Rock paper scissors game with javascript

rockpaperscissorsgame

var compare = function(userChoice, computerChoice) {
    if(userChoice == 'R') {
        if(computerChoice == 'S') {
            return 1;
        }
        else if(computerChoice == 'P') {
            return 2;
        }
        else {
            return 0;
        }
    }
    else if(userChoice == 'S') {
        if(computerChoice == 'P') {
            return 1;
        }
        else if(computerChoice == 'R') {
            return 2;
        }
        else {
            return 0;
        }
    }
    else {
        if(computerChoice == 'R') {
            return 1;
        }
        else if(computerChoice == 'S') {
            return 2;
        }
        else {
            return 0;
        }
    }
}

var takeComputerChoice = function() {
    var random = Math.floor((Math.random()*3)+1);
    if(random == 1) {
        return 'R';
    }
    else if(random == 2) {
      return 'P';  
    }
    else {
        return 'S';
    }
}

var takeUserChoice = function() {
    var choice = prompt("Rock(R) / Paper(P) / Scissors(S) / Exit(E)");
    if(choice == 'R' || choice == 'P' || choice == 'S' || choice == 'E') {
        return choice;
    }
    else {
        alert("You must write R or P or S...");
        return 'X';
    }
}

var rockPaperScissorsGame = function() {
    var userChoice = 'O';
    var computerChoice = 'O';
    while (userChoice != 'E'){
        userChoice = takeUserChoice();
        if(userChoice != 'X' && userChoice != 'E') {
            computerChoice = takeComputerChoice();
            var result = compare(userChoice, computerChoice);
            if(result == 1) {
                alert("User wins...");
            }
            else if(result == 2) {
                alert("Computer wins...");
            }
            else {
                alert("Draw...");
            }
        }
    } 
    alert("game is over, thank you");
}

rockPaperScissorsGame();