Async Batch Processing

index.js

// just a function to simulate IO
function getDouble(n) {
	const time = Math.floor(Math.random() * 10 * 1000)
	return new Promise((resolve) => {
		setTimeout(function() {
			console.log(n*2)
			resolve(n*2)
		}, time)
	})
}


// https://stackoverflow.com/a/41791149
function forEachPromise(items, fn) {
    return items.reduce(function (promise, item) {
        return promise.then(function () {
            return fn(item);
        });
    }, Promise.resolve());
}

const a = [1,2,3,4,5,6,7,8]

const N = a.length
const batchSize = 3
const batchStarts = Array.apply(0, new Array(Math.ceil(N / batchSize))).map((x, i) => {
	return i * batchSize
})

async function processBatch(start) {
	const batch = a.slice(start, start + batchSize)
	console.log("processing batch", batch)
	return Promise.all(batch.map(async n => {
		return await getDouble(n)
	}))
}

;(async () => {

	await forEachPromise(batchStarts, processBatch)
	console.log("done")
	
})()