Promise example. https://scotch.io/tutorials/javascript-promises-for-dummies

promise_example.js

const canStart = (response) => {
	return new Promise((resolve, reject) => {
		if (response) {
			resolve(response);
		} else {
			reason = new Error("cant start. forbiden");
			reject(reason);
		}
	});
}

const sumNumbersWithApi = (a, b) => {
	console.log(`a i b to: ${a} i ${b}`);
  const url = `https://stark-escarpment-75299.herokuapp.com/?a=${a}&b=${b}`;
  return fetch(url)
           .then(response => response.json())
           .then(response => parseInt(response['response']))
           .catch((error) => {
             console.log("there was error in fetch:", error.message);
           });
};

const putInConsole = (response) => {
 console.log("policzona suma: " + response);
 return Promise.resolve(response);
};

canStart(true)
.then(response => sumNumbersWithApi(3, 2))
.then(putInConsole)
.then(response => sumNumbersWithApi(3, response))
.then(putInConsole)
.then(response => sumNumbersWithApi(3, response))
.then(response => canStart(false))
.then(response => sumNumbersWithApi(3, response))
.then(putInConsole)
.catch(error => console.log(error.message))