JS Async Semaphore / Limit Concurrency

semaphore.js

function semaphore(n) {
    let running = 0;
    const queue = [];
    return (fn) => {
        return new Promise((resolve) => {
            const task = () => {
                running++;
                resolve(
                    fn().finally(() => {
                        running--;
                        if (queue.length > 0) {
                            queue.shift()();
                        }
                    }),
                );
            };
            if (running < n) {
                task();
            } else {
                queue.push(task);
            }
        });
    };
}

// Example:
const sem = semaphore(3);
function sleep(ms) {
    return new Promise((resolve) => setTimeout(resolve, ms));
}
sem(() => sleep(3000)).then(() => console.log("Task 1 done"));
sem(() => sleep(3000)).then(() => console.log("Task 2 done"));
sem(() => sleep(3000)).then(() => console.log("Task 3 done"));
sem(() => sleep(3000)).then(() => console.log("Task 4 done"));
sem(() => sleep(3000)).then(() => console.log("Task 5 done"));