promise-fun

I intend to use this space to document my promise modules, useful promise patterns, and how to solve common problems. For now though, you can see all my promise modules below.

Contents

• Packages
• FAQ

Packages

Not accepting additions, but happy to take requests.

• pify: Promisify a callback-style function
• delay: Delay a promise a specified amount of time
• yoctodelay: Delay a promise a specified amount of time
• p-map: Map over promises concurrently
• p-all: Run promise-returning & async functions concurrently with optional limited concurrency
• p-queue: Promise queue with concurrency control
• p-catch-if: Conditional promise catch handler
• p-if: Conditional promise chains
• p-tap: Tap into a promise chain without affecting its value or state
• p-log: Log the value/error of a promise
• p-event: Promisify an event by waiting for it to be emitted
• p-debounce: Debounce promise-returning & async functions
• p-throttle: Throttle promise-returning & async functions
• p-timeout: Timeout a promise after a specified amount of time
• p-finally: Promise#finally() ponyfill - Invoked when the promise is settled regardless of outcome
• p-retry: Retry a promise-returning or async function
• p-any: Wait for any promise to be fulfilled
• p-some: Wait for a specified number of promises to be fulfilled
• p-locate: Get the first fulfilled promise that satisfies the provided testing function
• p-limit: Run multiple promise-returning & async functions with limited concurrency
• p-series: Run promise-returning & async functions in series
• p-memoize: Memoize promise-returning & async functions
• p-pipe: Compose promise-returning & async functions into a reusable pipeline
• p-props: Like Promise.all() but for Map and Object
• p-waterfall: Run promise-returning & async functions in series, each passing its result to the next
• p-cancelable: Create a promise that can be canceled
• p-progress: Create a promise that reports progress
• p-reflect: Make a promise always fulfill with its actual fulfillment value or rejection reason
• p-filter: Filter promises concurrently
• p-reduce: Reduce a list of values using promises into a promise for a value
• p-settle: Settle promises concurrently and get their fulfillment value or rejection reason
• p-every: Test whether all promises passes a testing function
• p-one: Test whether some promise passes a testing function
• p-map-series: Map over promises serially
• p-each-series: Iterate over promises serially
• p-times: Run promise-returning & async functions a specific number of times concurrently
• p-lazy: Create a lazy promise that defers execution until .then() or .catch() is called
• p-whilst: While a condition returns true, calls a function repeatedly, and then resolves the promise
• p-do-whilst: Calls a function repeatedly while a condition returns true and then resolves the promise
• p-forever: Run promise-returning & async functions repeatedly until you end it
• p-wait-for: Wait for a condition to be true
• p-min-delay: Delay a promise a minimum amount of time
• p-try: Promise.try() ponyfill - Starts a promise chain
• p-race: A better Promise.race()
• p-immediate: Returns a promise resolved in the next event loop - think setImmediate()
• p-time: Measure the time a promise takes to resolve
• p-defer: Create a deferred promise
• p-break: Break out of a promise chain
• p-is-promise: Check if something is a promise
• p-state: Inspect the state of a promise
• make-synchronous: Make an asynchronous function synchronous

FAQ

How can I run 100 async/promise-returning functions with only 5 running at once?

This is a good use-case for p-map. You might ask why you can't just specify an array of promises. Promises represent values of a computation and not the computation itself - they are eager. So by the time p-map starts reading the array, all the actions creating those promises have already started running. p-map works by executing a promise-returning function in a mapper function. This way the promises are created lazily and can be concurrency limited. Check out p-all instead if you're using different functions to get each promise.

const pMap = require('p-map');

const urls = [
	'https://sindresorhus.com',
	'https://avajs.dev',
	'https://github.com',
	…
];

console.log(urls.length);
//=> 100

const mapper = url => {
	return fetchStats(url); //=> Promise
};

pMap(urls, mapper, {concurrency: 5}).then(result => {
	console.log(result);
	//=> [{url: 'https://sindresorhus.com', stats: {…}}, …]
});

How can I reduce nesting?

Let's say you want to fetch some data, process it, and return both the data and the processed data.

The common approach would be to nest the promises:

const getData = id =>
	Storage
		.find(id)
		.then(data => {
			return process(data).then(result => {
				return prepare(data, result);
			});
		});

But we can take advantage of Promise.all:

const getData = id =>
	Storage
		.find(id)
		.then(data => Promise.all([data, process(data)])
		.then(([data, result]) => prepare(data, result));

And even simpler with async functions: (Requires Babel or Node.js 8)

const getData = async id => {
	const data = await Storage.find(id);
	return prepare(data, await process(data));
};

What about something like Bluebird#spread()?

Bluebird:

Promise.resolve([1, 2]).spread((one, two) => {
	console.log(one, two);
	//=> 1 2
});

Instead, use destructuring:

Promise.resolve([1, 2]).then(([one, two]) => {
	console.log(one, two);
	//=> 1 2
});

What about something like Bluebird.join()?

Bluebird:

Promise.join(p1, p2, p3, (r1, r2, r3) => {
	// …
});

Instead, use an async function and destructuring:

const [r1, r2, r3] = await Promise.all([p1, p2, p3]);
// …

How do I break out of a promise chain?

You might think you want to break out ("return early") when doing conditional logic in promise chains.

Here you would like to only run the onlyRunConditional promises if conditional is truthy.

alwaysRun1()
	.then(() => alwaysRun2())
	.then(conditional => conditional || somehowBreakTheChain())
	.then(() => onlyRunConditional1())
	.then(() => onlyRunConditional2())
	.then(() => onlyRunConditional3())
	.then(() => onlyRunConditional4())
	.then(() => alwaysRun3());

You could implement the above by abusing the promise rejection mechanism. However, it would be better to branch out the chain instead. Promises can not only be chained, but also nested and unnested.

const runConditional = () =>
	onlyRunConditional1()
		.then(() => onlyRunConditional2())
		.then(() => onlyRunConditional3())
		.then(() => onlyRunConditional4());

alwaysRun1()
	.then(() => alwaysRun2())
	.then(conditional => conditional && runConditional())
	.then(() => alwaysRun3());