React.js Constructor in 2025

Simply called a constructor is a method that helps in the creation of objects. The constructor is no different in React. This can connect event handlers to the component and initialize the component’s local state. Before the component is mounted, the constructor() function is shot, and, like most things in React, it has a few rules that you can follow when using them. The concept of a constructor is the same in React.

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The constructor in a React component is called before the component is mounted. When you implement the constructor for a React component, you need to call the super(props) method before any other statement. Suppose you do not call the super(props) method, this.

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Props will be undefined in the constructor and can lead to bugs. If there is a constructor() function in your component, this function will be called when the component gets initiated. The constructor function is where you initiate the component’s properties.in react, component properties should be kept in an object called state.

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What is a Constructor?

Constructor is the method that is called automatically when we create the object from that class. In the context of React, it's essential for setting up the initial component state and for the binding method.

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• Super props called parent class constructor to ensure initialization

• Initialization of state with this.state = { count: 0};

• Bind methods this.handleClick = this.handleClick.bind(this);

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import React, { Component } from 'react';

class MyComponent extends Component {
  constructor(props) {
    super(props); // Call the parent class's constructor

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

    // Bind methods if needed
    this.handleClick = this.handleClick.bind(this);
  }

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

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

export default MyComponent;

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Call super(props) Before Using this.props

Before using this. props in a constructor is important to call its important to call super(props).If you want to implement the constructor for a React component, call the super(props) method before any other statement. Otherwise, this. props will be undefined in the constructor and create bugs. console.log(this.props) will safely access this.props.

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import React, { Component } from 'react';

class MyComponent extends Component {
  constructor(props) {
    super(props);  // You must call super(props) first!
    
    console.log(this.props);  // Now you can safely access this.props

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

  render() {
    return (
      <div>
        <p>{this.props.message}</p>  {/* Accessing props here is fine */}
        <p>Count: {this.state.count}</p>
      </div>
    );
  }
}

export default MyComponent;

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Never Call setState() Inside constructor()

Don't call setState after the component is created. It's not meant for updates. You should never call setState() inside the constructor() in React.

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import React, { Component } from 'react';

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

    // Don't call setState here! It's not meant for updates.
    this.setState({ count: 0 });  // This will lead to an error or unexpected behavior
  }

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

export default MyComponent;

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Bind Events All in One Place

This is necessary because, by default, event handler methods in JavaScript do not automatically bind to the class instance (this), which is why you use .bind(this).

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constructor(props) {
  super(props); // Call parent constructor

  this.state = {
    count: 0,
  };

  // Binding event handlers in one place (the constructor)
  this.onClick = this.onClick.bind(this);
  this.onKeyUp = this.onKeyUp.bind(this);
}

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Avoid Assigning Values from this.props to this.state

One issue is that we can not test it properly until an API call is made and we get a response. It encourages the developer not to assign a value directly from the props object to the state of a component.

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‍Props are for Passing Data: props are used to pass data from the parent component to the child component. The data in the state can change the component itself. Hence, props cannot modified inside components. 

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‍State Should Be Independent:  when we assign props to a state, it may couple between parent and child components. It may cause issues.Its better to props remain static and state data through dynamic.

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import React, { Component } from 'react';

class MyComponent extends Component {
  constructor(props) {
    super(props);
    this.state = {
      data: null,
      loading: true,
    };
  }

  // Make the API call after the component is mounted
  componentDidMount() {
    fetch('https://api.example.com/data')
      .then((response) => response.json())
      .then((data) => {
        this.setState({
          data: data,
          loading: false,
        });
      })
      .catch((error) => {
        console.error('Error fetching data:', error);
        this.setState({ loading: false });
      });
  }

  render() {
    const { data, loading } = this.state;
    if (loading) {
      return <div>Loading...</div>;
    }

    return <div>{data ? JSON.stringify(data) : 'No data available'}</div>;
  }
}

export default MyComponent;

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Conclusion

They play an important role and help us in initializing components, and states, and binding event handlers. A better understanding of the React component constructors helps in better creating and managing our React application and components.