How do I get javascript to return messages using if/else?

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 is maximum range - I'm not sure how to code this yet\\
             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.";
        }
    }    
}