Today we continue to discuss jigsaw.Efficient, low effort mashup isolation by James Mickens and Matthew Finifter. Mashups are web applications that contain code from different principals.
Jigsaw lets mashups be isolated with its framework  and lets them selectively expose private state. We were reviewing boxes and the choice of encapsul]ation syntax for principal objects.
This syntax can also be used not just as access modifiers but also visibility modifiers. They can be applied on property and anywhere on variables.
Jigsaw checks whether the statement in which public or private appears has visibility settings for the relevant prototype object. If there's a public method on 'obj', the surrogate wraps the this and binds it to the obj preventing attacks where the 'this' pointer can be changed. Jigsaw also does lazy evaluation because the objects graph can be big with most objects lightly used.
Surrogates automatically protect cross box data exchanges.

Jigsaw also virtualizes the DOM tree in each box. It lives inside each box's Javascript namespace. This is also an example of a set of predefined Javascript objects that jigsaw manages. As we can see, jigsaw improves the language and usage for its purpose.

Crossbox events are also supported. If a box X wants to register an event handler in a different box Y, it passes the handler to Y via Y's public interface. Y can then register the handler with the browser's event engine in the standard way. The difference is that the event object may not contain Y's references so its scrubbed before its passed externally. This prevents information leakage via foreign event handlers. The handler executes in the Javascript context of the box that created it.

Access from child to parents namespace is facilitated with Jigsaw.getParentPrincipal() and the access to the principal objects of its own children is facilitated with Jigsaw.principals Jigsaw.getSameDomainPrincipals. Jigsaw associates each principal with a unique id, and parent can restrict a child's access to a subset of the principal ids.

Similarly networking privileges and visual fields can be further restricted or relinquished but this has to be progressive down the hierarchy. This means that privileges decrease strictly down the hierarchy.

#codingexercise
uint factorial(uint n)
{
 if (n == 1) return 1;
 return n * factorial(n-1);
}
#codingexercise
uint GetEven (int n)
{
 If (n%2 != 0) return n+1;
Return n;
}