You can perform file I/O in JavaScript synchronously. The localStorage API in the browser is synchronous (and in Node via the --experimental-webstorage option), and of course requiring a CommonJS module is also synchronous (and there are many other sync filesystem APIs in Node as a sibling comment pointed out).

You just can't perform network I/O synchronously. Although a network attached file system allows for both network and file I/O technically, but that's a really pedantic point.