In Javascript, async is an optional parameter on the caller side. (Well, 'await' is, but I figure that's what you mean). It can be used with any function call,

In Javascript, async is an optional parameter on the caller side. (Well, 'await' is, but I figure that's what you mean). It can be used with any function call, not just those that are marked async. Though it only make sense to use it with Promise-returning functions. Alternatively, you can also use it with a plain Promise, which makes for a nice mutex. I've also used this for imitating async constructors, which is not currently possible.