It says that maximum call size stack exceeded, what am i missing?

Tell us what’s happening:

Your code so far

// Setup
function abTest(a, b) {
  // Only change code below this line
   abTest(a, b < 0) 
    return "undefined"
  // Only change code above this line

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

// Change values below to test your code

Your browser information:

User Agent is: Mozilla/5.0 (Windows NT 6.1; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/77.0.3865.90 Safari/537.36.

Link to the challenge:

What is your reasoning behind this line?

if a or b is less than 0 then the return is undefined

I don’t see an if statement in your code making such a comparison.

ok, here is the updated input:

abTest(a, b < 0) // If a or b is less than 0, then it is undefined
return “undefined”

it doesn’t seem you changed anything there.
that is still a function call…
a function calling itself without a condition that stop the thing, it will cause the stack overflow.
that’s not the way to solve this challenge