why( str === undefined) did not return anything

function hey(str) {
    for (let char of str){
        if (str.slice(-1) !== "?" && str === str.toUpperCase() && str !== " "){
            return 'Whoa, chill out!';
        }
        else if (str.slice(-1) === "?" && str === str.toUpperCase()){
            return "Calm down, I know what I'm doing!";
        }
        else if(str.slice(-1) === "?" && str !== str.toUpperCase()){
            return "Sure.";
        }
        else if (str === " " || str === undefined){
            return "Fine. Be that way!";
        }
        else {
          return 'Whatever.';
        }
    }
}

hey('');

link

Bob Bob is a lackadaisical teenager. In conversation, his responses are very limited.

Bob answers 'Sure.' if you ask him a question.

He answers 'Whoa, chill out!' if you yell at him.

He answers 'Calm down, I know what I'm doing!' if you yell a question at him.

He says 'Fine. Be that way!' if you address him without actually saying anything.

He answers 'Whatever.' to anything else.