Image Guess Game

HTML

<div id="wrapper">
    <img id="img" src="" />
    <div id="boxes"></div>
    <div id="message"></div>
</div>

CSS

html {
    color: #FFF;
    background-color: #000;
}

#wrapper {
    position: absolute;
    top: 0px;
    left: 0px;
    width: 640px;
    height: 480px;
}

#message {
    position: absolute;
    top: 100px;
    font: 32pt monospace;
}

#messgae, #boxes, #img {
    position: absolute;
    top: 0px;
    left: 0px;
    width: 640px;
    height: 480px;
    border: none;
}

#box {
    position: absolute;
    display: block;
    width: 80px;
    height: 60px;
    background-color: #000;
}

JavaScript

function message(msg) {
    document.getElementById('message').innerHTML = msg;
}

function image(imgUrl) {
    document.getElementById('img').style.width = '640px';
    document.getElementById('img').src = imgUrl;
}

function clearBoxes() {
    document.getElementById('boxes').innerHTML = '';
}

function box(x, y) {
    document.getElementById('boxes').innerHTML += '<div id="box" style="top:' + y + 'px;left:' + x + 'px"></div>';
}

message('Image Guess Game<br>By Tyler Keohane<br><br>Press any key to start!');

var currentImg = 0;
var w = 0;
var z = 0;

document.getElementById('img').style.width = '0px';

var imgList = [
    'https://upload.wikimedia.org/wikipedia/commons/thumb/d/d9/Node.js_logo.svg/1200px-Node.js_logo.svg.png',
    'https://upload.wikimedia.org/wikipedia/commons/thumb/d/d9/Node.js_logo.svg/1200px-Node.js_logo.svg.png',
    'https://upload.wikimedia.org/wikipedia/commons/thumb/d/d9/Node.js_logo.svg/1200px-Node.js_logo.svg.png',
    'https://upload.wikimedia.org/wikipedia/commons/thumb/d/d9/Node.js_logo.svg/1200px-Node.js_logo.svg.png'
    ];

function newGame(num) {
    message('');
    w = 0;
    z++;

    var map = [
        [1, 1, 1, 1, 1, 1, 1, 1],
        [1, 1, 1, 1, 1, 1, 1, 1],
        [1, 1, 1, 1, 1, 1, 1, 1],
        [1, 1, 1, 1, 1, 1, 1, 1],
        [1, 1, 1, 1, 1, 1, 1, 1],
        [1, 1, 1, 1, 1, 1, 1, 1],
        [1, 1, 1, 1, 1, 1, 1, 1],
        [1, 1, 1, 1, 1, 1, 1, 1]
        ];

    function step() {

        var i = 0;
        while (i !== 1) {
            var randX = Math.floor(Math.random() * 8);
            var randY = Math.floor(Math.random() * 8);
            if (map[randX][randY] === 1) {
                map[randX][randY] = 0;
                i = 1;
            }
        }

        image(imgList[num]);
        clearBoxes();

        for (y = 0; y < 8; y++) {
            for (x = 0; x < 8; x++) {
                if (map[x][y] === 1) {
                    box(x * 80, y * 60);
                }
            }
        }
   ...