Box2dWeb: Object Rotation - GetWorldCenter not found

Question

I'm trying to make a function for a skydiving game that'll make a sprite rotate left/right and then move that way depending on how angled it is, at least i would be if the object's get world centre parameter was recognised, code is as follows:

Dynamic circle creation:

function defineNewDynamicCircle(density, friction, restitution, x, y, r, objid) {
    var fixDef = new b2FixtureDef;
    fixDef.density = density;
    fixDef.friction = friction;
    fixDef.restitution = restitution;
    var bodyDef = new b2BodyDef;
    bodyDef.type = b2Body.b2_dynamicBody;
    bodyDef.position.x = x / SCALE;
    bodyDef.position.y = y / SCALE;
    // This creates a new circle shape
    fixDef.shape = new b2CircleShape(r/SCALE);
    var thisobj = world.CreateBody(bodyDef).CreateFixture(fixDef);
    thisobj.GetBody().SetUserData({id:objid})
    return thisobj;
}

object initialisation:

var capsule = defineNewDynamicCircle(1.0,0.2,0.8,400,400,20,"capsule");

problematic function:

function bankleft() {
    capsule.GetBody.ApplyImpulse(new b2Vec2(-0.5,0), capsule.GetBody.GetWorldCenter());
    if(capsule.GetBody.GetLinearVelocity().x < -5) {
        capsule.GetBody.SetLinearVelocity(new b2Vec2(-5,capsule.GetBody.GetLinearVelocity().y));
    }
}

Thanks for any insights you can provide!