when to fetch data react code example

Example: React get method

/* React get method.  */

componentWillMount(){
    fetch('/getcurrencylist',
    {
        /*
        headers: {
          'Content-Type': 'application/json',
          'Accept':'application/json'
        },
        */
        method: "get",
        dataType: 'json',
    })
    .then((res) => res.json())
    .then((data) => {
      var currencyList = [];
      for(var i=0; i< data.length; i++){
        var currency = data[i];
        currencyList.push(currency);
      }
      console.log(currencyList);
      this.setState({currencyList})
      console.log(this.state.currencyList);
    })
    .catch(err => console.log(err))
  }