how to get value from onclick handler in react

1// Render a block of three buttons
2[1, 2, 3].map(buttonId => (
3  // Pass a parameter in 'value' attribute
4  <button
5    key={buttonId}
6    value={buttonId}
7    onClick={this.handleButtonClicked}
8  >
9    button {buttonId}
10  </button>
11));
12...
13handleButtonClicked = ev => {
14  this.setState({
15    // Retrieve a passed parameter 'value' attribute
16    message: `Button ${ev.currentTarget.value} clicked`
17  });
18};
19