Edit in JSFiddle

var userChoice = prompt("Do you choose rock, paper or scissors?");
var computerChoice = Math.random();
if (computerChoice < 0.34) {
    computerChoice = "rock";
} else if (computerChoice <= 0.67) {
    computerChoice = "paper";
} else {
    computerChoice = "scissors";
}
var compare = function (choice1, choice2) {
    if (choice1 === choice2) {
        return "The result is a tie!"
    }

    if (choice1 === "rock") {
        if (choice2 === "scissors") {
            return "rock wins!";
        } else {
            return "paper wins";
        }
    }

    if (choice1 === "paper") {
        if (choice2 === "rock") {
            return "paper wins!";
        } else {
            return "scissors wins";
        }
    }

    if (choice1 === "scissors") {
        if (choice2 === "rock") {
            return "rock wins!";
        } else {
            return "scissors wins";
        }
    }
}
alert(compare(userChoice, computerChoice));