HTML 5 Canvas Tutorial circle,canvas animation code

Essential measures in animation
These are the steps you ought to take while designing a framework:

Unravel the canvas
Since the shapes you are drawing cover the whole canvas (for example a background image) you would need to remove all previously drawn shapes. The best way to achieve this is by using the form clearRect).
Save condition of the canvas

If the State has been restored, restore it until creating a new picture.