How to Execute Javascript Promises in Sequence

// An example of long running funtion returning a promise.
// for demonstration purposes we just wrap a promise around setTimeout.
function longRunningFunctionReturningAPromise(parameter) {
	const taskDuration = Math.floor(Math.random() * 110);

	return new Promise((resolve, reject) => {
		const calculationResult = Math.sin(parameter * parameter) * 2;

		if (parameter > 3) {
			// Example for an error.
			reject(new Error(`Parameters greater than 3 are not allowed.`));
		} else {
			setTimeout(() => {
				resolve(calculationResult);
			}, taskDuration);
		}
	});
}

// Create a promise sequence from an array of tasks.
function createSequence(tasks) {
	let sequenceResults = [];

	// The initial sequence Promise object:
	// Promise.resolve() returns a Promise. It also is "Thenable" so we
	// can append promises at the end of it by using sequence.then(...)
	let sequence = Promise.resolve();

	// Append all tasks to the promise
	// and execute them only after the previous task has finished.
	tasks.forEach((task, taskIndex) => {
		sequence = sequence
			.then(() => task())
			.then(taskResult => {
				// add the result of the task to the results array
				sequenceResults[taskIndex] = taskResult;
			});
	});

	// At the end of the sequence, return the sequence resutlts.
	return sequence.then(() => sequenceResults);
}

// Create a list of tasks. Note: we wrap each task in a separate anonymous function.
// This is important so we can invoke the tasks in order at a later point in time
// (instead of them being invoked immediately on creation)
const tasks = [
	() => longRunningFunctionReturningAPromise(1),
	() => longRunningFunctionReturningAPromise(2),
	() => longRunningFunctionReturningAPromise(3),
];

// Start a sequence that completes
createSequence(tasks).then(
	sequenceResults => console.log('Sequence completed:', sequenceResults),
	error => console.log('Sequence failed:', error)
);

const tasksThatWillFail = [
	() => longRunningFunctionReturningAPromise(2),
	() => longRunningFunctionReturningAPromise(3),
	() => longRunningFunctionReturningAPromise(4),
];

// Start a sequence that will throw an error
createSequence(tasksThatWillFail).then(
	sequenceResults => console.log('Sequence completed:', sequenceResults),
	error => console.log('Sequence failed:', error)
);