form.js

import React, {Component} from "react"

class App extends Component {
    constructor() {
        super()
        this.state = {
            firstName: "",
            lastName: "",
            isFriendly: false
        }
        this.handleChange = this.handleChange.bind(this)
    }
    
    handleChange(event) {
        const {name, value, type, checked} = event.target
        type === "checkbox" ? this.setState({ [name]: checked }) : this.setState({ [name]: value })
    }
    
    render() {
        return (
            <form>
                <input 
                    type="text" 
                    value={this.state.firstName} 
                    name="firstName" 
                    placeholder="First Name" 
                    onChange={this.handleChange} 
                />
                <br />
                <input 
                    type="text" 
                    value={this.state.lastName} 
                    name="lastName" 
                    placeholder="Last Name" 
                    onChange={this.handleChange} 
                />
                
                {
                    /**
                     * Other useful form elements:
                     * 
                     *  <textarea /> element
                     *  <input type="checkbox" />
                     *  <input type="radio" />
                     *  <select> and <option> elements
                     */
                }
                
                <textarea 
                    value={"Some default value"}
                    onChange={this.handleChange}
                />
                
                <br />
                
                <label>
                    <input 
                        type="checkbox" 
                        name="isFriendly"
                        checked={this.state.isFriendly}
                        onChange={this.handleChange}
                    /> Is friendly?
                </label>
                
                
                
                
                <h1>{this.state.firstName} {this.state.lastName}</h1>
            </form>
        )
    }
}

export default App