"join" polyfill

index.js

export default function using(...args) {
	const fn = args.at(-1)
	if (typeof fn !== 'function') {
		throw new Error('Missing expected function argument')
	}
	if (args.length < 2) {
		throw new Error('At least two arguments must be passed')
	}
	return Promise.all(args.slice(0, -1)).then((v) => fn(...v))
}

The typeof check might be controversial and diverges from e.g. Bluebird behaviour where e.g. join(1, 2, 3) would be equivalent to Promise.all([1, 2, 3]) (simply returning a promise for an array of the resolved parameters). However, I've seen bugs multiple times caused by the last parameter accidentally being e.g. undefined or null resulting in an array being passed to the next consumer rather than the desired result (often not an array). At the same time, I've rarely (if ever) used the join(1, 2, 3) version (I'd usually favor an object, and argue that a "fixed size" array is a smell that it's not actually a list).

Published as https://github.com/tjconcept/psjoin