Edit in JSFiddle

var c = document.getElementById('c');
c.width = 200;
c.height = 200;
var ctx = c.getContext('2d');

ctx.translate(100, 100);

// Math.PI * 2 is one way around the circle
for (var i = 0; i < Math.PI * 2; i += 0.1) {
    ctx.fillRect(
        // cos provides the x value
        Math.cos(i) * 90,
        // sin provides the y value
        Math.sin(i) * 90,
        2, 2);
}
<canvas id='c'></canvas>