RPS.js

var
    choices = ['rock','paper','scissors'], // Classic version
    // choices = ['cockroach','foot','nuke'], // Foot crushes cockroach, nuke blows up foot, cockroach survives nuke.
    // choices = ['rock','paper','scissors','spock','lizard'], // Rock-Paper-Scissor-Spock-Lizard version
    choicesCount = choices.length,
    promptString = (function(choices){
        var string = '',
            c = choicesCount - 1;
        for (var i = 0; i <= c; i++) {
            string += (i === 0) ? capitalize(choices[i]) : choices[i];
            string += (i === (c - 1)) ? ' or ' : (i != c) ? ', ' : '';
        }
        return string + '?';
    })(choices),
    gamesCount = prompt('Best of?', 3) * 1 || 1,
    humanChoiceIndex = -1,
    computerChoiceIndex = -1;

for (var i = gamesCount - 1; i >= 0; i--) {
    while (choices[humanChoiceIndex] === undefined) {
        humanChoiceIndex = choices.indexOf(prompt(promptString).toLowerCase());
        if (choices[humanChoiceIndex] === undefined) {
            alert('Not a valid choice. Try again.');
        }
    }

    computerChoiceIndex = Math.floor(Math.random() * choicesCount);

    document.write('<div>' + compare( choices[humanChoiceIndex], choices[computerChoiceIndex] ) + '</div>');

    humanChoiceIndex = -1;
    computerChoiceIndex = -1;
}

function compare( humanChoice, computerChoice ) {

    var humanChoiceIndex = choices.indexOf(humanChoice),
        computerChoiceIndex = choices.indexOf(computerChoice),
        // I wish I could take credit for this logic:
        // http://stackoverflow.com/questions/9553058/scalable-solution-for-rock-paper-scissor#9553712
        winner = (choicesCount + humanChoiceIndex - computerChoiceIndex) % choicesCount;

    if (winner){
        return (winner % 2) ?
            'You Win! <b>' +  capitalize(humanChoice) + '</b> beats ' + computerChoice + '.'
                :
            'Computer Wins! ' +  capitalize(computerChoice) + ' beats <b>' + humanChoice + '</b>.';
    }
    else{
        return 'Tie. <b>' +  capitalize(humanChoice) + '</b> equals ' + computerChoice + '.';
    }
}

function capitalize(string) {
    return string.charAt(0).toUpperCase() + string.slice(1);
}