Javascript - if with asynchronous case

My question is a bit regards concept.

A lot of times there is this such situation:

if(something){
    someAsyncAction();
}else{
    someSyncAction();
}

// Continue with the rest of code..
var a = 5;

The problem with this such case is clear, i don't want the var a = 5 to be call unless someAsyncAction() or someSyncAction() will done, now, cause soAsyncAction() is asynchronous the only way (i can think of) to solve this situation is something like that:

var after = function(){
    // Continue with the rest of code..
    var a = 5;
}

if(something){
    someAsyncAction(after);
}else{
    someSyncAction();
    after ();
}

BUT, this code is ugly, hard to read and looks like anti-pattern and problematic.

I trying to think maybe i can find some solution for that with Promises (using Bluebird at the backend) but can't find something.

Is anyone faced this before and can help me figure it out?

Thanks!