This won't really work, because if the function itself blocks (ie. it uses blocking APIs), then this will too.

Why would this not work? The function won't be called until everything on the event loop is dealt with, right?

Yes, but imagine this scenario:

setTimeout(() => console.log('It\'s been 1 second!'), 1000);
exec('sleep 10; echo "It\'s been 10 seconds!"', function noop() {}, { async: true });

Outputs:

It's been 10 seconds.
It's been 1 second.

@ariporad I just tried this example. I saw:

It's been 1 second!
It's been 10 seconds!

Also, exec()'s async version is totally different than the rest of the commands.

Another example:

> setTimeout(() => console.log('It\'s been 1 second!'), 5000);
> ls({async: true}, function() { echo('hi'); });

Output (as I would expect, since ls() is super fast) :

hi
It's been 1 second!

@nfischer: Whoops, my bad.

Try this (I know it's unrealistic times, but for argument's sake):

function sleep(time) {
    var start = Date.now()
    while (start + time > Date.now()) ;
};

setTimeout(() => console.log('1 sec'), 1000);
sleep(2000);
console.log('2 sec');

Expected:

1 sec
2 sec

Actual:

2 sec
1 sec

@ariporad your last example uses no shelljs. the actual output is what a JS programmer ought expect: you have to yield execution.