<div id="root"></div>
body {
  margin: 1em;
}
class App extends React.Component {
  constructor() {
    super()
    this.state = {
      email: '',
      username: '',
      password: '',
      comment: '',
      passwordConfirmation:'',
      formErrors: {
        email: '',
        username:'', 
        password: '', 
        passwordConfirmation: '',
      },
      formValidity: {
        email: false,
        username: false, 
        password: false, 
        passwordConfirmation: false,
      },
      canSubmit: false,
    };
    this.handleChange = this.handleChange.bind(this)
  }
  
  // triggered everytime value changes in our textboxes
  handleChange(event) {
    const {name, value} = event.target
    this.setState({
      // use dynamic name value to set our state object property
      [name]: value
    }, function(){ this.validateField(name, value)})
    
  }

  validateField(name, value) {
    if(Object.keys(this.state.formValidity).includes(name)){
      const fieldValidationErrors = this.state.formErrors
      const validity = this.state.formValidity
      const isEmail = name === "email"
      const isPassword = name === "password"
      const isPasswordConfirmation = name === "passwordConfirmation"
      const label = name === "passwordConfirmation"? 'password confirmation' : name
      const emailTest = /^[^\s@]+@[^\s@]+\.[^\s@]{2,}$/i
  
      validity[name] = value.length >0
      fieldValidationErrors[name] = validity[name] ? '': `${label} is required and cannot be empty`
  
      if(validity[name]) {
        if(isPassword){
          validity[name] = value.length >= 5;
          fieldValidationErrors[name] = validity[name] ? '': `${label} should be 5 characters or more`
        }
        if(isEmail){
          validity[name] = emailTest.test(value);
          fieldValidationErrors[name] = validity[name] ? '' : `${label} should be a valid email address`
        }
        if(isPasswordConfirmation){
          validity[name] = value === this.state.password
          fieldValidationErrors[name] = validity[name] ? '' : `${label} should match password`
        }
      }
    
      this.setState({
        formErrors: fieldValidationErrors,
        formValidity: validity,
      }, () => this.canSubmit())
    }
  }

  canSubmit() {
    this.setState({ canSubmit: this.state.formValidity.email && this.state.formValidity.username && this.state.formValidity.password && this.state.formValidity.passwordConfirmation })
  }

  errorClass(error) {
    return(error.length === 0 ? '' : 'is-invalid');
  }
   
  // triggered on submit
  handleSubmit = (event) => {
    // get our const values by destructuring state
    // see https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Destructuring_assignment#Object_destructuring
    event.preventDefault()
    const { email, username, password, comment } = this.state
    // regular javascript alert function
    alert(`Your registration detail: \n 
           Email: ${email} \n 
           Username: ${username} \n
           Password: ${password} \n
           Comment: ${comment || 'no comment'}`
    )
  }
  
  render() {    
    return (
      <React.Fragment>
      <h1>Registration Form</h1>
      <p>Please fill in all textboxes</p> 
      <form onSubmit={this.handleSubmit}>
        <div className="form-group">
          <label htmlFor="email">Email address</label>
          <input
            className={`form-control ${this.errorClass(this.state.formErrors.email)}`}
            id="email"
            name="email"
            type="text"
            placeholder="Enter email"
            value={this.state.email}
            onChange={this.handleChange}
          />
          <div className="invalid-feedback">{this.state.formErrors.email}</div>
        </div>
        
        <div className="form-group">
          <label htmlFor="username">Username</label>
          <input
            className={`form-control ${this.errorClass(this.state.formErrors.username)}`}
            id="username"
            name="username"
            type="text"
            placeholder="Enter username"
            value={this.state.username}
            onChange={this.handleChange}
          />
          <div className="invalid-feedback">{this.state.formErrors.username}</div>
        </div>
        
        <div className="form-group">
          <label htmlFor="password">Password</label>
          <input
            className={`form-control ${this.errorClass(this.state.formErrors.password)}`}
            id="password"
            name ="password"
            type="password"
            placeholder="Enter password"
            value={this.state.password}
            onChange={this.handleChange}
          />
          <div className="invalid-feedback">{this.state.formErrors.password}</div>
        </div> 

        <div className="form-group">
          <label htmlFor="passwordConfirmation">Password Confirmation</label>
          <input
            className={`form-control ${this.errorClass(this.state.formErrors.passwordConfirmation)}`}
            id="passwordConfirmation"
            name ="passwordConfirmation"
            type="password"
            placeholder="Enter password again"
            value={this.state.passwordConfirmation}
            onChange={this.handleChange}
          />
          <div className="invalid-feedback">{this.state.formErrors.passwordConfirmation}</div>
        </div> 

        <div className="form-group">
        <label htmlFor="comment"> Comment:</label>
          <textarea 
          className="form-control"
          id="comment"
          name="comment" 
          value={this.state.comment} 
          onChange={this.handleChange} />
        </div>       
        <button className="btn btn-success btn-block" disabled={!this.state.canSubmit}>Sign up</button>
      </form>
      </React.Fragment>
    )
  }
}

ReactDOM.render(<App />, document.getElementById('root'))
View Compiled

External CSS

  1. https://cdnjs.cloudflare.com/ajax/libs/twitter-bootstrap/4.1.2/css/bootstrap.min.css

External JavaScript

  1. https://cdnjs.cloudflare.com/ajax/libs/react/16.2.0/umd/react.development.js
  2. https://cdnjs.cloudflare.com/ajax/libs/react-dom/16.2.0/umd/react-dom.development.js