To call a backend API from a React app, you can use the fetch function or a library like axios.

Here is an example of how to call a backend API using the fetch function:

import React, {useState, useEffect} from 'react';

const App = () => {
    const [data, setData] = useState(null);

    useEffect(() => {
        const fetchData = async () => {
            const response = await fetch('https://my-api.com/endpoint');
            const json = await response.json();
            setData(json);
        };
        fetchData();
    }, []);

    return (
        <div>
            {data ? (
                <div>
                    <p>{data.id}</p>
                    <p>{data.name}</p>
                </div>
            ) : (
                'Loading...'
            )}
        </div>
    );
};

export default App;

In this example, we use the useEffect hook to fetch data from the API when the component mounts. The fetch function returns a promise, which we can await to get the API response. We then use the json method of the response object to parse the response into JSON format.

Here is an example of how to call a backend API using axios:

import React, {useState, useEffect} from 'react';
import axios from 'axios';

const App = () => {
    const [data, setData] = useState(null);

    useEffect(() => {
        const fetchData = async () => {
            const response = await axios.get('https://my-api.com/endpoint');
            setData(response.data);
        };
        fetchData();
    }, []);

    return (
        <div>
            {data ? (
                <div>
                    <p>{data.id}</p>
                    <p>{data.name}</p>
                </div>
            ) : (
                'Loading...'
            )}
        </div>
    );
};

export default App;

In this example, we use the axios.get method to make a GET request to the API, and we use the response.data property to access the response data.

It is important to note that you may need to handle errors and deal with issues like CORS (Cross-Origin Resource Sharing) when calling a backend API from a React app. You may also want to consider adding authentication and authorization to your API calls.

For more information on calling APIs in React, you can refer to the React documentation at https://reactjs.org/docs/faq-ajax.html.