Here is our example database:
As you see we can define functions with as many parameters as needed. Then you enter the body of the function with whatever code you have in your custom functions. We got a button for macOS to format the calculation with our Syntaxcoloring functions. Then we got a button to check errors, which will let FileMaker parse your code with the parameter definition included, so we can find syntax errors. Sadly we can't tell where the error is exactly, but we may tell you if the syntax is okay or what error code we got.
Our start script set ups a global dictionary named "CF" in our example using Dictionary.CreateNamed function. Then you add functions there with Dictionary.SetValueForKeyone by one or with Dictionary.AddSQL function as a block. Each function would have comments in front to name the parameters. We use "/// " & $name there, e.g. "/// value".
Then later you can call the custom function with our FM.CF function. You pass the dictionary identifier, "CF" in our example. Then you pass the name of the function (case sensitive) and the parameters. As many parameters as you need, so you may define your function with 10 parameters, but only pass 2 or 3 depending on what you do.
The dictionary is global for all files and all solutions opened in FileMaker and can be initialized in FileMaker Pro, Server (Scripting + Web Direct) as well as FileMaker iOS SDK based applications. Since we use a dictionary identifier, you can have different sets of custom functions in different namespaces.
Please try the JS.CF function with MBS FileMaker Plugin in version 11.3pr3 or later.