JavaScript Rock Paper Scissors

function rps() {
    if (!confirm('Do you want to play Rock Paper Scissors?'))
    return;
    playRPS();
    function playRPS() {
        var userChoice = prompt('Choose Rock, Paper, or Scissors!\n\nYou can shorten it to \"R\",\"P\", or \"S\"\nCapitals don\'t matter.')
        if (userChoice == null)
        return;
        userChoice = userChoice.toLowerCase();
        switch (Math.round(Math.random() * (3 - 1) + 1)) {
            case 1:
                var compChoice = "rock";
                break;
            case 2:
                var compChoice = "paper";
                break;
            case 3:
                var compChoice = "scissors";
                break;
        }
        switch (userChoice) {
            case 'r':
                userChoice = 'rock';
                break;
            case 'p':
                userChoice = 'paper';
                break;
            case 's':
                userChoice = 'scissors'
                break;
        }
        if (userChoice == 'rock') {
            switch (compChoice) {
                case 'rock':
                    if (!confirm('It was a tie!\nBoth opponents chose Rock.\n\nPlay again?'))
                    return;
                    playRPS();
                    break;
                case 'paper':
                    if (!confirm('You lost!\nThe opponent chose Paper, while you chose Rock.\n\nPlay again?'))
                    return;
                    playRPS();
                    break;
                case 'scissors':
                    if (!confirm('You won!\nThe opponent chose Scissors, while you chose Rock.\n\nPlay again?'))
                    return;
                    playRPS();
                    break;
            }
        } else if (userChoice == 'paper') {
            switch (compChoice) {
                case 'rock':
                    if (!confirm('You won!\nThe opponent chose Rock, while you chose Paper.\n\nPlay again?'))
                    return;
                    playRPS();
                    break;
                case 'paper':
                    if (!confirm('It was a tie!\nBoth opponents chose Paper.\n\nPlay again?'))
                    return;
                    playRPS();
                    break;
                case 'scissors':
                    if (!confirm('You lost!\nThe opponent chose Scissors, while you chose Paper.\n\nPlay again?'))
                    return;
                    playRPS();
                    break;
            }
        } else if (userChoice == 'scissors') {
            switch (compChoice) {
                case 'rock':
                    if (!confirm('You lost!\nThe opponent chose Rock, while you chose Scissors.\n\nPlay again?'))
                    return;
                    playRPS();
                    break;
                case 'paper':
                    if (!confirm('You won!\nThe opponent chose Paper, while you chose Scissors.\n\nPlay again?'))
                    return;
                    playRPS();
                    break;
                case 'scissors':
                    if (!confirm('It was a tie!\nBoth opponents chose Scissors.\n\nPlay again?'))
                    return;
                    playRPS();
                    break;
            }
        } else {
            alert('Invalid input!\nPlease type one of the valid choices.');
            return;
        }
    }
}
rps();