p-locate/index.js

import pLimit from 'p-limit';

class EndError extends Error {
	constructor(value) {
		super();
		this.value = value;
	}
}

// The input can also be a promise, so we await it.
const testElement = async (element, tester) => tester(await element);

// The input can also be a promise, so we `Promise.all()` them both.
const finder = async element => {
	const values = await Promise.all(element);
	if (values[1] === true) {
		throw new EndError(values[0]);
	}

	return false;
};

export default async function pLocate(
	iterable,
	tester,
	{
		concurrency = Number.POSITIVE_INFINITY,
		preserveOrder = true,
	} = {},
) {
	const limit = pLimit(concurrency);

	// Start all the promises concurrently with optional limit.
	const items = [...iterable].map(element => [element, limit(testElement, element, tester)]);

	// Check the promises either serially or concurrently.
	const checkLimit = pLimit(preserveOrder ? 1 : Number.POSITIVE_INFINITY);

	try {
		await Promise.all(items.map(element => checkLimit(finder, element)));
	} catch (error) {
		if (error instanceof EndError) {
			return error.value;
		}

		throw error;
	}
}