<canvas id="ctx" width="500" height="500" style="border:1px solid #000000;"></canvas>

<script>
var ctx = document.getElementById("ctx").getContext("2d");
ctx.font = '30px Arial';

var HEIGHT = 500;
var WIDTH = 500;
var timeWhenGameStarted = Date.now();	//return time in ms

var frameCount = 0;

var player = {
	x:50,
	spdX:30,
	y:40,
	spdY:5,
	name:'P',
	hp:10,
	width:20,
	height:20,
	color:'green',
};

var enemyList = {};


getDistanceBetweenEntity = function (entity1,entity2){	//return distance (number)
	var vx = entity1.x - entity2.x;
	var vy = entity1.y - entity2.y;
	return Math.sqrt(vx*vx+vy*vy);
}

testCollisionEntity = function (entity1,entity2){	//return if colliding (true/false)
	var rect1 = {
		x:entity1.x-entity1.width/2,
		y:entity1.y-entity1.height/2,
		width:entity1.width,
		height:entity1.height,
	}
	var rect2 = {
		x:entity2.x-entity2.width/2,
		y:entity2.y-entity2.height/2,
		width:entity2.width,
		height:entity2.height,
	}
	return testCollisionRectRect(rect1,rect2);

}

Enemy = function (id,x,y,spdX,spdY,width,height){
	var enemy3 = {
		x:x,
		spdX:spdX,
		y:y,
		spdY:spdY,
		name:'E',
		id:id,
		width:width,
		height:height,
		color:'red',
	};
	enemyList[id] = enemy3;

}

document.onmousemove = function(mouse){
	var mouseX = mouse.clientX - document.getElementById('ctx').getBoundingClientRect().left;
	var mouseY = mouse.clientY - document.getElementById('ctx').getBoundingClientRect().top;

	if(mouseX < player.width/2)
		mouseX = player.width/2;
	if(mouseX > WIDTH-player.width/2)
		mouseX = WIDTH - player.width/2;
	if(mouseY < player.height/2)
		mouseY = player.height/2;
	if(mouseY > HEIGHT - player.height/2)
		mouseY = HEIGHT - player.height/2;

	player.x = mouseX;
	player.y = mouseY;
}





updateEntity = function (something){
	updateEntityPosition(something);
	drawEntity(something);
}
updateEntityPosition = function(something){
	something.x += something.spdX;
	something.y += something.spdY;

	if(something.x < 0 || something.x > WIDTH){
		something.spdX = -something.spdX;
	}
	if(something.y < 0 || something.y > HEIGHT){
		something.spdY = -something.spdY;
	}
}

testCollisionRectRect = function(rect1,rect2){
	return rect1.x <= rect2.x+rect2.width
		&& rect2.x <= rect1.x+rect1.width
		&& rect1.y <= rect2.y + rect2.height
		&& rect2.y <= rect1.y + rect1.height;
}


drawEntity = function(something){
	ctx.save();
	ctx.fillStyle = something.color;
	ctx.fillRect(something.x-something.width/2,something.y-something.height/2,something.width,something.height);
	ctx.restore();
}



update = function(){
	ctx.clearRect(0,0,WIDTH,HEIGHT);
	frameCount++;

	if(frameCount % 100 === 0)	//every 4 sec
		randomlyGenerateEnemy();


	for(var key in enemyList){
		updateEntity(enemyList[key]);

		var isColliding = testCollisionEntity(player,enemyList[key]);
		if(isColliding){
			player.hp = player.hp - 1;
		}
	}
	if(player.hp <= 0){
		var timeSurvived = Date.now() - timeWhenGameStarted;
		console.log("You lost! You survived for " + timeSurvived + " ms.");
		startNewGame();
	}

	drawEntity(player);
	ctx.fillText(player.hp + " Hp",0,30);
}
startNewGame = function(){
	player.hp = 10;
	timeWhenGameStarted = Date.now();
	frameCount = 0;
	enemyList = {};
	randomlyGenerateEnemy();
	randomlyGenerateEnemy();
	randomlyGenerateEnemy();

}


randomlyGenerateEnemy = function(){
	//Math.random() returns a number between 0 and 1
	var x = Math.random()*WIDTH;
	var y = Math.random()*HEIGHT;
	var height = 10 + Math.random()*30;	//between 10 and 40
	var width = 10 + Math.random()*30;
	var id = Math.random();
	var spdX = 5 + Math.random() * 5;
	var spdY = 5 + Math.random() * 5;
	Enemy(id,x,y,spdX,spdY,width,height);

}

startNewGame();

setInterval(update,40);












</script>