Understanding State in React

Introduction

State is one of the most important concepts in React. It lies at the heart of dynamic and interactive user interfaces. While props allow data to be passed into components, state is what enables components to manage and respond to changes over time. Without state, applications would remain static and unable to react to user interactions, API responses, or internal events.

In this post, we will take a deep dive into understanding state in React. We will cover what state is, why it is needed, how to define and update it, differences between state and props, working with state in class components versus functional components, handling complex state, best practices, performance considerations, and real-world examples.

By the end of this guide, you will have a clear understanding of how to manage state effectively in React applications.

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What is State?

State in React refers to a built-in object that represents the current values or data inside a component. Unlike props, which are external and passed down from parent components, state is internal and owned by the component itself.

Whenever state changes, React re-renders the component, ensuring that the user interface always reflects the most up-to-date data.

Key Characteristics of State

1. Mutable – State can change over time, unlike props.
2. Component-local – Each component manages its own state.
3. Triggers re-render – Updating state forces React to re-render the UI.
4. Asynchronous updates – State updates may not happen immediately.
5. Controlled by React – Should always be updated using React’s methods (setState or useState).

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Why is State Important?

State enables interactivity. Without state, React apps would be static pages. Some examples of where state is necessary:

• A counter that increments when a button is clicked.
• A form where input fields update dynamically.
• A dropdown menu that expands and collapses.
• A shopping cart that stores selected products.
• A chat application that shows live messages.

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State vs Props

It’s important to distinguish between state and props because both affect a component’s behavior.

Feature	Props	State
Mutability	Immutable (cannot be changed)	Mutable (can change over time)
Source	Passed from parent component	Managed within the component
Ownership	Controlled externally	Controlled internally
Purpose	Configure components	Store dynamic data
Updates Cause	Re-render	Re-render

Example of Props vs State

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

// Using state
import React, { useState } from 'react';

function Counter() {
  const [count, setCount] = useState(0);
  return <h1>Count: {count}</h1>;
}

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State in Class Components

Before React introduced Hooks in version 16.8, state could only be used inside class components.

Declaring State

State is initialized inside the constructor.

class Counter extends React.Component {
  constructor(props) {
super(props);
this.state = { count: 0 };
  }

  render() {
return <h1>Count: {this.state.count}</h1>;
  }
}

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Updating State in Class Components

State should never be updated directly. Instead, this.setState() is used.

class Counter extends React.Component {
  constructor(props) {
super(props);
this.state = { count: 0 };
  }

  increment = () => {
this.setState({ count: this.state.count + 1 });
  };

  render() {
return (
  <div>
    <h1>Count: {this.state.count}</h1>
    <button onClick={this.increment}>Increment</button>
  </div>
);
  }
}

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setState is Asynchronous

React batches state updates for performance reasons. Therefore, relying directly on this.state when updating can cause issues.

Example of Incorrect Usage

this.setState({ count: this.state.count + 1 });
this.setState({ count: this.state.count + 1 });

You might expect count to increase by 2, but it only increases by 1.

Correct Usage with Function

this.setState((prevState) => ({
  count: prevState.count + 1,
}));
this.setState((prevState) => ({
  count: prevState.count + 1,
}));

This will correctly increment by 2 because it uses the previous state.

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State in Functional Components

With React Hooks, state can now be used inside functional components using the useState hook.

Declaring State

import React, { useState } from 'react';

function Counter() {
  const [count, setCount] = useState(0);
  return <h1>Count: {count}</h1>;
}

Here:

• count is the state variable.
• setCount is the function used to update it.
• useState(0) initializes the state with 0.

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Updating State

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

  const increment = () => setCount(count + 1);

  return (
<div>
  <h1>Count: {count}</h1>
  <button onClick={increment}>Increment</button>
</div>
  );
}

Each time the button is clicked, React re-renders the component with the updated state.

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Using Multiple State Variables

useState can be used multiple times in a single component.

function UserProfile() {
  const [name, setName] = useState("Alice");
  const [age, setAge] = useState(25);

  return (
<div>
  <h1>{name}</h1>
  <p>Age: {age}</p>
  <button onClick={() => setAge(age + 1)}>Increase Age</button>
</div>
  );
}

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Complex State Management

Sometimes state is more complex than a simple number or string. It may involve arrays or objects.

State as an Object

function Form() {
  const [form, setForm] = useState({ name: "", email: "" });

  const handleChange = (e) => {
setForm({ ...form, [e.target.name]: e.target.value });
  };

  return (
<div>
  <input
    name="name"
    value={form.name}
    onChange={handleChange}
    placeholder="Name"
  />
  <input
    name="email"
    value={form.email}
    onChange={handleChange}
    placeholder="Email"
  />
  <h2>{form.name} - {form.email}</h2>
</div>
  );
}

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State as an Array

function TodoList() {
  const [todos, setTodos] = useState([]);

  const addTodo = (todo) => {
setTodos([...todos, todo]);
  };

  return (
<div>
  <button onClick={() => addTodo("New Task")}>Add Todo</button>
  <ul>
    {todos.map((t, index) => (
      <li key={index}>{t}</li>
    ))}
  </ul>
</div>
  );
}

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Derived State

Sometimes, state depends on props or another piece of state. This is called derived state.

Example

class PriceCalculator extends React.Component {
  constructor(props) {
super(props);
this.state = { quantity: 1 };
  }

  render() {
const total = this.state.quantity * this.props.price;
return (
  <div>
    <p>Quantity: {this.state.quantity}</p>
    <p>Total: {total}</p>
    <button onClick={() => this.setState({ quantity: this.state.quantity + 1 })}>
      Increase
    </button>
  </div>
);
  }
}

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Best Practices for Using State

1. Keep state minimal – Avoid unnecessary state variables.
2. Derive values when possible – Compute values from existing state instead of duplicating.
3. Never mutate state directly – Always use setState or setX from useState.
4. Split complex state – Use multiple state variables instead of one huge object.
5. Use functional updates when the next state depends on the previous one.

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Common Mistakes with State

1. Directly modifying state

this.state.count = this.state.count + 1; // Wrong

2. Not providing unique keys for lists

todos.map(todo => <li>{todo}</li>) // Wrong

3. Using props as initial state incorrectly

this.state = { name: this.props.name }; // Can cause issues

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Real-World Example – Counter with History

Let’s create a more advanced example using state.

import React, { useState } from 'react';

function CounterWithHistory() {
  const [count, setCount] = useState(0);
  const [history, setHistory] = useState([]);

  const increment = () => {
setCount(count + 1);
setHistory([...history, count + 1]);
  };

  return (
<div>
  <h1>Count: {count}</h1>
  <button onClick={increment}>Increment</button>
  <h2>History</h2>
  <ul>
    {history.map((h, index) => (
      <li key={index}>{h}</li>
    ))}
  </ul>
</div>
  );
}

This demonstrates managing multiple pieces of state together.

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State and Side Effects

State is often used alongside side effects like API calls. In functional components, this is handled using the useEffect hook.

import React, { useState, useEffect } from 'react';

function Users() {
  const [users, setUsers] = useState([]);

  useEffect(() => {
fetch("https://jsonplaceholder.typicode.com/users")
  .then((res) => res.json())
  .then((data) => setUsers(data));
  }, []);

  return (
<ul>
  {users.map((u) => (
    <li key={u.id}>{u.name}</li>
  ))}
</ul>
  );
}

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State Performance Considerations

1. Batching Updates – React batches multiple setState calls for performance.
2. Large State Objects – Break large state into smaller parts.
3. Expensive Computations – Use useMemo or derived state to avoid recalculations.
4. Frequent Updates – Minimize unnecessary re-renders with memoization (React.memo).

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Advanced State Management

As applications grow, managing state with useState or class components may not be enough. In such cases, React provides additional solutions:

• Context API – For sharing state across multiple components.
• Reducers (useReducer) – For more structured state management.
• Third-party libraries – Redux, MobX, or Zustand for global state.