IO in what? The browser? Node? If you mean node, what IO specifically? `http.createServer`? `fs.readFileSync`? `fs.createReadStream`? `fs.open`? `fs.readFile` w

IO in what? The browser? Node? If you mean node, what IO specifically? `http.createServer`? `fs.readFileSync`? `fs.createReadStream`? `fs.open`? `fs.readFile` with node callbacks? Promises? Streams? Observables? Without context it sounds like you saw a bunch of nested callbacks in a Node.js example and judge the entire language based on that.