Centering text inside a canvas rectangle (button)

Question

Is there a dynamic way to center the text based on the height, width, X, Y, or a rectangle and the text length? I'm having a bear trying to manually figuring out the X/Y coordinates for a button text.

Ignoring that I'm using a method roundRect to make rounded edges, what is a good way to center the text?

ctx.lineWidth = 4;
ctx.strokeStyle = "#000000";
ctx.fillStyle = "#abc";
roundRect(ctx, 10, 10, 100, 50, 10, true);
ctx.font="20px Georgia";
ctx.fillStyle = "#000000";
var rectHeight = 50;
var rectWidth = 100;
var rectX = 10;
var rectY = 10;
ctx.fillText("Attack!",rectX+(rectWidth/2),rectY+(rectHeight/2));

See fiddle: http://jsfiddle.net/vu7dZ/1/

Answer 1

The answer lays mostly in textAlign="center" and textBaseline="middle". These 2 properties align the text in the horizontal space and vertical space respectively.

ctx.lineWidth = 4;
ctx.strokeStyle = "#000000";
ctx.fillStyle = "#abc";
roundRect(ctx, 10, 10, 100, 50, 10, true);
ctx.font="20px Georgia";
ctx.textAlign="center"; 
ctx.textBaseline = "middle";**
ctx.fillStyle = "#000000";
var rectHeight = 50;
var rectWidth = 100;
var rectX = 10;
var rectY = 10;
ctx.fillText("Attack!",rectX+(rectWidth/2),rectY+(rectHeight/2));

Demo: http://jsfiddle.net/vu7dZ/4/