Vitor L Cavalcanti VitorLuizC

async/await is just the do-notation of the Promise monad

CertSimple just wrote a blog post arguing ES2017's async/await was the best thing to happen with JavaScript. I wholeheartedly agree.

In short, one of the (few?) good things about JavaScript used to be how well it handled asynchronous requests. This was mostly thanks to its Scheme-inherited implementation of functions and closures. That, though, was also one of its worst faults, because it led to the "callback hell", an seemingly unavoidable pattern that made highly asynchronous JS code almost unreadable. Many solutions attempted to solve that, but most failed. Promises almost did it, but failed too. Finally, async/await is here and, combined with Promises, it solves the problem for good. On this post, I'll explain why that is the case and trace a link between promises, async/await, the do-notation and monads.

First, let's illustrate the 3 styles by implementing

GitHub GraphQL API Starred Repositories Examples With Pagination

You can play with the GraphQL API live here. It's pretty nice and has autocompletion based on the GraphQL schema.

The first example gets your first 3 starred repos, the cursor values can be used for pagination.

	# Moving from Ava to Jest

	---

	## Pros and Cons

	Benefits:

	- Locally it has sped up tests from about 40-50 seconds for Ava, to 8-10 for jest
	- On Travis it has cut test runs down about 9-10 minutes

	// Async compose
	const compose = (…functions) => input => functions.reduceRight((chain, func) => chain.then(func), Promise.resolve(input));

	// Functions fn1, fn2, fn3 can be standard synchronous functions or return a Promise
	compose(fn3, fn2, fn1)(input).then(result => console.log(`Do with the ${result} as you please`))

	/**
	* Updates the passed dialog to retain focus and restore it when the dialog is closed. Won't
	* upgrade a dialog more than once. Supports IE11+ and is a no-op otherwise.
	* @param {!HTMLDialogElement} dialog to upgrade
	*/
	var registerFocusRestoreDialog = (function() {
	if (!window.WeakMap \|\| !window.MutationObserver) {
	return function() {};
	}
	var registered = new WeakMap();

	const R = new LaRamda()

	/**
	* A subset of custom implementations of functions from
	* the Ramda library. (all in Lamda form)
	* - thanks to @xgrommx for uniq, intersection, where, evolve,
	* applySpec, defaultTo, both, either, cond, zipWith
	*/
	function LaRamda () {
	const I = x => x

	const R = new LaRamda()

	/**
	* A subset of custom implementations of functions from
	* the Ramda library. (all in Lamda form)
	* - thanks to @xgrommx for uniq, intersection, where, evolve,
	* applySpec, defaultTo, both, either, cond, zipWith
	*/
	function LaRamda () {
	const I = x => x

Vitor L Cavalcanti VitorLuizC

async/await is just the do-notation of the Promise monad

GitHub GraphQL API Starred Repositories Examples With Pagination

MOOCs

Self paced

Non self paced

	const App = observer(({ store }) => {

	const { authenticated } = store
	if (!authenticated ) {
	return <Login
	doLogin={store.login}
	postLogin={store.showHomepage}
	/>
	}

	/** Shows or hides a Bootstrap 3 modal and returns a Promise object.
	*
	* Bootstrap's modal functions return before the modal is shown/
	* hidden, which can cause glitches if subsequent code doesn't
	* expect that.
	*
	* Unfortunately, there is no built-in way to defer an
	* operation until after the modal is shown/hidden, so
	* this function does a little monkey-patching to make
	* it work.