Drawing a rectangle in Canvas with user input
<!doctype html>
<html>
<head>
<link rel="stylesheet" type="text/css" media="all" href="css/reset.css" /> <!-- reset css -->
<script type="text/javascript" src="http://code.jquery.com/jquery.min.js"></script>
<style>
body{ background-color: ivory; }
canvas{border:1px solid red;}
</style>
<script>
$(function(){
// get a reference to the canvas and context
var canvas=document.getElementById("canvas");
var ctx=canvas.getContext("2d");
// vars for current width and height of a canvas rectangle
var width=50;
var height=35;
// references to the input-text elements
// used to let user change the rect width & height
var $width=document.getElementById('width');
var $height=document.getElementById('height')
// set the initial input-text values to the width/height vars
$width.value=width;
$height.value=height;
// call the draw command
draw();
// listen for keyup events on width & height input-text elements
// Get the current values from input-text & set the width/height vars
// call draw to redraw the rect with the current width/height values
$width.addEventListener("keyup", function(){
width=this.value;
draw();
}, false);
$height.addEventListener("keyup", function(){
height=this.value;
draw();
}, false);
// draw() clears the canvas and redraws the rect
// based on user input
function draw(){
ctx.clearRect(0,0,canvas.width,canvas.height);
ctx.fillRect(40,40,width,height);
}
}); // end $(function(){});
</script>
</head>
<body>
Width:<input type="text" id="width"><br>
height:<input type="text" id="height"><br>
<canvas id="canvas" width=300 height=300></canvas>
</body>
</html>