Edit in JSFiddle

// get the canvas element
var canvas = document.getElementById('canvas-1');
// get a context to draw in
var ctx = canvas.getContext('2d');
function tick() {
  // clear what was drawn in the previous frame
  canvas.width = canvas.width;
  // begin a new path: arc is a line instruction like lineTo
  ctx.beginPath();
  // define the circle: position according to time, 50px radius
  ctx.arc((Date.now() / 10) % 500 + 50, 50, 50, 0, 2 * Math.PI);
  // draw the circle
  ctx.fill();
  // request a chance to draw the circle again as soon as possible
  requestAnimationFrame(tick);
}
tick();

<canvas width=600 height=100 id='canvas-1'></canvas>