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Dynamic Centering and Alignment on the Canvas

Published : November 17, 2014   •   Last Edited : November 24, 2025   •   Author : Adam Khoury
In this JavaScript programming exercise you can learn the logic behind dynamic centering and alignment for dynamically positioning your canvas assets. 
<!DOCTYPE html>
<html>
<head>
<style>
body{ margin:10px; background:#666; }
#my_canvas{ background:#FFF; border:#000 1px solid; }
</style>
<script>
function draw(){
    var ctx = document.getElementById('my_canvas').getContext('2d');
    ctx.fillStyle = "magenta";
    var rectW = 100;
    var rectH = 100;
    var rectX = (ctx.canvas.width * .5) - (rectW * .5);
    var rectY = (ctx.canvas.height * .5) - (rectH * .5);
    ctx.fillRect(rectX, rectY, rectW, rectH);
}
window.onload = draw;
</script>
</head>
<body>
<canvas id="my_canvas" width="500" height="350"></canvas>
</body>  
</html>
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