Example use of Javascript Fetch

fetch-ex.js

function call_api(url, args, rsp_json, result_func) {
	// Start the fetch chain of promises
	fetch(url, {
		method: 'POST',
		headers: {'Content-Type': 'application/json'},
		body: JSON.stringify(args)
	})
	.then(rsp => {
		// rsp.ok true if rsp.status is 200-299
		if (rsp.ok) {
			// Return a new promise to next .then section
			return rsp_json ? rsp.json() : rsp.text();
		} else {
			// Return a rejected promise and execution ends in .catch
			return Promise.reject('API HTTP Error ' + rsp.status + ': ' + rsp.statusText);
		}
	})
	.then(data => {
		// Use the data returned from previous .then
		result_func(data);
	})
	.catch(err => {
		// We end here if fetch fails (fx connection error) but NOT if server answers with any rsp.status
		// We also end here if we return a Promise.reject in .then
		alert(err);
	});
}