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Stop user from scrolling using arrow keys while canvas based game is running

I have a canvas game in my webpage and it operates with arrow key controls, but whenever somebody tries to play it the page scrolls. Is there any way I can prevent it? The code for my game is here. All I need is for the page to not be bouncing up and down whenever somebody is trying to play. I know it would be easier to just use the wasd keys but to me that feels like putting duct tape on a leak on the hull of a boat instead of actually fixing it. Any suggestions? I'm using google apps script HTML service so I'm not sure if using jquery is possible or if it is whether its going going to be hours and hours of work that are eventually ditched in favor of a quicker solution. Anyways, hope somebody can help.
<h1> example html</h1>
<p>exampleHTMLexampleHTMLexampleHTMLexampleHTMLexampleHTMLexampleHTMLexampleHTMLexampleHTMLexampleHTMLexampleHTMLexampleHTMLexampleHTML</p>
<h2>example html </h2>
<canvas id='my' width = '640' height = '480' style = 'display: none;'></canvas>
<script>
var paused = false
function PausePlay(){
if (paused === false)
{paused = true;}
else{paused = false;}
}
var canvas = document.getElementById("my");
var ctx = canvas.getContext("2d");
function paddle(x, y, width, height) {
this.x = x;
this.y = y;
this.width = width;
this.height = height;
this.speedModifier = 0;
this.hasCollidedWith = function(ball) {
var paddleLeftWall = this.x;
var paddleRightWall = this.x + this.width;
var paddleTopWall = this.y;
var paddleBottomWall = this.y + this.height;
if (ball.x > paddleLeftWall &&
ball.x < paddleRightWall &&
ball.y > paddleTopWall &&
ball.y < paddleBottomWall) {
return true;
}
return false;
};
this.move = function(keyCode) {
var nextY = this.y;
if (keyCode == 40) {
nextY += 5;
this.speedModifer = 1.5;
} else if (keyCode == 38) {
nextY += -5;
this.speedModifier = 1.5;
} else {
this.speedModifier = 0;
}
nextY = nextY < 0 ? 0 : nextY;
nextY = nextY + this.height > 480 ? 480 - this.height : nextY;
this.y = nextY;
};
}
var player = new paddle(5, 200, 25, 100);
var ai = new paddle(610, 200, 25, 100);
var ball = {
x: 320,
y: 240,
radius: 7,
xSpeed: 2,
ySpeed: 0,
playerscore: 0,
aiscore: 0,
reverseX: function() {
this.xSpeed *= -1;
},
reverseY: function() {
this.ySpeed *= -1;
},
reset: function() {
alert('The score is now ' + this.playerscore + ' to ' + this.aiscore);
this.x = 20;
this.y = 24;
this.xSpeed = 2;
this.ySpeed = 0;
},
isBouncing: function() {
return ball.ySpeed != 0;
},
modifyXSpeedBy: function(modification) {
modification = this.xSpeed < 0 ? modification * -1 : modification;
var nextValue = this.xSpeed + modification;
nextValue = Math.abs(nextValue) > 9 ? 9 : nextValue;
this.xSpeed = nextValue;
},
modifyYSpeedBy: function(modification) {
modification = this.ySpeed < 0 ? modification * -1 : modification;
this.ySpeed += modification;
}
};
function tick() {
updateGame();
draw()
window.setTimeout("tick()", 1000 / 60);
}
function updateGame() {
if (paused === false){
ball.x += ball.xSpeed;
ball.y += ball.ySpeed;
if (ball.x < 0) {
ball.reset();
ball.aiscore = ball.aiscore + 1;
}
if (ball.x > 640) {
ball.reset();
ball.playerscore = ball.playerscore + 1
}
if (ball.y <= 0 || ball.y >= 480) {
ball.reverseY();
}
var collidedWithPlayer = player.hasCollidedWith(ball);
var collidedWithAi = ai.hasCollidedWith(ball);
if (collidedWithPlayer || collidedWithAi) {
ball.reverseX();
ball.modifyXSpeedBy(0.25);
var speedUpValue = collidedWithPlayer ? player.speedModifier : ai.speedModifier;
ball.modifyYSpeedBy(speedUpValue);
}
for (var keyCode in heldDown) {
player.move(keyCode);
}
var aiMiddle = ai.y + (ai.height / 2);
if (aiMiddle < ball.y) {
ai.move(40);
}
if (aiMiddle > ball.y) {
ai.move(38);
}
}
}
function draw() {
if(paused === false){
ctx.fillStyle = "black";
ctx.fillRect(0, 0, 640, 480);
renderPaddle(player);
renderPaddle(ai);
renderBall(ball);
}
}
function renderPaddle(paddle) {
ctx.fillStyle = "blue";
ctx.fillRect(paddle.x, paddle.y, paddle.width, paddle.height);
}
function renderBall(ball) {
ctx.beginPath();
ctx.arc(ball.x, ball.y, ball.radius, 0, 2 * Math.PI, false);
ctx.fillStyle = "pink";
ctx.fill();
}
var heldDown = {};
window.addEventListener("keydown", function(keyInfo) {
heldDown[event.keyCode] = true;
}, false);
window.addEventListener("keyup", function(keyInfo) {
delete heldDown[event.keyCode];
}, false);
function playPong(){
canvas.style.display = 'block';
tick()
}
function show(){
var canvas = document.getElementById('my')
canvas.style.display = 'block';
}
</script>
<div>
<button onclick = 'hide()'> Hide or show the games</button>
<br>
<button onclick = 'PausePlay()'> Pause/play games</button>
<br>
</div>
<br><br><br><br>
<button onclick = 'playPong()'> Play pong </button>

You may use overflow: hidden; CSS property in order to control the ability of scrolling. If you need to prevent vertical/horizontal scrolling only, so you may use overflow-x and overflow-y More about overflow property here.

add rectangle dynamically on button click inside canvas

function Shape(x, y, w, h, fill) {
this.x = x || 0;
this.y = y || 0;
this.w = w || 1;
this.h = h || 1;
this.fill = fill || '#AAAAAA';
}
Shape.prototype.draw = function(ctx) {
ctx.fillStyle = this.fill;
ctx.fillRect(this.x, this.y, this.w, this.h);
}
Shape.prototype.contains = function(mx, my) {
return (this.x <= mx) && (this.x + this.w >= mx) &&
(this.y <= my) && (this.y + this.h >= my);
}
function CanvasState(canvas) {
this.canvas = canvas;
this.width = canvas.width;
this.height = canvas.height;
this.ctx = canvas.getContext('2d');
var stylePaddingLeft, stylePaddingTop, styleBorderLeft, styleBorderTop;
if (document.defaultView && document.defaultView.getComputedStyle) {
this.stylePaddingLeft = parseInt(document.defaultView.getComputedStyle(canvas, null)['paddingLeft'], 10) || 0;
this.stylePaddingTop = parseInt(document.defaultView.getComputedStyle(canvas, null)['paddingTop'], 10) || 0;
this.styleBorderLeft = parseInt(document.defaultView.getComputedStyle(canvas, null)['borderLeftWidth'], 10) || 0;
this.styleBorderTop = parseInt(document.defaultView.getComputedStyle(canvas, null)['borderTopWidth'], 10) || 0;
}
var html = document.body.parentNode;
this.htmlTop = html.offsetTop;
this.htmlLeft = html.offsetLeft;
this.valid = false;
this.shapes = [];
this.dragging = false;
this.selection = null;
this.dragoffx = 0;
this.dragoffy = 0;
var myState = this;
canvas.addEventListener('selectstart', function(e) { e.preventDefault(); return false; }, false);
canvas.addEventListener('mousedown', function(e) {
var mouse = myState.getMouse(e);
var mx = mouse.x;
var my = mouse.y;
var shapes = myState.shapes;
var l = shapes.length;
for (var i = l-1; i >= 0; i--) {
if (shapes[i].contains(mx, my)) {
var mySel = shapes[i];
myState.dragoffx = mx - mySel.x;
myState.dragoffy = my - mySel.y;
myState.dragging = true;
myState.selection = mySel;
myState.valid = false;
return;
}
}
if (myState.selection) {
myState.selection = null;
myState.valid = false;
}
}, true);
canvas.addEventListener('mousemove', function(e) {
if (myState.dragging){
var mouse = myState.getMouse(e);
myState.selection.x = mouse.x - myState.dragoffx;
myState.selection.y = mouse.y - myState.dragoffy;
myState.valid = false;
}
}, true);
canvas.addEventListener('mouseup', function(e) {
myState.dragging = false;
}, true);
canvas.addEventListener('dblclick', function(e) {
var mouse = myState.getMouse(e);
myState.addShape(new Shape(mouse.x - 10, mouse.y - 10, 20, 20, 'rgba(0,255,0,.6)'));
}, true);
this.selectionColor = '#CC0000';
this.selectionWidth = 2;
this.interval = 30;
setInterval(function() { myState.draw(); }, myState.interval);
}
CanvasState.prototype.addShape = function(shape) {
this.shapes.push(shape);
this.valid = false;
}
CanvasState.prototype.clear = function() {
this.ctx.clearRect(0, 0, this.width, this.height);
}
CanvasState.prototype.draw = function() {
if (!this.valid) {
var ctx = this.ctx;
var shapes = this.shapes;
this.clear();
var l = shapes.length;
for (var i = 0; i < l; i++) {
var shape = shapes[i];
if (shape.x > this.width || shape.y > this.height ||
shape.x + shape.w < 0 || shape.y + shape.h < 0) continue;
shapes[i].draw(ctx);
}
if (this.selection != null) {
ctx.strokeStyle = this.selectionColor;
ctx.lineWidth = this.selectionWidth;
var mySel = this.selection;
ctx.strokeRect(mySel.x,mySel.y,mySel.w,mySel.h);
}
this.valid = true;
}
}
CanvasState.prototype.getMouse = function(e) {
var element = this.canvas, offsetX = 0, offsetY = 0, mx, my;
if (element.offsetParent !== undefined) {
do {
offsetX += element.offsetLeft;
offsetY += element.offsetTop;
} while ((element = element.offsetParent));
}
offsetX += this.stylePaddingLeft + this.styleBorderLeft + this.htmlLeft;
offsetY += this.stylePaddingTop + this.styleBorderTop + this.htmlTop;
mx = e.pageX - offsetX;
my = e.pageY - offsetY;
return {x: mx, y: my};
}
function init() {
var s = new CanvasState(document.getElementById('canvas1'));
s.addShape(new Shape(40,40,50,50)); // The default is gray
}
<canvas id="canvas1" width="200" height="200" style="border:1px solid #000000;">
This text is displayed if your browser does not support HTML5 Canvas.
</canvas>
<button onclick= "init()">click </button>
On click of a button, I am creating a rectangular element which can be drag and drop inside the canvas element restricted area.
however, Is it possible to create a new rectangular element dynamically on every click of a button which can further be drag and drop inside the canvas boundary.
And the previous rectangular element won't lost.

Doing this from scratch will be a lot of work. I suggest you use a library like fabric.js which has everything you asked plus a lot more to offer. Here is a demo page.

On every button click you was calling init(),.. Causing you CanvasState to reset.
I've just made another function called addShape, and just called init once.
I've also kept a reference to the CanvasState, called s.. So I could do s.addShape.. So changes were very minimal.
Hope this helps..
function Shape(x, y, w, h, fill) {
this.x = x || 0;
this.y = y || 0;
this.w = w || 1;
this.h = h || 1;
this.fill = fill || '#AAAAAA';
}
Shape.prototype.draw = function(ctx) {
ctx.fillStyle = this.fill;
ctx.fillRect(this.x, this.y, this.w, this.h);
}
Shape.prototype.contains = function(mx, my) {
return (this.x <= mx) && (this.x + this.w >= mx) &&
(this.y <= my) && (this.y + this.h >= my);
}
function CanvasState(canvas) {
this.canvas = canvas;
this.width = canvas.width;
this.height = canvas.height;
this.ctx = canvas.getContext('2d');
var stylePaddingLeft, stylePaddingTop, styleBorderLeft, styleBorderTop;
if (document.defaultView && document.defaultView.getComputedStyle) {
this.stylePaddingLeft = parseInt(document.defaultView.getComputedStyle(canvas, null)['paddingLeft'], 10) || 0;
this.stylePaddingTop = parseInt(document.defaultView.getComputedStyle(canvas, null)['paddingTop'], 10) || 0;
this.styleBorderLeft = parseInt(document.defaultView.getComputedStyle(canvas, null)['borderLeftWidth'], 10) || 0;
this.styleBorderTop = parseInt(document.defaultView.getComputedStyle(canvas, null)['borderTopWidth'], 10) || 0;
}
var html = document.body.parentNode;
this.htmlTop = html.offsetTop;
this.htmlLeft = html.offsetLeft;
this.valid = false;
this.shapes = [];
this.dragging = false;
this.selection = null;
this.dragoffx = 0;
this.dragoffy = 0;
var myState = this;
canvas.addEventListener('selectstart', function(e) { e.preventDefault(); return false; }, false);
canvas.addEventListener('mousedown', function(e) {
var mouse = myState.getMouse(e);
var mx = mouse.x;
var my = mouse.y;
var shapes = myState.shapes;
var l = shapes.length;
for (var i = l-1; i >= 0; i--) {
if (shapes[i].contains(mx, my)) {
var mySel = shapes[i];
myState.dragoffx = mx - mySel.x;
myState.dragoffy = my - mySel.y;
myState.dragging = true;
myState.selection = mySel;
myState.valid = false;
return;
}
}
if (myState.selection) {
myState.selection = null;
myState.valid = false;
}
}, true);
canvas.addEventListener('mousemove', function(e) {
if (myState.dragging){
var mouse = myState.getMouse(e);
myState.selection.x = mouse.x - myState.dragoffx;
myState.selection.y = mouse.y - myState.dragoffy;
myState.valid = false;
}
}, true);
canvas.addEventListener('mouseup', function(e) {
myState.dragging = false;
}, true);
canvas.addEventListener('dblclick', function(e) {
var mouse = myState.getMouse(e);
myState.addShape(new Shape(mouse.x - 10, mouse.y - 10, 20, 20, 'rgba(0,255,0,.6)'));
}, true);
this.selectionColor = '#CC0000';
this.selectionWidth = 2;
this.interval = 30;
setInterval(function() { myState.draw(); }, myState.interval);
}
CanvasState.prototype.addShape = function(shape) {
this.shapes.push(shape);
this.valid = false;
}
CanvasState.prototype.clear = function() {
this.ctx.clearRect(0, 0, this.width, this.height);
}
CanvasState.prototype.draw = function() {
if (!this.valid) {
var ctx = this.ctx;
var shapes = this.shapes;
this.clear();
var l = shapes.length;
for (var i = 0; i < l; i++) {
var shape = shapes[i];
if (shape.x > this.width || shape.y > this.height ||
shape.x + shape.w < 0 || shape.y + shape.h < 0) continue;
shapes[i].draw(ctx);
}
if (this.selection != null) {
ctx.strokeStyle = this.selectionColor;
ctx.lineWidth = this.selectionWidth;
var mySel = this.selection;
ctx.strokeRect(mySel.x,mySel.y,mySel.w,mySel.h);
}
this.valid = true;
}
}
CanvasState.prototype.getMouse = function(e) {
var element = this.canvas, offsetX = 0, offsetY = 0, mx, my;
if (element.offsetParent !== undefined) {
do {
offsetX += element.offsetLeft;
offsetY += element.offsetTop;
} while ((element = element.offsetParent));
}
offsetX += this.stylePaddingLeft + this.styleBorderLeft + this.htmlLeft;
offsetY += this.stylePaddingTop + this.styleBorderTop + this.htmlTop;
mx = e.pageX - offsetX;
my = e.pageY - offsetY;
return {x: mx, y: my};
}
var s;
function init() {
s = new CanvasState(document.getElementById('canvas1'));
}
function addShape() {
s.addShape(new Shape(40,40,50,50)); // The default is gray
}
init();
<canvas id="canvas1" width="400" height="300" style="border:1px solid #000000;">
This text is displayed if your browser does not support HTML5 Canvas.
</canvas>
<button onclick= "addShape()">click </button>

How would I create a trail behind my shapes in canvas?

Right now I have some shapes (I only included the triangle as they're all generated the same way) that are generated when the user spins the mouse wheel. I want to leave a trail behind the shapes that slowly disappear. I've looked around and tried a few different ways but I can't seem to get any of them to work. Here's the code:
<html>
<head>
<script>
var canvas;
var context;
var triangles = [];
var timer;
function init() {
canvas = document.getElementById('canvas');
context = canvas.getContext("2d");
resizeCanvas();
window.addEventListener('resize', resizeCanvas, false);
window.addEventListener('orientationchange', resizeCanvas, false);
canvas.onwheel = function(event) {
handleClick(event.clientX, event.clientY);
};
var timer = setInterval(resizeCanvas, 30);
}
function Triangle(x,y,triangleColor) {
this.x = x;
this.y = y;
this.triangleColor = triangleColor;
this.vx = Math.random() * 30 - 15;
this.vy = Math.random() * 30 - 15;
this.time = 100;
}
function handleClick(x,y) {
var colors = [[0,170,255], [230,180,125], [50,205,130]];
var triangleColor = colors[Math.floor(Math.random()*colors.length)];
triangles.push(new Triangle(x,y,triangleColor));
for (var i=0; i<triangles.length; i++) {
drawTriangle(triangles[i]);
}
}
function drawTriangle(triangle) {
context.beginPath();
context.moveTo(triangle.x,triangle.y);
context.lineTo(triangle.x+25,triangle.y+25);
context.lineTo(triangle.x+25,triangle.y-25);
var c = triangle.triangleColor
context.fillStyle = 'rgba(' + c[0] + ', ' + c[1] + ', ' + c[2] + ', ' + (triangle.time / 100) + ')';
context.fill();
}
function resizeCanvas() {
canvas.width = window.innerWidth-20;
canvas.height = window.innerHeight-20;
fillBackgroundColor();
for (var i=0; i<triangles.length; i++) {
var t = triangles[i];
drawTriangle(t);
if (t.x + t.vx > canvas.width || t.x + t.vx < 0)
t.vx = -t.vx
if (t.y + t.vy > canvas.height || t.y + t.vy < 0)
t.vy = -t.vy
if (t.time === 0) {
triangles.splice(i,1);
}
t.time -= 1;
t.x += t.vx;
t.y += t.vy;
}
}
function fillBackgroundColor() {
context.fillStyle = "black";
context.fillRect(0,0,canvas.width,canvas.height);
}
window.onload = init;
</script>
</head>
<body>
<canvas id="canvas" width="500" height="500"></canvas>
</body>
</html>

The way I managed to do this is by storing past locations of my objects (for example 10 past locations, and draw line from one to the next one.
1.Add arrays pastX and pastY to your triangle object.
this.pastX = [];
this.pastY = [];
2.Each tick put every element 1 place further and add current location as first.
Right before you update triangle positions
for(var k = t.pastX.length; k > 0; k--){
if(k < 10){
t.pastX[k] = pastX[k-1];
t.pastY[k] = pastY[k-1];
}
t.pastX[0] = t.x;
t.pastY[0] = t.y;
draw lines between them like this (from 0 to 1, from 1 to 2 and so on);
for(var k = 0; k < t.pastX.length - 1; k++){
//draw line from pastX[k], pastY[k] to pastX[k + 1], pastY[k + 1]
}
Hope I helped you.

HTML5: canvas animated not work in action

I work with this code for show HTML5 canvas animated in over my div:
HTML:
<div class="ls-slide" id="asb">
<div id="smile" style="width:100%;height:359px;">
<canvas style="position: absolute; width: 100%; z-index: 3; display: block;" height="359" width="2111">
</canvas>
</div>
</div>
JS:
window.onload = function() {
travers();
var lastHeight = $("#asb").height();
var canvas = document.querySelector('canvas');
ctx = canvas.getContext('2d');
color = '#aaa';
var check = respondCanvas();
canvas.width = check[1];
canvas.height = lastHeight;
canvas.style.display = 'block';
ctx.fillStyle = color;
ctx.lineWidth = .1;
ctx.strokeStyle = color;
function respondCanvas() {
var width = [];
width[0] = $('#smile').width(); //max width
width[1] = $('canvas').width(); //max width
return width;
//Call a function to redraw other content (texts, images etc)
}
function checkheight() {
var height = $('#asb').height(); //max width
//console.log(height);
// return height;
}
var mousePosition = {
x: 30 * canvas.width / 100,
y: 30 * canvas.height / 100
};
if (canvas.width <= 1000) {
var numdot = 100;
} else if (canvas.width <= 800) {
var numdot = 80;
} else if (canvas.width <= 500) {
var numdot = 35;
} else {
var numdot = 300;
}
var dots = {
nb: numdot,
distance: 70,
d_radius: 50,
array: []
};
function Dot() {
this.x = Math.random() * canvas.width;
this.y = Math.random() * canvas.height;
this.vx = -.5 + Math.random();
this.vy = -.5 + Math.random();
this.radius = Math.random();
}
Dot.prototype = {
create: function() {
ctx.beginPath();
ctx.arc(this.x, this.y, this.radius, 0, Math.PI * 2, false);
ctx.fill();
},
animate: function() {
for (i = 0; i < dots.nb; i++) {
var dot = dots.array[i];
if (dot.y < 0 || dot.y > canvas.height) {
dot.vx = dot.vx;
dot.vy = -dot.vy;
} else if (dot.x < 0 || dot.x > canvas.width) {
dot.vx = -dot.vx;
dot.vy = dot.vy;
}
dot.x += dot.vx;
dot.y += dot.vy;
}
},
line: function() {
for (i = 0; i < dots.nb; i++) {
for (j = 0; j < dots.nb; j++) {
i_dot = dots.array[i];
j_dot = dots.array[j];
if ((i_dot.x - j_dot.x) < dots.distance && (i_dot.y - j_dot.y) < dots.distance && (i_dot.x - j_dot.x) > -dots.distance && (i_dot.y - j_dot.y) > -dots.distance) {
if ((i_dot.x - mousePosition.x) < dots.d_radius && (i_dot.y - mousePosition.y) < dots.d_radius && (i_dot.x - mousePosition.x) > -dots.d_radius && (i_dot.y - mousePosition.y) > -dots.d_radius) {
ctx.beginPath();
ctx.moveTo(i_dot.x, i_dot.y);
ctx.lineTo(j_dot.x, j_dot.y);
ctx.stroke();
ctx.closePath();
}
}
}
}
}
};
function createDots() {
ctx.clearRect(0, 0, canvas.width, canvas.height);
for (i = 0; i < dots.nb; i++) {
dots.array.push(new Dot());
dot = dots.array[i];
dot.create();
}
dot.line();
dot.animate();
}
$("canvas").mousemove(function(parameter) {
mousePosition.x = parameter.pageX - 0;
mousePosition.y = parameter.pageY - 300;
});
setInterval(createDots, 1000 / 30);
};
But, In action not work and I can't see dotted animation(I checked in chrome,ff). How do fix this?!
demo : https://jsfiddle.net/5pzvh8ko/

You can try it better by giving an ID to you canvas tag, and then use "document.getElementById()", to access your canvas tag, i think it is a better method and even simpler.here is an example:
canvas tag with id:
<canvas ID="canvas_id" style="position: absolute; width: 100%; z-index:3; display: block;" height="359" width="2111">
then use JavaScript to access it this way:
var canvas = document.getElementById("canvas_id");
This might not be all you need but you can at least try it that way.

Try to implement shooting by bullets

Hello I try to implement shooting by drawing bullets. I created function SHOOt and just called it from the keyboard detection condition.... However, when i press key to shoot everything is stopped and no shooting, no bullets....Whats happening?! Any help appreciated thanks.
<html>
<head>
<title>Spaceman Invaders</title>
<script>
window.onload = function() {
var posx = 20;
var posy = 0;
var go_right = true;
var go_down = false;
var canvas = document.getElementById("screen");
context = canvas.getContext("2d");
context2 = canvas.getContext("2d");
var Alien = function(x, y) {
this.x = x;
this.y = y;
this.posx = 30 + x*30;
this.posy = 90 + y*30;
this.go_right = true;
this.go_down = false;
}
function Player() {
this.x=0, this.y = 0, this.w = 20, this.h = 20;
this.render = function (){
context.fillStyle = "orange";
context.fillRect(this.x, this.y, this.w, this.h);
}
}
var X2=223;
var Y2=320;
function shoot(){
context2.fillStyle = "white";
context2.fillRect = (X2, Y2--, 5,10);
context2.fillStyle = "yellow";
context2.fillRect = (X2, Y2, 5,10);
if (Y2>=0) {
timer=setTimeout("shoot()", 5);
}
}
var player = new Player();
Alien.prototype.move = function() {
if (!this.go_down) {
if(this.posx + (2-this.x) * 30 < 250 && this.go_right) {
this.posx += 3;
} else if(this.posx < 30 + this.x*30) {
this.go_right = true;
this.go_down = true;
} else if(!this.go_right) {
this.posx -= 3;
} else {
this.go_right = false;
this.go_down = true;
}
} else {
//if(posy <= 30)
this.posy += 30;
this.go_down = false;
}
}
Alien.prototype.draw = function(context) {
if(this.x == 0) {
context.fillStyle = "red";
} else if(this.x == 1) {
context.fillStyle = "yellow";
} else {
context.fillStyle = "blue";
}
context.beginPath();
context.fillRect(this.posx, this.posy, 20 , 20);
context.fill();
}
var canvas = document.getElementById("screen");
context = canvas.getContext("2d");
if (canvas.getContext) {
//init the aliens array
var aliens = [];
for(var i = 0; i < 3; i++) {
for(var j = 0; j < 3; j++) {
aliens.push(new Alien(j, i));
}
}
player.x=100;
player.y= 480;
setInterval( function() {
context.fillStyle="black";
context.fillRect(0,0,canvas.width, canvas.height);
/*context.fillStyle = "white";
context.fillRect(100, 460, 30 , 30);*/
player.render();
//move all aliens & draw all aliens
for(var i = 0; i < 9; i++) {
aliens[i].move(),
aliens[i].draw(context);
}
}, 20);
document.addEventListener('keydown', function(event){
var key_press = String.fromCharCode(event.keyCode);
// alert(event.keyCode + " | " + key_press);
if (key_press == "D") {
if (player.x >=(280)){
player.x=(280);
}
else {
player.x +=3;
}
} else
if (key_press=="A"){
if (player.x<0){
player.x=(0);
}
else {player.x -=3;}
} else
if (key_press="W") {
//alert("Pah-pah");
shoot();
}
});
}
};
</script>
</head>
<body>
<canvas id="screen" width="300" height="500"/>
</body>
</html>

In your shoot() function, you're setting fillRect to the parameters you're meaning to pass to fillRect().
function shoot(){
context2.fillStyle = "white";
//context2.fillRect = (X2, Y2--, 5,10); -- This is a bad line. Correct:
context2.fillRect(X2, Y2--, 5,10);
context2.fillStyle = "yellow";
//context2.fillRect = (X2, Y2, 5,10); -- This is a bad line. Correct:
context2.fillRect(X2, Y2, 5,10);
if (Y2>=0) {
timer=setTimeout("shoot()", 5);
}
}
Has some weird behavior, and there's a lot that could be improved and cleaned up here, but this should get you on the right track.
And for your future reference, if you're using Chrome, open up the Dev Tools (CTRL/CMD + SHIFT + J) and you could see the error:
Uncaught TypeError: Property 'fillRect' of object #<CanvasRenderingContext2D> is not a function
That clued me in to know it was getting overwritten somewhere, as we know it IS normally a function of the CanvasRenderingContext2D.