Manage API Rate Limits with a Client-Side Request Queue

class ApiRequestQueue {
    constructor(delayMs = 1000) {
        this.queue = [];
        this.delayMs = delayMs;
        this.isProcessing = false;
    }

    addRequest(requestFn) {
        return new Promise((resolve, reject) => {
            this.queue.push({ requestFn, resolve, reject });
            this.processQueue();
        });
    }

    async processQueue() {
        if (this.isProcessing || this.queue.length === 0) {
            return;
        }

        this.isProcessing = true;
        const { requestFn, resolve, reject } = this.queue.shift();

        try {
            const result = await requestFn();
            resolve(result);
        } catch (error) {
            reject(error);
        } finally {
            // Wait for the delay before processing the next request
            await new Promise(res => setTimeout(res, this.delayMs));
            this.isProcessing = false;
            this.processQueue(); // Process next request
        }
    }
}

// --- Usage Example ---
const apiQueue = new ApiRequestQueue(500); // 500ms delay between requests

async function fetchUser(userId) {
    console.log(`Fetching user ${userId}...`);
    // Simulate an API call
    return new Promise(resolve => setTimeout(() => {
        resolve({ id: userId, name: `User ${userId}` });
    }, Math.random() * 300 + 100)); // Simulate variable API response time
}

async function fetchDataWithQueue() {
    try {
        const user1 = await apiQueue.addRequest(() => fetchUser(1));
        console.log('User 1 data:', user1);

        const user2 = await apiQueue.addRequest(() => fetchUser(2));
        console.log('User 2 data:', user2);

        // Even if called immediately, requests will be processed with a delay
        const user3 = await apiQueue.addRequest(() => fetchUser(3));
        console.log('User 3 data:', user3);

        console.log('All users fetched via queue.');
    } catch (error) {
        console.error('Error fetching data:', error);
    }
}

fetchDataWithQueue();