if statement not evaluating but console log shows it should

I am trying to write a function to get the length of a string without using the .length property nor any loops or built in methods. I am using recursion, however, the stopping statement is not evaluating and causing a stack overflow. I tried console logging string[i] and sure enough, once the length is reached, it console logs "undefined" but the if statement still won’t evaluate.

const getLength = (string, i = 0) => {
    if (string[i] === 'undefined') return 0;
    return 1 + getLength(string, i+1);
}

console.log(getLength("what is going on??")); //18

>Solution :

the if-state should be string[i] === undefined