React State and setState in Class Components

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In this tutorial, we will learn how to use the state object to store the data and setState() method to update the data in class components.

A state is a built-in object in React class components. In the state object, we store property values that belong to the component. When the state object changes, the component re-renders. We use the setState() method to change the state object in a class component.

Example 1: State and setState in Class Components

Create State Object in Employee Component

Let's create an Employee class component and create and initialize the state object in the constructor:

import React from "react"

class Employee extends React.Component {
    constructor(props) {
        super(props)

        this.state = {
            firstName: "Ramesh",
            lastName: "Fadatare",
            email: "ramesh@gmail.com"
        }
    }

    render(){
        return (
            <div className="center">
                <h1> Employee Details</h1> <hr />
                <p>{this.state.firstName}</p>
                <p>{this.state.lastName}</p>
                <p>{this.state.email}</p>
            </div>
        )
    }
}

export default Employee

Let's import the above component into the App component and see the result in the browser:

import './App.css'
import Employee from './components/Employee'

function App() {
  return (
    <div>
      <Employee />
    </div>
  )
}

export default App

Using setState() Method to Update the State Object

To change a value in the state object, use this.setState() method. When a value in the state object changes, the component will re-render, meaning that the output will change according to the new value(s). 

For example, let's add a button with an onClick event that will change the lastName and email properties of the employee:
import React from "react"

class Employee extends React.Component {
    constructor(props) {
        super(props)

        this.state = {
            firstName: "Ramesh",
            lastName: "Fadatare",
            email: "ramesh@gmail.com"
        }
    }

    updateEmployee(){
        this.setState({
            lastName: "jadhav",
            email: "ram@gmail.com"
        })
    }

    render(){
        return (
            <div className="center">
                <h1> Employee Details</h1> <hr />
                <p>{this.state.firstName}</p>
                <p>{this.state.lastName}</p>
                <p>{this.state.email}</p>

                <button onClick={() => this.updateEmployee()}>Update Employee</button>
            </div>
        )
    }
}

export default Employee

Example 2: state and setState()

Creating the state Object

Let's create a StudentComponent and create and initialize the state object in the constructor:
import React, { Component } from 'react'

class StudentComponent extends Component {
    constructor(props) {
        super(props)

        this.state = {
                 firstName: "Ramesh"
        }
    }

    render() {
        return (
            <div>
                <h1> Hello Student</h1>
            </div>
        )
    }
}

export default StudentComponent
The state object can contain as many properties. For example, let's specify all the properties your component need:
import React, { Component } from 'react'

class StudentComponent extends Component {
    constructor(props) {
        super(props)

        this.state = {
                 firstName: "Ramesh",
                 lastName:"Fadatare",
                 rollNo: 123,
                 age: 20,
                 books: ["C programming", "C++ programming", "Data Structure and Algorithms"]
        }
    }

    render() {
        return (
            <div>
                <h1> Hello Student</h1> <hr/>
            </div>
        )
    }
}

export default StudentComponent
Now, we have seen how to create state and initiate the state object in a component. 
Let's see how to use state object in the component.

Using the state Object

Refer to the state object anywhere in the component by using the following syntax:
this.state.propertyname
Example: Refer to the state object in the render() method:
import React, { Component } from 'react'

class StudentComponent extends Component {
    constructor(props) {
        super(props)

        this.state = {
                 firstName: "Ramesh",
                 lastName:"Fadatare",
                 rollNo: 123,
                 age: 20,
                 books: ["C programming", "C++ programming", "Data Structure and Algorithms"]
        }
    }

    render() {
        return (
            <div>
                <h1> Student Details</h1> <hr/>
                <h3> First Name: {this.state.firstName }</h3>
                <h3> Last Name: {this.state.lastName }</h3>
                <h3> Roll No: {this.state.rollNo } </h3>
                <h3> Age: {this.state.age }</h3>
                <h3> {this.state.books} </h3>
            </div>
        )
    }
}

export default StudentComponent
Let's import above component into the App component and see the result in the browser:
import React from 'react';
import logo from './logo.svg';
import './App.css';
import StudentComponent from './components/StudentComponent';

function App() {
  return (
    <div className="App">
         <header className="App-header">
            <StudentComponent />
        </header>
    </div>
  );
}

export default App;

Hit URL http://localhost:3000/ in Browser

Changing the state Object using setState() Method

To change a value in the state object, use this.setState() method.
When a value in the state object changes, the component will re-render, meaning that the output will change according to the new value(s).
For example, let's add a button with an onClick event that will change the rollNo and age of properties of the student:
import React, { Component } from 'react'

class StudentComponent extends Component {
    constructor(props) {
        super(props)

        this.state = {
                 firstName: "Ramesh",
                 lastName:"Fadatare",
                 rollNo: 123,
                 age: 20,
                 books: ["C programming", "C++ programming", "Data Structure and Algorithms"]
        }
    }

    updateStudent(){
        this.setState({
            rollNo: 124,
            age: 21
        })
    }
    render() {
        return (
            <div>
                <h1> Student Details</h1> <hr/>
                <h3> First Name: {this.state.firstName }</h3>
                <h3> Last Name: {this.state.lastName }</h3>
                <h3> Roll No: {this.state.rollNo } </h3>
                <h3> Age: {this.state.age }</h3>
                <h3> {this.state.books} </h3>
                <button type="button" onClick={() => this.updateStudent()} >Update Student Details</button>
            </div>
        )
    }
}

export default StudentComponent
Click on the "Update Student Details" button will update student details in the browser:

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