fix(get-static-value): incorrect return when variable has object mutation

[ota-meshi]

This PR fixes getStaticValue to not retrieve the value if the variable has an object mutation.

Previously, for example, the value retrieved by getStaticValue for the a identifier in the following script returned {foo: 'a'}, which is incorrect.

const a = {foo: 'a'};
a.bar = 'b';
a; // <-- ?

Ideally, it would return {foo: 'a', bar: 'b'}, but since the result differs depending on when the value is retrieved and the a identifier is not static, I have fixed it so that getStaticValue returns null, assuming the value cannot be retrieved.

[fisker]

I'm not sure about this, there are too many ways to mutate an object, make an exception for property assignment not really helping IMHO. Maybe we should add extra property the result to indicate if the value is absolutely safe to use?

[ota-meshi]

Thank you for your comment!

there are too many ways to mutate an object

I agree with it, but I think it's at least a step forward from where we are now 😅

Maybe we should add extra property the result to indicate if the value is absolutely safe to use?

I think we could add a property, but under what specific conditions would that be absolutely safe?
Either way, I think that can be handled in a separate PR. This PR would allow us to return null for non-static values.

[fisker]

but under what specific conditions would that be absolutely safe?

Only defined, no other reference, except property access(read primitive value).

[ota-meshi]

I think the a variable in the last statement of the script below won't get that mark, is that correct?
So, I think the user should consider that if the value is an object, it is potentially dangerous (not absolutely safe).

const a = {foo: 'a'};

// ... 

a; // <--

This PR finds mutations of a in the following script, so I think it would be better to make adding new properties a separate task.

const a = {foo: 'a'};
a.bar = 'b';
a; // <--