React Props vs. State: The Key Differences You Must Know

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When developing React applications, two fundamental concepts play a crucial role in managing data: Props and State. These concepts determine how data flows within components, impacting component reusability, performance, and maintainability.

Understanding the key differences between React props and state is essential for writing efficient and scalable React applications. In this article, we’ll explore:

• What props and state are in React.
• Their core differences and use cases.
• Best practices for using them effectively.

By the end, you’ll have a clear understanding of when to use props and when to use state in your React components.

1. What Are Props in React?

1.1 Definition of Props

In React, props (short for properties) are read-only inputs that a parent component passes to a child component. Props allow data and event handlers to be passed down the component tree, enabling reusable and modular components.

1.2 Characteristics of Props

• Immutable: Once a prop is passed to a child component, it cannot be modified by that component.
• Passed from Parent to Child: Props are always received from a parent component and cannot be changed inside the child component.
• Promote Reusability: Components can be reused with different props, making them more flexible.

1.3 Example of Props in React

function Greeting(props) {
  return <h1>Hello, {props.name}!</h1>;
}

function App() {
  return <Greeting name="John" />;
}

Here, the Greeting component receives a name prop from the App component and displays it.

2. What Is State in React?

2.1 Definition of State

State is a built-in object that allows a component to maintain and manage dynamic data. Unlike props, which are passed from parent to child, state is managed within a component and can be updated over time.

2.2 Characteristics of State

• Mutable: State can be modified within the component using the setState function (in class components) or useState hook (in functional components).
• Local to the Component: State is private to the component and not directly accessible by other components.
• Triggers Re-renders: When state changes, React re-renders the component to reflect the new state.

2.3 Example of State in React

import { useState } from "react";

function Counter() {
  const [count, setCount] = useState(0);

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

Here, the Counter component maintains a count state, which updates when the button is clicked.

3. Key Differences Between Props and State

3.1 Comparison Table

3.2 When to Use Props vs. State

3.2.1 When to Use Props

• When passing static or dynamic data from a parent to a child component.
• When handling event callbacks (e.g., passing a function from a parent to a child).
• When building reusable components that can work with different data.

3.2.2 When to Use State

• When managing user interactions (e.g., button clicks, form inputs).
• When tracking real-time updates (e.g., timers, counters, API responses).
• When handling component-specific behavior that doesn’t need to be passed down.

4. Best Practices for Using Props and State

4.1 Best Practices for Props

• Keep props pure—do not modify props inside the child component.

• Use destructuring to make props more readable:

  function Greeting({ name }) {
    return <h1>Hello, {name}!</h1>;
  }

• Use defaultProps to provide default values:

  Greeting.defaultProps = {
    name: "Guest",
  };

4.2 Best Practices for State

• Keep state local and avoid lifting it unless necessary.

• Use functional updates when updating state based on the previous value:

  setCount((prevCount) => prevCount + 1);

• Avoid unnecessary state—if a value can be derived from props, don’t store it in state.

5. Common Mistakes and How to Avoid Them

5.1 Modifying Props Inside a Component

Wrong approach:

function Greeting(props) {
  props.name = "Alice"; // ❌ This is not allowed
  return <h1>Hello, {props.name}!</h1>;
}

Solution: Use state if modification is needed within the component.

5.2 Overusing State When Props Are Sufficient

Wrong approach:

function Message({ text }) {
  const [message, setMessage] = useState(text); // ❌ Unnecessary state
  return <p>{message}</p>;
}

Solution: Directly use props unless local modification is needed.

5.3 Not Keeping State Immutable

Wrong approach:

function App() {
  const [user, setUser] = useState({ name: "John", age: 25 });

  function updateAge() {
    user.age = 26; // ❌ Direct mutation
    setUser(user);
  }

  return <button onClick={updateAge}>Update Age</button>;
}

Solution: Always create a new object when updating state:

setUser((prevUser) => ({ ...prevUser, age: 26 }));

6. Conclusion

Both props and state are essential for managing data in React applications. Understanding their differences and best use cases will help you build more efficient and scalable applications.

• Use props when passing data from parent to child components.
• Use state when managing component-specific data that changes over time.