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parallel ​

Process an array with an async callback with controlled parallelism. Similar to Promise.all, but limits how many items are processed concurrently.

Signature ​

typescript
function parallel<T, R>(
  array: T[],
  callback: (item: T, index: number, array: T[]) => Promise<R>,
  options?: { limit?: number; signal?: AbortSignal },
): Promise<R[]>;

Parameters ​

  • array – Array of items to process
  • callback – Async function to process each item
  • options.limit – Maximum number of concurrent operations (defaults to array length)
  • options.signal – Optional AbortSignal to cancel processing

Returns ​

Promise resolving to an ordered array of results.

Examples ​

Basic Usage ​

typescript
import { parallel } from '@vielzeug/toolkit';
const urls = ['url1', 'url2', 'url3', 'url4', 'url5'];
// Process 3 URLs at a time
const results = await parallel(
  urls,
  async (url) => {
    const response = await fetch(url);
    return response.json();
  },
  { limit: 3 },
);
console.log(results); // Array of JSON responses in order

With AbortSignal ​

typescript
import { parallel } from '@vielzeug/toolkit';
const controller = new AbortController();
// Cancel after 5 seconds
setTimeout(() => controller.abort(), 5000);
try {
  const results = await parallel(items, async (item) => processItem(item), {
    limit: 2,
    signal: controller.signal,
  });
} catch (error) {
  console.log('Processing aborted');
}

Rate Limiting API Calls ​

typescript
import { parallel } from '@vielzeug/toolkit';
const userIds = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10];
// Fetch users with max 3 concurrent requests
const users = await parallel(
  userIds,
  async (id) => {
    const response = await fetch(`/api/users/${id}`);
    return response.json();
  },
  { limit: 3 },
);

With Index and Array Parameters ​

typescript
import { parallel, sleep } from '@vielzeug/toolkit';
const items = ['a', 'b', 'c', 'd', 'e'];
const results = await parallel(
  items,
  async (item, index, array) => {
    console.log(`Processing ${item} (${index + 1}/${array.length})`);
    await sleep(100);
    return item.toUpperCase();
  },
  { limit: 2 },
);
console.log(results); // ['A', 'B', 'C', 'D', 'E']
  • queue – Task queue with monitoring