Testing JavaScript allways giving me it is not a function

i got task to do, as I'm new in the academy.

I have a function, which is working, but during my test, it is allways saying: ● Test if it is right

  ReferenceError: splitString is not defined

      13 |         'Test if it is right',
      14 |     (fullName, splitString) => {
    > 15 |         expect(testSamoUloha(fullName)).toBe(splitString);
         |                                              ^
      16 |     }
      17 | );
      18 | 

My question is,what parameter should I put into toBe(function).

Here is my test :

const testSamoUloha = require('../samostatnaUloha.js');

    test("Should return a full name, where last name is anonymized by *", () => {
    expect(testSamoUloha(fullName)).toBe('string');
    })

    test.each([["Richard Tezbir","Richard T*****"],
    ["Richard Tezbir","Richard T*****"],
    ["Slavomir Slovenkai","Slavomir S********"],
    ["Marek Mihok","Marek M****"],
    ["Maros Lukac","Maros L****"],
    ["Bernadet Molnarova","Bernadet M********"]])(
        'Test if it is right',
    (fullName, splitString) => {
        expect(testSamoUloha(fullName)).toBe(splitString);
    }
);

All others possibilities I have tried to put into toBe, it was saying (variable) is not a function. And here is my function:

const fullName = "Jozko Baci";

function testSamoUloha(fullName) {

  var splitString = fullName.split(' ');
  var tempString = '';

  for (var i = 0; i < splitString[1].length; i++) {
    if (i > 0) {
      tempString += '*';

      tempString += splitString[1][i];
    }
  }
  return splitString[0] + ' ' + tempString;
}
testSamoUloha(fullName);

module.exports = testSamoUloha;

How could I get this done ?