Currency Exchange Calculator

I'll help you build a comprehensive currency converter! Let's start with the HTML structure featuring dual currency selectors and input fields, then implement JavaScript to fetch real-time rates from an API like ExchangeRate-API or Fixer.io. We'll use localStorage for conversion history and favorites, Chart.js for historical rate visualization, and service workers for offline caching. I'll include a calculator module, rate alert system using the Notification API, and export functions leveraging jsPDF and PapaParse libraries.

This is quite an ambitious project with multiple interconnected features. I'd recommend building this incrementally: start with core conversion functionality using a free tier API (ExchangeRate-API offers 1,500 requests/month), then layer in history tracking and favorites. Some concerns: rate alerts with notifications require user permissions and persistent background checks, and comparing multiple provider rates may be limited by API costs. I can provide starter code for the MVP, but note that full cryptocurrency support and multi-provider comparison significantly increase complexity.

Here's a modular approach using the ExchangeRate-API (free tier supports 170+ currencies). The interface uses CSS Grid for responsive layout, Fetch API with error handling for rate retrieval, and IndexedDB for storing history and cached rates. For charts, integrate Chart.js with historical data endpoints. Service workers enable offline mode by serving cached rates with timestamps. Calculator functionality uses JavaScript's eval() alternative (Function constructor) for security. Export features leverage browser APIs: window.print() for PDF, Blob API for CSV/JSON downloads.