map-limit

What is map-limit?

The map-limit npm package allows you to map over an array of items with a limit on the number of concurrent asynchronous operations. This is useful for controlling the rate of operations, especially when dealing with APIs or other resources that have rate limits.

What are map-limit's main functionalities?

Basic Usage

This example demonstrates the basic usage of map-limit. It processes an array of numbers, doubling each one, but only allows two asynchronous operations to run concurrently.

const mapLimit = require('map-limit');
const tasks = [1, 2, 3, 4, 5];
const limit = 2;

function asyncTask(item, callback) {
  setTimeout(() => {
    callback(null, item * 2);
  }, 1000);
}

mapLimit(tasks, limit, asyncTask, (err, results) => {
  if (err) throw err;
  console.log(results); // [2, 4, 6, 8, 10]
});

Error Handling

This example shows how to handle errors in map-limit. If an error occurs during the processing of an item, it is passed to the final callback.

const mapLimit = require('map-limit');
const tasks = [1, 2, 3, 4, 5];
const limit = 2;

function asyncTask(item, callback) {
  setTimeout(() => {
    if (item === 3) {
      callback(new Error('An error occurred'));
    } else {
      callback(null, item * 2);
    }
  }, 1000);
}

mapLimit(tasks, limit, asyncTask, (err, results) => {
  if (err) {
    console.error(err.message); // 'An error occurred'
  } else {
    console.log(results);
  }
});

Other packages similar to map-limit

async

The async package provides a wide range of asynchronous utilities, including the mapLimit function, which offers similar functionality to map-limit. It is more feature-rich and widely used, making it a good alternative if you need more than just map limiting.

p-limit

The p-limit package allows you to run multiple promise-returning & async functions with a concurrency limit. It is more modern and uses promises, making it a good choice for projects that prefer promise-based APIs over callback-based ones.

bluebird

Bluebird is a fully-featured promise library with a focus on performance and innovative features. It includes a map function with concurrency control, similar to map-limit, but also offers a wide range of other utilities for working with promises.

README

map-limit

async.mapLimit's functionality available as a standalone npm module.

I often find myself pulling in async for this method alone, so in the spirit of breaking things into smaller pieces here's that method as a single thing you can require.

Usage

mapLimit(arr, limit, iterator, callback)

The same as map only no more than "limit" iterators will be simultaneously running at any time.

Note that the items are not processed in batches, so there is no guarantee that the first "limit" iterator functions will complete before any others are started.

Arguments

• arr - An array to iterate over.
• limit - The maximum number of iterators to run at any time.
• iterator(item, callback) - A function to apply to each item in the array. The iterator is passed a callback(err, transformed) which must be called once it has completed with an error (which can be null) and a transformed item.
• callback(err, results) - A callback which is called after all the iterator functions have finished, or an error has occurred. Results is an array of the transformed items from the original array.

License

MIT. See LICENSE.md for details.