Let's add a fixed object so that our falling circle has something to bounce on. The construction follows the standard pattern - create a body definition, create a fixture definition and use these to create a body and its fixture:
In this case, the only real difference is that we are defining the shape to be a rectangle 20 meters long and 0.4 meters thick. Note that we specify the half-width and half-height in the function call.
If you add this to the program just before the end then you will see the ball fall from the "sky" and bounce a few times on the "ground".
Make sure that you understand what is going on. Try changing the restitution to see how it affects the way the ball bounces.
You can also try giving the body an initial velocity:
var body=world.CreateBody(bodyDef); body.CreateFixture(fixDef); body.SetLinearVelocity(new b2Vec2(2,2));
Now you will see it move sideways bounce and then move off to the left. Notice that the "ball" picks up rotational motion from the collisions as it would in real life.
You can also add some side walls to constrain the circle to stay withing a region: