process async items with concurrency

index.js

const worker = async (queue, concurrency = 25) => {
    let index = 0;
    let results = [];
    let threads = [];

    const execThread = async () => {
        while (index < queue.length) {
            const currentIndex = index++;
            
            // Use of `currentIndex` is important because `index` may change after await is resolved
            results[currentIndex] = await queue[currentIndex]();
        }
    };

	for (let i = 0; i < concurrency; i++) {
		threads.push(execThread());
	}

    await Promise.all(threads);
    
    return results;
};

const items = [new Array(1000)];

const queue = items.map((item, index) => {
	return async () => {
		try {
			console.log(`Processing item ${index}`);

			return item;
		} catch(err) {
			return {
				error: err.message
			};
		}
	};
}

const results = await worker(queue, 25);