textFamily property of span problem

Hello,
My problem is that I don't understand why the fontFamily property of the SPAN object doesn't accept a method that returns an array, but if the fontFamily property is passed an array directly, everything works. To make a long story short. Here are two cases:

objA = {
	text: "Hello, World",
	alignment: "left",
	fontFamily: ["Helvetica", "sans-serif"]
};
getField("myTextFieldName").richValue = [objA] // WORKS!


objA = {
	text: "Hello, World",
	alignment: "left",
	fontFamily: function(){return ["Helvetica", "sans-serif"]}
};
getField("myTextFieldName").richValue = [objA] // NOT WORKING!

I need to randomly generate a font type in a form based on a button press. Therefore I need to pass the fontFamily property the result of the function, unfortunately the console doesn't tell me anything and I can't find why it doesn't work. The text appears, other properties like alignment and textSize.. too, but fontFamily does not.

var objSpan = {
	text: "Hello, World",
	alignment: "left",
	textColor: ["RGB", 0/255, 0/255, 0/255],
	textSize: 12,
	linespacing: 20,
	fontFamily: 	function(){
					var randomFont = [
						["Calibri", "sans-serif"], //1
						["Helvetica", "sans-serif"], //2
						["Myriad Pro", "sans-serif"], //3
						["Adobe Caslon Pro", "serif"], //4
						["Adobe Garamond Pro", "serif"], //5
						["Arno Pro", "serif"], //6
						["Arno Pro SmText", "serif"], //7
						["Century", "serif"], //8
						["Corbel", "sans-serif"], //9
						["Georgia", "serif"], //10
						["Gill Sans MT", "sans-serif"], //11
						["Chaparral Pro", "serif"], //12
						["Segoe UI", "sans-serif"], //13
						["Times New Roman", "serif"] //14
					];
					var fo1 = Math.random()*(randomFont.length);//fo1 je náhodné reálné číslo v intervalu <0;14)
					var fo2 = Math.floor(fo1);// fo2 is a natural number from the interval <0;13>
					return randomFont[fo1]// returns the 0th to 13th element of the array randomFont
					},

	fontWeight: 400,
	fontStyle: "normal",
	fontStretch: "normal"
};
getField("myTextFieldName").richValue = [objSpan];

Thanks to much!