/**
 * Fetch API helper function
 * @param {string} endpoint Endpoint URL
 * @param {string} method Method (GET, POST, etc.)
 * @param {Object} headers Headers to include in the request
 * @param {Object} body Body of the request
 * @param {Function} cbOK Callback function for successful response
 * @param {Function} cbError Callback function for error response
 */
function fetchAPI(endpoint, method, headers = {}, body = null, cbOK, cbError) {
    let url = window.location.origin + "/api/" + endpoint;
    let options = {
        method: method,
        headers: headers
    }
    if (body !== null) {
        options.body = JSON.stringify(body);
        if (!options.headers['Content-Type']) {
            options.headers['Content-Type'] = 'application/json';
        }
    }
    fetch(url, options)
        .then(async (response) => {
            if (!response.ok) {
                let msg;
                try {
                    let _xxx = await response.json();
                    msg = _xxx.message || response.statusText;
                } catch {
                    msg = await response.statusText;
                }
                throw new Error(msg);
            }
            return response.json();
        })
        .then(data => {
            cbOK(data);
        })
        .catch(error => {
            cbError(error);
        });
}