~andreicek


Working with promises in batches

Wednesday, December 4, 2019; Reading time 3 minutes.

Have you ever neded to do some work in batches? How about promises? Let’s say you have an API that can’t take more than 2 API calls at once. This is a really simple way to do the work you need.


We are going to be using typescript for this demonstration since I think it conveys the idea the best. In order to do so we have to define two types used in this example. First one is a function that returns a promise with a type T, and the other is an array of arrays of T (think 2D matrix).

type CurriedPromise<T> = () => Promise<T>;
type ChunkedArray<T> = Array<Array<T>>;

First we have a function that returns another function with a promise. We use currying here a) so we can start the promise (e.g. a fetch call) later, and b) since I wanted an easy way to show order of execution.

function getData(msg: string): CurriedPromise<string> {
	return () => Promise.resolve(msg);
}

Next we store the getters in a array called fifo. That means that we are not going to be popping the items for the array but we are going to be splitting the arrays into chunks. Take note that the type of fifo array is an array of functions that return a string (() => Promise<string>)[].

const fifo: Array<CurriedPromise<string>> = [
	getData('1'),
	getData('2'),
	getData('3'),
	getData('4'),
];

Let’s say we limit the chunks to two (2) items:

const PER_CHUNK = 2;

And now we can split the fifo array to contain PER_CHUNK (2) items in every array, resulting in an array containg two arrays with functions. Main part of this function is Math.floor(i / PER_CHUNK). This returns following values for our fifo array for each iteration: 0, 0, 1, 1. Next up is just creating a chunk array if it doesn’t exists, or adding to it. Note that we create new array with one element already to avoid doing push. Be sure to return the acc for every iteration.

const chunks = fifo.reduce<ChunkedArray<CurriedPromise<string>>>((acc, curr, i) => {
	const chunkIndex = Math.floor(i / PER_CHUNK);

	if (!acc[chunkIndex]) {
		acc[chunkIndex] = [curr];
	} else {
		acc[chunkIndex].push(curr);
	}

	return acc;
}, []);

Next up is starting all promises in a chunk. We use Promise.all here and wait for the result. We also map the every chunk with the instance of the promise. In the end we log for every chunk.

(async function doWork() {
	for (const chunk of chunks) {
		const res = await Promise.all(chunk.map((f) => f()));
		console.log(res);
	}
})();

This is the result:

['1', '2']
['3', '4']

Play with the example


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