Chartjs-Cannot access before initialization React Error

you can keep the initial state as undefined (by default if nothing is provided)

Make the structure you want before setting it to state from the API response ( I am not sure of the response structure but you can make it as you need)

Added a loading state till the response is available (for demo purpose).

function App() {
    const [chartjs, setChartjs] = useState();

    const url =
      "https://datausa.io/api/data?drilldowns=Nation&measures=Population";

    useEffect(() => {
      axios.get(url).then((response) => {
        const data = {
          labels: response.data.data.map((item) => item.Year),
          datasets: [
            {
              labels: "Population",
              data: response.data.data.map((item) => item.Population),
            },
          ],
        };
        setChartjs(data);
      });
    }, [url]);

    if(!chartjs) return "loading ...."

    return (
      <div>
        <BarChart chartData={chartjs} />
      </div>
    );
}