paperscissorsrock.js

var outcomes = ['rock', 'scissors', 'paper'];

// Creates random outcome
var randomOutcome = function () {
  return outcomes[Math.floor(Math.random() * outcomes.length)];
};

// Init prompter (ask user for answer)
var prompter = function () {
    var answer = prompt('Select your outcome');
    return (outcomes.indexOf(answer) !== -1) ? answer : prompter();
};

// Get user answer
var userChoice = prompter();

// Generate computers answer
var computerChoice = randomOutcome();

var userChoiceIndex = outcomes.indexOf(userChoice);
var computerChoiceIndex = outcomes.indexOf(computerChoice);

console.log('Your choice: '+ userChoice);
console.log('Computer choice: ' + computerChoice);

if (userChoiceIndex === computerChoiceIndex) {
    console.log('Draw');
}
else if (outcomes.splice(userChoiceIndex - 1, 1)[0] === computerChoice) {
    console.log('Computer wins')    
}
else {
    console.log('You win');
}