What is an API Gateway, and what role does it play in front of a collection of Node.js microservices?

An API Gateway acts as a single entry point for external clients, routing incoming requests to the appropriate backend microservice, while also centralizing cross-cutting concerns like authentication, rate limiting, request/response transformation, and aggregating responses from multiple services into a single response tailored for a specific client (like a mobile app needing a combined payload from three separate services) -- sparing individual microservices and external clients from needing to handle all of this routing and cross-cutting logic themselves.

// Simplified API Gateway routing logic
app.use('/api/users', authenticate, proxy('http://user-service:3001'));
app.use('/api/orders', authenticate, proxy('http://order-service:3002'));
app.use('/api/products', proxy('http://product-service:3003')); // no auth needed for public data