to establish communication between modules and perform actions asynchronously.

For example, create a module that emits events:

// emitter.js
const EventEmitter = require('events');

class MyEmitter extends EventEmitter {}

module.exports = new MyEmitter();

Now, listen for the event in other modules:

// listener.js
const myEmitter = require('./emitter');

myEmitter.on('myEvent', (data) => {
  console.log(`Received data: ${data}`);
});

Emit the event:

// app.js
const myEmitter = require('./emitter');

myEmitter.emit('myEvent', 'Hello, world!');

By following these practices, you’ll be able to design a highly modular and extensible Node.js application that is scalable and maintainable. This will ease integration of new features and components in future development.