HTML5 Dynamically create Canvas
JavascriptHtmlCanvasJavascript Problem Overview
Hi there I have a question about dynamically creating a canvas using javascript.
I create a canvas like this:
var canvas = document.createElement('canvas');
canvas.id = "CursorLayer";
canvas.width = 1224;
canvas.height = 768;
canvas.style.zIndex = 8;
canvas.style.position = "absolute";
canvas.style.border = "1px solid";
but when I try to locate it, I get a null value:
cursorLayer = document.getElementById("CursorLayer");
Am I doing it wrong? Is there a better way to create a canvas using JavaScript?
Javascript Solutions
Solution 1 - Javascript
The problem is that you do not insert your canvas element in the document body.
Just do the following:
document.body.appendChild(canvas);
Example:
var canvas = document.createElement('canvas');
canvas.id = "CursorLayer";
canvas.width = 1224;
canvas.height = 768;
canvas.style.zIndex = 8;
canvas.style.position = "absolute";
canvas.style.border = "1px solid";
var body = document.getElementsByTagName("body")[0];
body.appendChild(canvas);
cursorLayer = document.getElementById("CursorLayer");
console.log(cursorLayer);
// below is optional
var ctx = canvas.getContext("2d");
ctx.fillStyle = "rgba(255, 0, 0, 0.2)";
ctx.fillRect(100, 100, 200, 200);
ctx.fillStyle = "rgba(0, 255, 0, 0.2)";
ctx.fillRect(150, 150, 200, 200);
ctx.fillStyle = "rgba(0, 0, 255, 0.2)";
ctx.fillRect(200, 50, 200, 200);
Solution 2 - Javascript
Via Jquery:
$('<canvas/>', { id: 'mycanvas', height: 500, width: 200});
Solution 3 - Javascript
It happens because you call it before DOM has loaded. Firstly, create the element and add atrributes to it, then after DOM has loaded call it. In your case it should look like that:
var canvas = document.createElement('canvas');
canvas.id = "CursorLayer";
canvas.width = 1224;
canvas.height = 768;
canvas.style.zIndex = 8;
canvas.style.position = "absolute";
canvas.style.border = "1px solid";
window.onload = function() {
document.getElementById("CursorLayer");
}
Solution 4 - Javascript
Alternative
Use element .innerHTML= which is quite fast in modern browsers
document.body.innerHTML = "<canvas width=500 height=150 id='CursorLayer'>";
// TEST
var ctx = CursorLayer.getContext("2d");
ctx.fillStyle = "red";
ctx.fillRect(100, 100, 50, 50);
canvas { border: 1px solid black }
Solution 5 - Javascript
<html>
<head></head>
<body>
<canvas id="canvas" width="300" height="300"></canvas>
<script>
var sun = new Image();
var moon = new Image();
var earth = new Image();
function init() {
sun.src = 'https://mdn.mozillademos.org/files/1456/Canvas_sun.png';
moon.src = 'https://mdn.mozillademos.org/files/1443/Canvas_moon.png';
earth.src = 'https://mdn.mozillademos.org/files/1429/Canvas_earth.png';
window.requestAnimationFrame(draw);
}
function draw() {
var ctx = document.getElementById('canvas').getContext('2d');
ctx.globalCompositeOperation = 'destination-over';
ctx.clearRect(0, 0, 300, 300);
ctx.fillStyle = 'rgba(0, 0, 0, 0.4)';
ctx.strokeStyle = 'rgba(0, 153, 255, 0.4)';
ctx.save();
ctx.translate(150, 150);
// Earth
var time = new Date();
ctx.rotate(((2 * Math.PI) / 60) * time.getSeconds() + ((2 * Math.PI) / 60000) *
time.getMilliseconds());
ctx.translate(105, 0);
ctx.fillRect(10, -19, 55, 31);
ctx.drawImage(earth, -12, -12);
// Moon
ctx.save();
ctx.rotate(((2 * Math.PI) / 6) * time.getSeconds() + ((2 * Math.PI) / 6000) *
time.getMilliseconds());
ctx.translate(0, 28.5);
ctx.drawImage(moon, -3.5, -3.5);
ctx.restore();
ctx.restore();
ctx.beginPath();
ctx.arc(150, 150, 105, 0, Math.PI * 2, false);
ctx.stroke();
ctx.drawImage(sun, 0, 0, 300, 300);
window.requestAnimationFrame(draw);
}
init();
</script>
</body>
</html>