Sounds like you want to rotate the rect around its centerpoint.
Red rectangle is original, Yellow rectangle is rotated around the centerpoint.
To do that you need to first context.translate to the rect's centerpoint before rotating.
// move the rotation point to the center of the rect
ctx.translate( x+width/2, y+height/2 );
// rotate the rect
ctx.rotate(degrees*Math.PI/180);
Note that the context is now in its rotated state.
That means drawing position [0,0] is visually at [ x+width/2, y+height/2 ].
So you must draw the rotated rect at [ -width/2, -height/2 ] (not at the original unrotated x/y).
// draw the rect on the transformed context
// Note: after transforming [0,0] is visually [-width/2, -height/2]
// so the rect needs to be offset accordingly when drawn
ctx.rect( -width/2, -height/2, width,height);
Here is code and a Fiddle: http://jsfiddle.net/m1erickson/z4p3n/
<!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(){
var canvas=document.getElementById("canvas");
var ctx=canvas.getContext("2d");
var startX=50;
var startY=80;
// draw an unrotated reference rect
ctx.beginPath();
ctx.rect(startX,startY,100,20);
ctx.fillStyle="blue";
ctx.fill();
// draw a rotated rect
drawRotatedRect(startX,startY,100,20,45);
function drawRotatedRect(x,y,width,height,degrees){
// first save the untranslated/unrotated context
ctx.save();
ctx.beginPath();
// move the rotation point to the center of the rect
ctx.translate( x+width/2, y+height/2 );
// rotate the rect
ctx.rotate(degrees*Math.PI/180);
// draw the rect on the transformed context
// Note: after transforming [0,0] is visually [x,y]
// so the rect needs to be offset accordingly when drawn
ctx.rect( -width/2, -height/2, width,height);
ctx.fillStyle="gold";
ctx.fill();
// restore the context to its untranslated/unrotated state
ctx.restore();
}
}); // end $(function(){});
</script>
</head>
<body>
<canvas id="canvas" width=300 height=300></canvas>
</body>
</html>
