Implement a Sequential API Request Queue in JavaScript

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

    /**
     * Adds a request function to the queue.
     * The request function should return a Promise.
     * @param {Function} requestFn - A function that returns a Promise for an API call.
     * @returns {Promise} - A promise that resolves with the result of the request function.
     */
    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 {
            this.isProcessing = false;
            // Process the next item in the queue
            this.processQueue();
        }
    }
}

// Example Usage:
const requestQueue = new ApiRequestQueue();

// Simulate API calls
const makeApiCall = (id, delay) => {
    return () => new Promise(resolve => {
        setTimeout(() => {
            console.log(`API Call ${id} completed after ${delay}ms`);
            resolve(`Result from API Call ${id}`);
        }, delay);
    });
};

// Add requests to the queue
console.log('All requests added to queue. Processing sequentially...');
requestQueue.addRequest(makeApiCall(1, 1000))
    .then(result => console.log('Handler for Call 1:', result))
    .catch(error => console.error('Error for Call 1:', error));

requestQueue.addRequest(makeApiCall(2, 500)) // This will wait for call 1
    .then(result => console.log('Handler for Call 2:', result))
    .catch(error => console.error('Error for Call 2:', error));

requestQueue.addRequest(makeApiCall(3, 1500)) // This will wait for call 2
    .then(result => console.log('Handler for Call 3:', result))
    .catch(error => console.error('Error for Call 3:', error));