/**
* definition of core function
*
* @param  {String} url          URL to acess the API
* @param  {Object} initOptions  Request body and options to be sent
* @param  {Object} Promise  		Promise of the fetch to be resolved or rejected
* @return {Object} options     extra options
*/
core = (url, initOptions, promiseForFetch, options) => {
    fetch(url, initOptions).then((response) => {
        if (response.ok) {
            response.json().then((responseJSON) => {
                promiseForFetch.resolve(responseJSON);
            }).catch((parseError) => {
                // Parsing errors are caught and handled here
                if (options.parseErrorHandler) {
                    options.parseErrorHandler(parseError);
                }else{
                    this.dispatch(this.parseErrorHandler(parseError));
                }
            });
        } else {
            // Backend errors are caught and handled here
            if (options.backendErrorHandler) {
                options.backendErrorHandler(response);
            }else{
                this.dispatch(this.backendErrorHandler(response));
            }
            promiseForFetch.reject(response.text());
        }
    })
    .catch((networkError) => {
        // Network errors are caught and handled here
        if (options.networkErrorHandler) {
            options.networkErrorHandler(networkError);
        }else{
            this.dispatch(this.networkErrorHandler(networkError));
        }

        // Re fetch the failed api once again in an exponential time frame
        const sleepTime = options.sleepTime || 1000;
        this.sleep(sleepTime).then(() => {
            const updatedOptions = { ...options, sleepTime: sleepTime * 2 };
            this.core(url, initOptions, promiseForFetch, updatedOptions);
        });

    });
}