Javascript won't display message

I'm trying to create a simple program where a user needs to pick a maximum number, then try to guess a number between 1 and that maximum number. When they enter their guess, I need to validate the input, and offer a choice of feedback: different messages if it's (1) not a number, (2) a number not within the range, (3) the correct guess, (4) too high, but within range, and (5) too low, but within range.

function do_guess(prompt) {
    let valid_input = false;
    let guess = Number(document.getElementById("guess").value);

    let message = document.getElementById("message");

    while(!valid_input) {
        input = window.prompt(prompt);

        guess = Number(input);
    
        if (guess == NaN) {
            message.innerHTML = "That is not a number!";
        }
        else if (guess < 1 || guess > val) { \\val = max_num - I wasn't sure how to code this.
             message.innerHTML = "That number is not in the range, try again.";
        }
        else if (guess == num) {
            valid_input = true;
            message.innerHTML = "You got it!";
        }
         else if (guess > num) {
            message.innerHTML = "No, try a lower number.";
        }
        else {
             message.innerHTML = "No, try a higher number.";
        }
    }    
}