React

HTML

<div id="app"></div>

CSS

body {
  background: #20262E;
  padding: 20px;
  font-family: Helvetica;
}

#app {
  background: #fff;
  border-radius: 4px;
  padding: 20px;
  transition: all 0.2s;
}

li {
  margin: 8px 0;
}

h2 {
  font-weight: bold;
  margin-bottom: 15px;
}

input {
  margin-right: 5px;
}

.error {
  color: #f00;
}

React

class SampleApp extends React.Component {
  constructor(props) {
    super(props)
    this.state = {
    	errors: {}
    }
  }
  
  handleInput = (event) => {
  	const target = event.target
    const errors = Object.assign({}, this.state.error)
    
    delete errors[target.name]
    this.setState({errors})
  }
  
  handleSubmit = (event) => {
 		const username = document.getElementById('username')
    const errors = {}
    if (!username.value.trim()) {
      errors[username.name] = 'Username is required'
    }
    console.log(errors)
    this.setState({ errors })
  }
  
  render() {
    return (
      <div>
        <label>Username: </label>
        <input type="text" id="username" name="username" onChange={this.handleInput} />         {this.state.errors['username'] &&
          <div class="error">{this.state.errors['username']}</div>
        }
        
        <br/><button onClick={this.handleSubmit}>Submit</button>
      </div>
    )
  }
}

ReactDOM.render(<SampleApp />, document.querySelector("#app"))