callback-refs.js

class MyComponent extends React.Component {

  constructor(props) {
    super(props);
    this.toggleInputCase = this.toggleInputCase.bind(this);
    this.state = { uppercase: false };
  }
  
  toggleInputCase() {
    const isUpper = this.state.uppercase;
    
    // Accessing the ref using this.inputField
    const value = this.inputField.value;
    
    this.inputField.value =
      isUpper
        ? value.toLowerCase()
        : value.toUpperCase();
        
    this.setState({ uppercase: !isUpper });
  }

  render() {
    return (
      <div>
        {/* Creating a callback ref and storing it in this.inputField */}
        <input type="text" ref={elem => this.inputField = elem} />
        
        <button type="button" onClick={this.toggleInputCase}>
          Toggle Case
        </button>
      </div>
    );
  }
  
}