Because you forgot to assign the ref to some dom element. You are only creating it.
Write it like this:
class App extends React.PureComponent {
constructor(props) {
super(props);
this.test = React.createRef();
}
handleClick = () => alert(this.test.current.value)
render() {
return (
<div className="App">
<input ref={this.test} />
<button onClick={this.handleClick}>Get Value</button>
</div>
)
}
}
Working Example.