Node.js used Claudia API Gateways between a browser request and AWS Lambda functions. It also discussed various services and routines to route event handlers.
As a note, XMLHttpRequest can also be sent/received either synchronous or asynhronous manner. Have a at code as below.
Similiarly, used to get server response as stream.
Claudia API Builder is just a Node module that does simple routing and few other necessary things, but the layer between browser and your Lambda function is Amazon API Gateway. It listens to HTTP requests and then triggers your AWS Lambda function synchronously (it waits for the reply). But it also does much more, for example, auto-scaling, security, authorization, etc.
It's not like AJAX XMLHttpRequest, as that one is used to invoke HTTP request asynchronously on the front end side. API Gateway is on the backend, and it invokes your Lambda function synchronously.
Second chapter of our book talks a bit more about API Gateway integration.