Monster On The Mind

My son received the Mastermind “board” game for Easter and he and his sister started playing it (nonstop) after figuring out how it worked. They came up with rules for whether or not they could use two pegs of a single color in their code (rarely) and whether it was “fair” to use a blank instead of a color (no way!).

Having previously stumbled upon Anko’s JavaScript example (search the page for “Same in JS”) when looking for a simple, functional platform for running a JS text adventure game (for a programming session with my daughter and some classmates at school), I thought it would work nicely as a base for recreating Mastermind as a simple web app.

The HTML (index.html) to display the game and handle input and output is very straightforward:

<!DOCTYPE html>
<html lang="en">
		<meta charset="utf-8">
		<script src="js/game.js"></script>
		<textarea id="id_txaOutputWindow" rows="30" cols="55" readonly="readonly"></textarea>
		<form onSubmit="parse(); return false;">
			<input type="text" id="id_txtGuessPosn0" maxlength="1" size="1" required="required" />
			<input type="text" id="id_txtGuessPosn1" maxlength="1" size="1" required="required" />
			<input type="text" id="id_txtGuessPosn2" maxlength="1" size="1" required="required" />
			<input type="text" id="id_txtGuessPosn3" maxlength="1" size="1" required="required" />
			<input type="submit" value="Submit Guess" />

My first approach for managing game state involved placing the data in hidden form elements and madly passing them around via post. However, I quickly realized this was cumbersome and presented me with a nice opportunity to experiment with localStorage.

Here’s the JavaScript (game.js) that powers the game; I’ve commented it relatively thoroughly so I don’t think it requires much additional commentary beyond directing your attention to the fact that the game state is loaded each time the page loads and that each time the user hits the “Submit Guess” button the parse() function is called:

// Declare global vars to keep track of game state
var gameState;
var codeArr = [];

window.onload = function WindowLoad(event) {

	// Check for an existing game
	var gameStateJSON = window.localStorage.getItem("monstermindGameState"); 
	if (gameStateJSON) { 

		// Populate game state from localStorage
		gameState = JSON.parse(gameStateJSON);
		codeArr = gameState["codeArr"];
		if (gameState["codeNotBroken"] == 0) {
			// Game was won on user's previous guess, so start a new game
	else {
		// Game is NEW

function newGame() {
	// Pick random secret 4-digit code comprised of letters A - F
	// NOTE: I gave up on trying to make the codes numeric as JSON.parse apparently casts everything as a char
	//		so then indexOf() fails to behave properly (e.g., 2 != 2 and the like)
	for (var i = 0; i < 4; i++) {
		// NOTE: Pick a number between 65 and 70 (inclusive) and then convert that ASCII value into a character
		codeArr[i] = String.fromCharCode(Math.floor((Math.random() * 6) + 65));

	// Initialize game state
	gameState = { "codeArr": codeArr, "codeNotBroken": 1, "guessCount": 0 };

	// Put game state into localStorage
	localStorage.setItem("monstermindGameState", JSON.stringify(gameState));
	// Blank out game board and welcome player to game
	var textOut = document.getElementById("id_txaOutputWindow");
	textOut.value = "";
	textOut.value += "Welcome to Monstermind.  You have 10 guesses to get the code right.  The code is 4 characters long and consists of letters 'A' through 'F'.  You will get a black marker for each letter that you guess correctly and a white marker for each letter that is contained in the code but in the wrong position.  Feel free to make your first guess!\n\n";
	// Blank out player's guesses from text input fields (if they exist from a previous game)
	document.getElementById("id_txtGuessPosn0").value = "";
	document.getElementById("id_txtGuessPosn1").value = "";
	document.getElementById("id_txtGuessPosn2").value = "";
	document.getElementById("id_txtGuessPosn3").value = "";

function processGuess(guessArr) {
	var blackMarkerCount = 0;
	var whiteMarkerCount = 0;
	var i;
	var searchArr = [];
	for (i = 0; i < 4; i++) {
		if (guessArr[i] == codeArr[i]) {
			// Character chosen in this position matches code, so increment black marker count
		else {
			// Character chosen in this position does NOT match code, so add this position's code to the search array for 
			//	white marker candidates
			searchArr[i] = codeArr[i];
	if (blackMarkerCount != 4) {
		// Player has NOT guessed the code correctly
		for (i = 0; i < 4; i++) {
			if (searchArr[i]) {
				// Look for character match, but in wrong position, and ONLY in positions where the guess did NOT match
				//	the code up in the black marker search section of code
				var matchFoundIndex = searchArr.indexOf(guessArr[i]);
				if (matchFoundIndex !== -1) {
					// Character match found in wrong location AND wrong location has NOT already been matched with the
					//	correct character (i.e. a black marker) so this gets a white marker
					// Mangle search array value just used so it is not available to use in subsequent searches (which
					//	would lead to false white markers)
					searchArr[matchFoundIndex] = "x";
	return { "blackMarkerCount": blackMarkerCount, "whiteMarkerCount": whiteMarkerCount };

function inputErrors(guessArr) {

	var errorMessage = "";
	for (var i = 0; i < 4; i++) {
		if ((guessArr[i].charCodeAt() < 65 ) || (guessArr[i].charCodeAt() > 70 )) {
			// Invalid input
			errorMessage += "'" + guessArr[i] + "', ";			
	if (errorMessage.length) {
		errorMessage = errorMessage.substring(0, errorMessage.length - 2);
	return errorMessage;

function parse() {

	var errorMessage;

	// Increment guess # by 1
	gameState["guessCount"] = Number(gameState["guessCount"]) + 1;
	if ((gameState["guessCount"] == 11) || (gameState["codeNotBroken"] == 0)) {
		// Player hit Submit Guess button instead of reloading page - start new game for them!

	// Assign variable for writing to game board
	var textOut = document.getElementById("id_txaOutputWindow");

	// Retrieve player's guesses from text input fields and force them to uppercase
	var guessArr = [ document.getElementById("id_txtGuessPosn0").value.toUpperCase()
		, document.getElementById("id_txtGuessPosn1").value.toUpperCase()
		, document.getElementById("id_txtGuessPosn2").value.toUpperCase()
		, document.getElementById("id_txtGuessPosn3").value.toUpperCase()

	// Validate input
	if ((errorMessage = inputErrors(guessArr)).length > 0) {
		textOut.value += "Sorry but the following input is invalid: " + errorMessage + "\n\n";
		// Don't count invalid input as a guess
		gameState["guessCount"] = Number(gameState["guessCount"]) - 1;
	// Process guesses
	var newStateArray = processGuess(guessArr);
	// Render results of current guess to game board
	textOut.value += "Guess #" + gameState["guessCount"] + ": " 
    	+ guessArr[0] 
    	+ guessArr[1] 
    	+ guessArr[2] 
    	+ guessArr[3] 
    	+ "\nblack count: " + newStateArray["blackMarkerCount"] 
    	+ ", white count: " + newStateArray["whiteMarkerCount"] 
    	+ "\n\n";
	if (newStateArray["blackMarkerCount"] == 4) {
		// Player has guessed the code correctly!
		gameState["codeNotBroken"] = 0;
		textOut.value += "You WIN!!\n\nReload the page to play again";
	else if (gameState["guessCount"] == 10) {
		// Player has exhausted all of their guesses, reveal code
		gameState["codeNotBroken"] = 0;
		textOut.value += "Sorry, your 10 guesses are up!\n\nThe correct code was: "
			+ codeArr[0] 
			+ codeArr[1]
			+ codeArr[2] 
			+ codeArr[3] 
			+ "\n\nReload the page to play again";
	// Write the updated game state to localStorage so that, on page reload, the game knows to restart
	localStorage.setItem('monstermindGameState', JSON.stringify(gameState));

This program works well (enough) and has been lots of fun for both me and the kids to play whenever we feel like it, particularly because it requires neither the tedium of the setup/cleanup of the physical game board nor the cooperation of a second human to serve as the code master. 🙂

There is certainly room for improvement, however. More accurately recreating the experience of the board game by, for instance, having the game use colors in the text input/output (e.g, “green” instead of “a” and “red” instead of “b”), not to mention actually switching from the current text interface to an actual graphical experience, complete with colored pegs and markers, and mouse-clicks to position each guess, would make Monstermind more like Mastermind.

But there are many online implementations of Mastermind that aim to be true, literal recreations of the original, so my personal interests lie more in improvements to the approach I’ve chosen. Of particular interest to me is discovering a means to obscure localStorage contents from scrutiny via browser developer tools, which would make the game immune to the temptation of technically-savvy players to cheat before they hit their 10th and final guess. I have been unable, thus far, to find any real info on the web about how to do this. If you have any suggestions, please do leave them in the comments!

Here is Monstermind on GitHub1 if you’re interested in pulling down a copy for your own use (versus copying-and-pasting from this post). Either way, enjoy – it IS a fun game any way you play it!

  1. I felt it was only proper to put our family’s spin on the name of the game in honor of our dog, Monty, a husky Yorkie whose passion for consumption of all things (even remotely) edible (including Mastermind pegs!) combined with his frequent (and lengthy) high-decibel monologues has earned him the nickname “Monster.”