Let me give you an example from the IDE source code, Check this expression: https://github.com/firejs/fire-ide/blob/master/FireIde.Core....

You will see how I check if the file "package.json" exists, if it exists then I parse the content of the file. If it doesn't exists then I return false.

Because I didn't use javascript then I didn't have to write any callbacks to achieve the behavior. It's a lot simplier with JSONScript.