v1.2 of Elisabeth Robson ( https://github.com/bethrobson )'s Battleship game from her book Head First JavaScript program, which I am using as the textbook at college. I call it v1.2 as I made it slightly more advanced than it should be at the end of Chapter 2. 😃

battleship-v1-2.js

/* jshint -W097 */
/* global prompt, alert */
"use strict";
var guess,
    loc1    = Math.floor(Math.random() * 5),
    loc2    = loc1 + 1,
    loc3    = loc1 + 2,
    hits    = 0,
    isSunk  = false,
    guesses = [];

// Loop until the ship is sunk
while (!isSunk) {
  guess = parseInt(prompt("Ready, aim, fire! (enter as number between 0-6)"));

  // The user entered an invalid guess or a cell that has been guessed already
  if (guess < 0 || guess > 6 || guesses.indexOf(guess) > -1) {
    alert("Please enter a valid cell number!");

    // The guess was valid
  } else {
    // The user correctly guessed the ship's location
    switch (guess) {
      case loc1:
      case loc2:
      case loc3:
        hits += 1;
        alert("Hit!");
        break;

        // ... Or not
      default:
        alert("Miss!");
    }

    // The user found and sunk the battleship
    if (hits === 3) {
      isSunk = true;
      alert("You sunk my battleship!");
    }
  }

  // Keep track of the cells the user has guessed
  guesses.push(guess);
}

// Give the user the round's results, round shooting accuracy to nearest percent
var stats = "You took " + guesses.length + " guesses to sink the battleship. " +
    "This gives you a shooting accuracy of " + Math.round((3 / guesses.length) * 100) + "%.";
alert(stats);