Solution: Rock, Paper, Scissors

rockpaperscissors.js

const game = {
  choices: ["rock", "paper", "scissors"],
  rock: "scissors",
  paper: "rock",
  scissors: "paper"
 };

const play = playerChoice => {
  if (!game.choices.includes(playerChoice)) {
    return `Player hasn't selected valid choice`;
  }
  const computerChoice = game.choices[Math.floor(Math.random()*3)];
  if (computerChoice === playerChoice) {
    return `Tie!`;
  }
  const outcome = game[playerChoice].includes(computerChoice) ? 'won' : 'lost';
  return `You ${outcome} with ${playerChoice}! Computer picked ${computerChoice}.`;
}

console.log(play("rock"));
console.log(play("paper"));
console.log(play("scissors"))