Math browser game with prompts and alert to guess user's number

digit_game.js

var startGame = function() {
    alert("Welcome to the Guess Your Digit Game!");
    var startOrNot = confirm("Would you like to start?");
    if (startOrNot === true) {
        alert("Okay, let's have you start by choosing a 4 digit number.  Click OK when you've written it down.");
        alert("Now, you're going to add all the digits of your number together and record it.");
        alert("Next, subtract your new sum from the original number.  Press OK when ready.");
        var numberInput = prompt("Finally, remove a digit from this number that's not a zero " +
            "- I'm going to try to guess what the digit you removed is.  I'll need one hint though, what are" +
            "the 3 digits that are remaining in your number?");
    }
    function findDigitRemoved() {

        var sum = 0;
        for (var i = 0; i < numberInput.length; i++) {
            sum += parseInt(numberInput.charAt(i), 10);
        }
        var findMultNine = function() {
            var multNine = sum;
            while (multNine % 9 !== 0 && multNine !== 0) {
                multNine++;
            }
            return multNine;
        };

        var digitRemoved = findMultNine() - sum;
            console.log("The digitRemoved var is equal to" + digitRemoved);
            console.log("The sum variable is equal to" + sum);
            console.log(findMultNine() + "is the multNine variable");
        return digitRemoved;

    }

    var endGame = confirm("Hmmm.  The digit that you removed was " + findDigitRemoved() + ".  Was I right?" +
        " Press OK to play again.");

    if (endGame === true) {
        startGame();
    } else
        alert("Thanks for playing!");

};
startGame();