Form validation

"Shawn, I think we should also add basic validation to the BookList component so that the user can't go to the next step without selecting a book," said Mike.

"Agree. Let me give it a try." answered Shawn.

// src/BookStore.js
// Updating BookList component

var BookList = React.createClass({
  getInitialState() {
    return (
      { books: [
        { id: 1, name: 'Zero to One', author: 'Peter Thiel' },
        { id: 2, name: 'Monk who sold his Fearrary', author: 'Robin Sharma' },
        { id: 3, name: 'Wings of Fire', author: 'A.P.J. Abdul Kalam' }
      ],
        selectedBooks: [],
        error: false
      }
    );
  },

  _renderError() {
    if (this.state.error) {
      return (
        <div className="alert alert-danger">
          {this.state.error}
        </div>
      );
    }
  },

  handleSubmit(event) {
    event.preventDefault();

    if(this.state.selectedBooks.length === 0) {
      this.setState({error: 'Please choose at least one book to continue...