Wait for async task to finish

This will never work, because the JS VM has moved on from that async_call and returned the value, which you haven’t set yet.

Don’t try to fight what is natural and built-in the language behaviour. You should use a callback technique or a promise.

function f(input, callback) {
    var value;

    // Assume the async call always succeed
    async_call(input, function(result) { callback(result) };

}

The other option is to use a promise, have a look at Q. This way you return a promise, and then you attach a then listener to it, which is basically the same as a callback. When the promise resolves, the then will trigger.