Logic concerning negative num inputs being converted to positive num inputs

Maybe I'm asking google all the wrong questions but I scoured the internet and could not find posts similar to my unique problem.

This is a simple coding challenge from FreeCodeCamp: I need to work on the function abTest(a,b) where when a OR b is less than 0, the function will exit with the value of undefined. This is because we run the value a and b through a square root function. And you cannot take the square root of a negative number. Below is the code.

// Setup
function abTest(a, b) {
 
  if (a < 0) {
    a = -a;
   return undefined;
} else if (b < 0) {
    b = -b;
    return undefined;
}

  return Math.round(Math.pow(Math.sqrt(a) + Math.sqrt(b), 2));
}

// Change values below to test your code
abTest(2,2);

This code I used above works fine. However I have to specify that when a or b is < 0, a is assigned -a and b is assigned -b. Why is this necessary? In addition I want to convert this into a Switch statement, but I run into this problem of negative num values. Below are two switch statements I wrote but when using negative num input the abTest(-2,2) or abTest(2,-2) does not return the value undefined.

My First Switch Statement

switch (a,b) {
    case a<0 || b<0:
      return undefined;
  }
  

My Second Switch Statement

  switch (a,b) {
    case a < 0: 
      a = -a;
      console.log(undefined);
      break;
    case b < 0: 
      b = -b;
      console.log(undefined);
      break;
  }

Where is this hole in my logic concerning switch statements and negative num values?

Thank you guys,