Canvas al centro
HTML
<canvas></canvas>
JavaScript
var canvas = document.querySelector('canvas');
var context = canvas.getContext('2d');
var x0 = canvas.width/2 - 15;
var y0 = canvas.height/2 -15;
//prima setto le caratteristiche
context.fillStyle = 'black';
// poi creo lo spazio
context.fillRect(x0, y0,30,30);