wycats
Reflect mechanism to determine if a slot is accessible or not
Having to try/catch in order to prevent a runtime ReferenceError doesn't seem like an elegant or performant approach - could there also be a Reflect.hasDataSlot or similar, that takes an object and a data slot name, and returns true or false?
Assuming #2, we'd only want one method that returned false when the slot was either absent or inaccessible.
One of our starting requirements was that there would be no reflection APIs for accessing private slots. This was security motivated.
Note that a "name" would not be sufficient for making that query. The "key" of a private slot (called a "slot key") in the document is a pair consisting of a name and a declaration site. "Declaration sites" are not reified values. They are only implicitly accessible from within the class body that declares a private slot (or the body of a class that inherits a protected slot).
A class could expose a method for testing for the existence of a slot it declares:
class Foo {
private #bar;
static hasBar(obj) {
try {
obj.#bar;
return true;
} catch (e) {
return false
}
}
}
Foo.hasBar( {} ); //should answer false
There are a lot of methods that Reflect offers that return a boolean where the Object method would throw - would the same motivation not apply here? Forcing try/catch to protect against an almost inevitable runtime error doesn't seem like a good API choice - there are many complaints about JSON.parse in the same vein.
Note that this isn't a request for a general reflection mechanism, merely a non-try/catch-based way to protect against runtime errors.
No reflection is a starting position. If some reflection operation can be demonstrated to be security safe and sufficiently useful, there should be a problem.
Note that Reflect methods exist in order to fully rectify MOP calls. The Boolean results weren't a primary requirement but something that turned out to be reasonable and useful.
What you are really asking for, I think, is a nominal type test. One option might be to (later) introduce a new context-sensitive binary operator to perform such a test.
