They don't block the event loop, no. But they do remove callbacks and thus asynchronity from libraries. i.e aws-sdk S3.putObject(..., function callback(r

They don't block the event loop, no. But they do remove callbacks and thus asynchronity from libraries. i.e aws-sdk S3.putObject(..., function callback(result){}) would become (with _wrapAsync): result = S3.putObjectSync(...) In the below example: var x = 5; x = S3.putObjectSync(...); console.log(x); The console will never log "5". That is what I mean... maybe I'm confusing the terminology though :\