queue.js

const max_pending_promises = 3

export default class Queue {
	static queue = [];
	static nb_pending_promises = 0;

	static enqueue (promise) {
		return new Promise((resolve, reject) => {
			this.queue.push({
				promise,
				resolve,
				reject,
			});
			this.dequeue();
		});
	}

	static dequeue () {
		// If max pending promises is reached, return
		if (this.nb_pending_promises >= max_pending_promises) {
			return false;
		}

		const item = this.queue.shift();
		// If all promises are done, return
		if (!item) {
			return false;
		}

		// Try to perform the next promise
		try {
			this.nb_pending_promises++;
			item.promise()
				.then((value) => {
					item.resolve(value);
				})
				.catch(err => {
					item.reject(err);
				})
		} catch (err) {
			item.reject(err);
		} finally {
			// In all cases: decrement and try to perform the next promise
			this.nb_pending_promises--;
			this.dequeue();
		}
		return true;
	}
}