Introduction: Why the ReactJS Constructor Still Matters

While React functional components with hooks dominate modern development, class components still play a crucial role—especially in large codebases or legacy projects.

At the core of every class component lies the constructor method. It’s the first function called when an instance of a class is created. In React, the constructor is where you:

• Initialize state

• Bind event handlers

• Prepare your component before it's rendered

Understanding the React constructor helps you write cleaner, more maintainable class-based components—and it also clarifies how components are initialized internally.

What Is the Constructor in React?

In JavaScript, a constructor is a special method used to create and initialize an object created with a class.

In React class components, the constructor is used to:

• Call the parent class’s constructor using super(props)

• Initialize component state

• Bind event handler methods to the component instance

Only use the constructor if you need to initialize state or bind methods. For most modern scenarios, using class properties is preferred.

Step-by-Step: Using a Constructor in React

✅ 1. Define a Class Component

import React, { Component } from 'react';

class MyComponent extends Component {
  constructor(props) {
    super(props); // Always call super(props) before accessing 'this'

    // Initialize state
    this.state = {
      count: 0,
    };

    // Bind event handlers
    this.handleClick = this.handleClick.bind(this);
  }

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

  render() {
    return (
      <div>
        <p>You've clicked {this.state.count} times.</p>
        <button onClick={this.handleClick}>Click Me</button>
      </div>
    );
  }
}

✅ 2. Key Concepts in the Constructor

super(props)

• Required to call the parent class constructor (Component) before using this.

• Passes props to the Component constructor to ensure this.props is defined in the component.

this.state

• Initialize component-level state.

• Can only be set directly in the constructor. Outside the constructor, use this.setState().

Method Binding

• JavaScript class methods don’t bind this automatically.

• Use bind() to ensure this refers to the correct instance in event handlers.

Constructor vs. Class Field Syntax

You can avoid the constructor entirely by using class field syntax (if supported in your setup):

class MyComponent extends React.Component {
  state = {
    count: 0,
  };

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

  render() {
    return (
      <button onClick={this.handleClick}>Click me: {this.state.count}</button>
    );
  }
}

✅ Complete Functional Code Example

import React, { Component } from 'react';

class Counter extends Component {
  constructor(props) {
    super(props);

    // Initial state
    this.state = {
      count: 0,
    };

    // Binding the handler
    this.increment = this.increment.bind(this);
  }

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

  render() {
    return (
      <div style={{ textAlign: 'center', padding: '20px' }}>
        <h2>React Class Component with Constructor</h2>
        <p>Count: {this.state.count}</p>
        <button onClick={this.increment}>Increment</button>
      </div>
    );
  }
}

export default Counter;

This example showcases:

• constructor() to initialize state

• Method binding

• this.setState() usage to update the UI

⚠️ Tips & Common Pitfalls

✅ Tips

• Always call super(props) before accessing this.

• Bind methods in the constructor to avoid undefined errors.

• Use class fields for cleaner, modern syntax if supported.

• Initialize all necessary state at once inside the constructor.

❌ Common Pitfalls

• Forgetting super(props) → causes a runtime error.

• Not binding methods → results in this being undefined.

• Modifying state directly (e.g., this.state.count++) → breaks React’s rendering logic.

• Using constructors when not needed—modern React favors simpler patterns.

Use Cases for Constructor in React

Conclusion: Mastering ReactJS Constructors

While not as commonly used today with the rise of hooks and functional components, understanding the ReactJS constructor is still valuable—especially when maintaining or refactoring legacy code.

Key Takeaways:

• Use constructor() to set initial state and bind methods.

• Always call super(props) before accessing this.

• Prefer class field syntax or hooks in new codebases.

• Keep constructors clean—initialize, bind, and get out!