JavaScript Guessing Game

function guessinggame() {
    if (!confirm('Do you want to play a number guessing game?'))
        return;
    var minnum,
        maxnum,
        totalguesses = 0,
        guessnumber,
        guess = 0;
    definevalues();

    function definevalues() {
        minnum = prompt('Please input the smallest possible number.');
        if (minnum == null)
            return;
        if (Number.isInteger(Number(minnum))) {
            maxnum = prompt('Please input the largest possible number.');
            if (maxnum == null)
                return;
            if (Number.isInteger(Number(maxnum))) {
                if (Number.isSafeInteger(Number(minnum)) && Number.isSafeInteger(Number(maxnum))) {
                    guessnumber = Math.round(Math.random() * (maxnum - minnum) + minnum);
                    totalguesses = 0,
                        guess = '';
                    startgame();
                } else {
                    alert(`Integer Overflow!\n\nPlease keep your 'Smallest possible number' over ${Number.MIN_SAFE_INTEGER} and your 'Largest possible number' under ${Number.MAX_SAFE_INTEGER}`);
                }
            } else {
                alert('Invalid Input!\nPlease type a whole number.')
            }

        } else {
            alert('Invalid Input!\nPlease type a whole number.');
        }
    }


    function startgame() {
        totalguesses += 1
        guess = prompt(`The number is between ${minnum} and ${maxnum}\n\nTry to guess the number!`, guess);
        if (guess == '\x64\x65\x76') {
            totalguesses -= 1;
            guess = guessnumber;
            startgame();
            return;
        }
        if (guess == guessnumber) {
            if (guess == null)
                return;
            if (totalguesses == 1) {
                var playagainornah = confirm(`Congrats! You have guessed the number in 1 guess!\nThat's some real luck!\n\nThe number was ${guessnumber}.\nDo you want to play again?`);
                if (!playagainornah)
                    return;
                totalguesses = 0
                definevalues();
            } else {
                var playagainornah = confirm(`Congrats! You have guess the number in ${totalguesses} guesses!\n\nThe number was ${guessnumber}.\nDo you want to play again?`);
                if (!playagainornah)
                    return;
                totalguesses = 0
                definevalues();
            }

        } else if (guess > guessnumber) {
            alert('Smaller!');
            startgame();
            return;
        } else if (guess < guessnumber) {
            alert('Bigger!');
            startgame();
            return;
        } else {
            alert('Invalid Input!\nIf you are trying to make a guess with numbers in a word form, please type the numbers in number form.');
            totalguesses -= 1
            startgame();
            return;
        }
    }

}
guessinggame();