Reference javascript variable in a field name script

I've built an Acroform in Acrobat DC Pro. There are two linked dropdown lists, the 2nd (Subcategory) dependent on the 1st (Category). A javascript defines both the first list's items and the corresponding list's items as well. Here's the script, in the Custom Format Script of the field Category:

var dependentListBoxFieldName = "Subcategory";
var dependentListValues = 
{
	"Computer": [
		["-Select-", "-Select-"],
		["Desktop", "Desktop"],
		["Laptop", "Laptop"],
		["Smart phone", "Smart phone"],
		["Tablet", "Tablet"],
		["Other", "Other"] 
	],
	"Hardware": [
		["-Select-", "-Select-"],
		["Headset", "Headset"],
		["Keyboard", "Keyboard"],
		["Monitor", "Monitor"],
		["Mouse", "Mouse"],
		["Printer", "Printer"],
		["Scanner", "Scanner"],
		["Other", "Other"] 
	],
	"Software": [
		["-Select-", "-Select-"],
		["AV app", "AV app"],
		["Browser", "Browser"],
		["Email app", "Email app"],
		["Media app", "Media app"],
		["Office app", "Office app"],
		["Social app", "Social app"],
		["Utility", "Utility"],
		["Other", "Other"] 
	],
};
if ((event.target.type == "combobox" && event.name == "Format") || (event.target.type == "listbox" && event.name == "Keystroke")) {
	if (event.target.type == "combobox") {
		if (dependentListValues.hasOwnProperty(event.target.value)) {
this.getField(dependentListBoxFieldName).setItems(dependentListValues[event.target.value]);
		}
		else {
			this.getField(dependentListBoxFieldName).clearItems();
		}
	}
	if (event.target.type == "listbox" && dependentListValues.hasOwnProperty(event.changeEx)) {
this.getField(dependentListBoxFieldName).setItems(dependentListValues[event.changeEx]);	
	}
}
else {
	app.alert("This script was not intended for this field type or event.");	
}

The issue I have is when the form is re-created for additional data entry. A button spawns a Form template and the hard-coded list name breaks because the field is renamed (a.spawn(numPages, true, false);). I have been able to determine the page number to get the new field name when the user clicks in the first text field of the form. This setup as a javascript run when the 1st text field gets focus:

var currentPage = this.pageNum;
var newSub = this.getField("P" + currentPage + ".Form." + "Subcategory");

And I wind up with something like P1.Form.Subcategory. But how do I code the result into the P1.Form.Category script on this page  (and subsequent pages)?

Does the line var dependentListBoxFieldName = "Subcategory"; become var dependentListBoxFieldName = "newSub"; because that doesn't seem to work. I get an error in the console: 

TypeError: this.getField(...) is null
229:AcroForm:P1.Form.Category:Format

That points to this line: this.getField(dependentListBoxFieldName), which is newSub. 

I'm assuming my syntax is wrong - how do I reference the output of the newSub variable into the Category script? Should it be referenced as a string somehow?

Thanks,

Laner61