For when you don't know the response type

fetch-data.js

/*
    Needed in evaluation/Form.js because successful calls to server but
    w/ empty data (null from java which fetch retrieves as empty string)
    will throw errors because fetch can't parse it as json.

    Retrieves either json or text depending on what's available

    Didn't change already existing fetchJson to not
    risk breaking any exisitng api calls
*/

export default async function fetchData({ url, throw: throwError = true, ...conf }) {

    let response = await fetch(url, conf).catch(e => e);

    if (response instanceof Error) {
        console.error(`Fetching ${url} failed`, response.message);
        
        if (throwError) throw response;
        return response;
    }

    const
        text = await response.text(),
        json = await Promise.resolve(text).then(JSON.parse).catch(() => ({}));

    if (!response.ok) {
        console.error(`${url} gave non-ok server response`, `status: ${response.status}`, text);
        
        if (throwError) throw new Error(text);
        return new Error(text);
    }

    return !Object.keys(json).length && text ? text : json;
}