Document title can't show a 0 / function fillin

The thing about numbers is that they don't start with 0. Only strings can start with a "0" character.  A number is just a number. So you have to be careful about how you think about number representation and how strings are built from numbers. 

How does you create the document title?

function fillin(s1, s2, s3, sep = "") {

/*

Purpose: concatenate up to 3 strings with an optional separator

inputs:

s1: required input string text or empty string

s2: required input string text or empty string

s3: required input string text or empty string

sep: optional separator sting

returns:

sResult concatenated string

*/

// variable to determine how to concatenate the strings

var test = 0; // all strings null

var sResult; // reslut string to return

// force any number string to a character string for input variables

s1 = s1.toString();

s2 = s2.toString();

s3 = s3.toString();

//if(sep.toString() == undefined) sep = ''; // if sep is undefined force to null

/*

assign a binary value for each string present

so the computed value of the strings will indicate which strings are present

when converted to a binary value

*/

if (s1 != "") test += 1; // string 1 present add binary value: 001

if (s2 != "") test += 2; // string 2 present add binary value: 010

if (s3 != "") test += 4; // string 3 present add binary value: 100

/* return appropriate string combination based on

calculated test value as a binary value

*/

switch (test.toString(2)) {

case "0": // no non-empty strings passed - binary 0

sResult = "";

break;

case "1": // only string 1 present - binary 1

sResult = "Bruikleen" + sep + s1;

break;

case "10": // only string 2 present - binary 10

sResult = "Bruikleen" + sep + s2;

break;

case "11": // string 1 and 2 present - binary 10 + 1

sResult = "Bruikleen" + sep + s1 + sep + s2;

break;

case "100": // only string 3 present - binary 100

sResult = "Bruikleen" + sep + s3;

break;

case "101": // string 1 and 3 - binary 100 + 001

sResult = "Bruikleen" + sep + s1 + sep + s3;

break;

case "110": // string 2 and 3 - binary 100 + 010

sResult = "Bruikleen" + sep + s2 + sep + s3;

break;

case "111": // all 3 strings - binary 100 + 010 + 001

sResult = "Bruikleen" + sep + s1 + sep + s2 + sep + s3;

break;

default: // any missed combinations

sResult = "";

break;

}

return sResult;

}

the script