React: fetch JSON to state

export default class Ticker extends React.Component {
state = {
value: 0
}

constructor(props){
super(props);
var val=this.props.pair.toUpperCase().split("_");
console.log(val);
fetch("https://min-api.cryptocompare.com/data/price?fsym="+val[0]+"&tsyms=USD").then(r => r.json()).then(res => {this.setState({value:  res.USD})});
}

render(){
const { pair } = this.props;
return (
<div className="ticker">
<p>{pair.toUpperCase().replace('_', ' to ')}</p>
<p>{this.state.value}</p>
</div>
)
}

}