phpengine/ui/admin/assets/js/OMGUtils.js

const fetchHeaders = {
	'Accept': 'text/html',
	'Content-Type': 'text/html',
	'X-Requested-With': 'XMLHttpRequest',
	'Cache-Control': 'no-cache',
	'Pragma': 'no-cache',
	'Expires': '0'
};

/**
 * Make an AJAX request.
 *
 * If error, catch return a promise error object:
 *
 *  .catch(err => alert(`${err.url}\n\n${err.status} (${err.statusText})`));
 *
 * @param url URL of RESTful api to call
 * @param options the fetch() options
 * @returns {Promise<Response | never>}
 */
export function AjaxReq(url, options) {
	let defaults = {headers: fetchHeaders};
	// Extend default options
	Object.assign(defaults, options);

	return fetch(url, defaults)
		.then(resp => {
			if (resp.status === 200)
				return resp.json();
			else
				return Promise.reject(resp);
		});
}