SOLID principle is a set of five design principles in object-oriented programming that helps in developing more maintainable and scalable code. The SOLID principle stands for the single-responsibility principle, open-closed principle, Liskov substitution principle, interface segregation principle, and dependency inversion principle.

In this blog, we will discuss how the SOLID principle can be applied in React to build efficient and maintainable applications.

Single Responsibility Principle (SRP) This principle states that each module or class should have only one reason to change. In terms of React, each component should be responsible for only one thing. This makes it easier to test, maintain, and reuse components.

Take an example of a login form. Ideally, a login form component should only handle the user authentication process and not perform any other unrelated task, like fetching user information from a database. Otherwise, any change in the database structure will require changing the login form component also. To follow the SRP principle, we can create a separate service layer that handles the data and the component will only handle the user interface.

import { login } from "../services/authService";

function LoginForm() {
  const [username, setUsername] = useState("");
  const [password, setPassword] = useState("");

  const handleSubmit = async (event) => {
    event.preventDefault();

    try {
      await login(username, password);
      console.log("Successfully logged in");
    } catch (error) {
      console.log(`Error: ${error.message}`);
    }
  };

  return (
    <form onSubmit={handleSubmit}>
      <input
        type="text"
        placeholder="Username"
        value={username}
        onChange={(event) => setUsername(event.target.value)}
      />
      <input
        type="password"
        placeholder="Password"
        value={password}
        onChange={(event) => setPassword(event.target.value)}
      />
      <button type="submit">Submit</button>
    </form>
  );
}

Open-Closed Principle (OCP) This principle states that software entities (classes, modules, functions, etc.) should be open for extension but closed for modification. That means any new features should be added to the application without modifying the existing code.

In terms of React, this principle can be applied when creating reusable components. By creating a single reusable component that can be extended, we can avoid code duplication and make the application more scalable.

Consider a user avatar component. We can create a reusable component that accepts a prop for the image URL, size, and shape.

function UserAvatar(props) {
  const { image, size, shape } = props;

  return (
    <img src={image} style={{ width: size, height: size, borderRadius: shape }} />
  );
}

Now, if we want to extend this component to show the user's name and description, we don't need to modify the existing component but can create a new component that extends the UserAvatar component.

import UserAvatar from "./UserAvatar";

function UserCard(props) {
  const { user } = props;

  return (
    <div>
      <UserAvatar image={user.avatar} size="150px" shape="50%" />
      <h2>{user.name}</h2>
      <p>{user.description}</p>
    </div>
  );
}

Liskov Substitution Principle (LSP) This principle states that objects of a superclass should be able to be replaced with objects of its subclasses, without affecting the correctness of the program.

In React, this principle can be applied when creating components with similar behavior. For example, consider a Button component that displays a button element with some styles. We can create different types of buttons like PrimaryButton, SecondaryButton, etc., that inherit the base Button component's properties and behavior.

function Button(props) {
  const { children, ...rest } = props;

  return (
    <button {...rest}>
      {children}
    </button>
  );
}

function PrimaryButton(props) {
  return (
    <Button style={{ backgroundColor: "blue", color: "white" }} {...props} />
  );
}

function SecondaryButton(props) {
  return (
    <Button style={{ backgroundColor: "gray", color: "white" }} {...props} />
  );
}

Interface Segregation Principle (ISP) This principle states that a client should not be forced to implement an interface that it doesn't use. In React, this principle can be applied when creating reusable components that can be used in different parts of the application.

Consider a form component that has multiple input fields. We can create individual input field components that can be used in the form component. This way, if we want to use only one of the input fields in any other part of the application, we can do so without adding unnecessary code to the application.

function Input(props) {
  const { label, ...rest } = props;

  return (
    <div>
      <label>{label}</label>
      <input {...rest} />
    </div>
  );
}

function LoginForm() {
  const [username, setUsername] = useState("");
  const [password, setPassword] = useState("");

  const handleSubmit = async (event) => {
    event.preventDefault();

    try {
     // Login logic
    } catch (error) {
      console.log(`Error: ${error.message}`);
    }
  };

  return (
    <form onSubmit={handleSubmit}>
      <Input type="text" label="Username" value={username} onChange={(event) => setUsername(event.target.value)} />
      <Input type="password" label="Password" value={password} onChange={(event) => setPassword(event.target.value)} />
      <button type="submit">Submit</button>
    </form>
  );
}

Dependency Inversion Principle (DIP) This principle states that high-level modules should not depend on low-level modules. Instead, both should depend on abstractions.

In React, this principle can be applied when creating reusable components that depend on external libraries. We can create an abstraction layer between the component and the external library.

For example, consider a component that uses the moment.js library to format a date. Instead of directly depending on moment.js, we can create a utility function that formats the date and use it in the component.

import { formatDate } from "../helpers/dateUtils";

function BlogPost(props) {
  const { title, content, timestamp } = props;

  return (
    <div>
      <h2>{title}</h2>
      <p>{content}</p>
      <small>{formatDate(timestamp)}</small>
    </div>
  );
}

// dateUtils.js

import moment from "moment";

export function formatDate(timestamp) {
  return moment(timestamp).format("MMM DD, YYYY");
}

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By applying SOLID principles in React, we can create more scalable, maintainable, and testable applications. It encourages better code structure, separation of concerns, and more flexible application design.

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This article was written by Rix.