facebook
Makes HermesRuntimeImpl::isArray proxy-compatible
When using hermes engine, dynamicFromValue in React Native calls isArray here to check if the input object is an array.
Consider this piece of code:
const list = ['a', 'b', 'c', 'd'];
const proxyed_list = new Proxy(list, {});
The proxyed_list behaves exactly the same as list in Javascript. But when been passed to native and converted to folly::dynamic by dynamicFromValue, proxyed_list will be converted to {"0": "a", "1": "b", "2": "c", "3": "d"} but not the expected ["a", "b", "c", "d"].
This patch implements similar routines in commit 26840ed441d614a07809c612521074ca9544086c and makes isArray proxy-compatible.
Summary
Test Plan
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Hey @dorentus, thanks for submitting this. This is a little tricky because Hermes today assumes that any jsi::Array must be an instance of a vm::JSArray. Supporting this would require changing this assumption throughout hermes.cpp. Given that the API is documented to be equivalent to Array.isArray(), that is probably the right thing to do, but we'll look into it a bit first.
https://github.com/facebook/react-native/blame/321f7dbcadb78dede9048500ab8abe86af863061/packages/react-native/ReactCommon/jsc/JSCRuntime.cpp#L993
JSC used to implement isArray by calling Array.isArray when _JSC_FAST_IS_ARRAY is defined, but now it only calls JSValueIsArray.
And JSValueIsArray behaves the same as HermesRuntimeImpl::isArray and returns false for new Proxy([], {}).
So yes, changes suggested by this PR might cause more problems than expected.
As for the dynamicFromValue problem, I'm trying the Array.isArray approach in my local copy of code to see if is has any performance glitches.
Closing for now, since no progress. Please feel free to re-open when there are updates.
