Experiment with objects in JavaScript.

// Create the canvas
var canvas = document.createElement("canvas");
var ctx = canvas.getContext("2d");
canvas.width = 512;
canvas.height = 480;
document.body.appendChild(canvas);

// Background image
var bgReady = false;
var bgImage = new Image();
bgImage.onload = function () {
    bgReady = true;
};
bgImage.src = "images/background.png";

// Hero image
var heroReady = false;
var heroImage = new Image();
heroImage.onload = function () {
    heroReady = true;
};
heroImage.src = "images/hero.png";

// Monster image
var monsterReady = false;
var monsterImage = new Image();
monsterImage.onload = function () {
    monsterReady = true;
};
monsterImage.src = "images/monster.png";

// Kill sound
var snd = new Audio("audio/kill.wav")

/* Game objects */

// Hero object
var hero = {
    speed: 250 // movement in pixels per second
};

// Monster object
var monster = {
    speed: 200,
    directions: [null, null]
};
var monstersCaught = 0;

// Wave counter
var currentWave = 0;

// Handle keyboard controls
var keysDown = {};

addEventListener("keydown", function (e) {
    keysDown[e.keyCode] = true;
}, false);

addEventListener("keyup", function (e) {
    delete keysDown[e.keyCode];
}, false);

// Reset the game when the player catches a monster
var reset = function () {
    hero.x = canvas.width / 2;
    hero.y = canvas.height / 2;

    // Change wave number and change speed accordingly
    if (monstersCaught % 10 == 0){
        currentWave++;
        hero.speed += 25;
        monster.speed += 50;
    }

    // Reset speed on waves that are multiples of five
    if (currentWave % 5 == 1){
        hero.speed = 250;
        monster.speed = 250;
    }

    placeMonster();
};

var placeMonster = function () {
    // Throw the monster somewhere on the screen randomly
    monster.x = 32 + (Math.random() * (canvas.width - 96));
    monster.y = 32 + (Math.random() * (canvas.height - 96));
};

var moveEntity = function (entity, modifier, direction) {
    switch (direction) {
    case 38:
    case 'up':
        if (entity.y >= 32) { // Check collision with top wall
            entity.y -= entity.speed * modifier;
        }
        break;
    case 40:
    case 'down':
        if (entity.y <= canvas.height - 64) { // Check collision with bottom wall
            entity.y += entity.speed * modifier;
        }
        break;
    case 37:
    case 'left':
        if (entity.x >= 32) { // Check collision with left wall
            entity.x -= entity.speed * modifier;
        }
        break;
    case 39:
    case 'right':
        if (entity.x <= canvas.width - 64) {
            entity.x += entity.speed * modifier;
        }
        break;
    }
};

var keyToDirection = {
    38: 'up',
    40: 'down',
    37: 'left',
    39: 'right'
};

var directions = [37, 38, 39, 40];

// Update game objects
var update = function (modifier) {
    for (key in keysDown) {
        var direction = keyToDirection[key];
        if (direction) moveEntity(hero, modifier, keyToDirection[key]);
    }

    for (var i = 0; i < 2; i++) {
        moveEntity(monster, modifier, monster.directions[i]);
    }

    // Are they touching?
    if (
        hero.x <= (monster.x + 32)
        && monster.x <= (hero.x + 32)
        && hero.y <= (monster.y + 32)
        && monster.y <= (hero.y + 32)
    ) {
        ++monstersCaught;

        // Resets sound if it is still playing
        if(snd.currentTime > 0) {
            snd.currentTime = 0;
        }
        snd.play();

        placeMonster();
    }
};

var changeMonsterDirections = function() {
    for (var i = 0; i < 2; i++) {
        monster.directions[i] = directions[Math.floor(Math.random() * directions.length)];
    }
    // If both directions are the same, null out one of them so that
    // the monster doesn't move twice as fast
    if (monster.directions[0] == monster.directions[1]) {
        monster.directions[1] = null;
    }
}

// Draw everything
var render = function () {
    if (bgReady) {
        ctx.drawImage(bgImage, 0, 0);
    }

    if (heroReady) {
        ctx.drawImage(heroImage, hero.x, hero.y);
    }

    if (monsterReady) {
        ctx.drawImage(monsterImage, monster.x, monster.y);
    }

    // Score
    ctx.fillStyle = "rgb(250, 250, 250)";
    ctx.font = "14px Helvetica";
    ctx.textAlign = "left";
    ctx.textBaseline = "top";
    ctx.fillText("Goblins caught: " + monstersCaught, 32, 32);
    ctx.fillText("Current wave: " + currentWave, 32, 46);
};

// The main game loop
var main = function () {
    var now = Date.now();
    var delta = now - then;

    update(delta / 1000);
    render();

    then = now;
};

// Let's play this game!
reset();
var then = Date.now();
setInterval(main, 1); // Execute as fast as possible
setInterval(changeMonsterDirections, 1000);