Fix for 1512: sandbox.stub() fails on prototype props

[fatso83]

Purpose (TL;DR) - mandatory

This PR restores the functionality in sandboxes that made it possible to stub methods that were higher on the prototype chain.

Background

The bug introduced in 74263dd happened as the check for a property only checked on the instance, not on the properties further up the prototype chain. That meant it was no longer possible to stub out methods on instances of a "class". See issue #1512 for background info.

Solution

Simply checks for existence of the (enumerable) property.

How to verify - mandatory

1. Check out this branch
2. npm install
3. npm test (will run the testcase)
4. git checkout HEAD~1 && npm test should fail

[fatso83]

The test only checks for enumerable properties though ... I can change it to use Object.hasOwnProperty, but it will need to recurse up the prototype chain to the top. Do we have such a utility? Wrong See below.

[lucasfcosta]

Hi @fatso83, excellent work, as always.

We do not have that exact utility, but we have get-property-descriptor, which goes up the prototype chain until it finds a property descriptor, otherwise it returns undefined.

I think we could use that and check if the descriptor returned is defined or not.

[fatso83]

Ha, I was wrong: the original code was 100% correct, and I just learned how to read the EcmaScript specification 😄

Some exploratory tests using prop in object on non-enumerable objects made me confused, as it showed that it actually could tell if a non-enumerable property was present. I thought that it was the complement of the for ... in, but then I

1. found the definition of how the in operator works, which lead me to
2. [[HasOperator]], which again led me to
3. [[GetProperty]], which essentially is exactly what our internal util does - a recursive lookup on the prototype to find the descriptor!

So, basically I'll just merge this as it is, as it does exactly what was intended (somewhat by chance) 🙈

[mroderick]

👍

[lucasfcosta]

@fatso83 awesome! I haven't noticed that before.

Btw, if you're interested in doing some more research into the MetaObject Protocol I highly recommend you this link. Which explains them and relates them to how Proxies work.