Add TextEncoder.prototype.encoding property

Summary: Adds the read-only `encoding` property to TextEncoder

Reviewed By: avp

Differential Revision: D53212867

fbshipit-source-id: 010875ff359aaaa8e84a185c8d716bc4689a1538

include/hermes/VM/InternalProperties.def

@@ -31,6 +31,7 @@ NAMED_PROP(IntlNativeType)
 NAMED_PROP(NativeState)
 NAMED_PROP(ArrayBufferExternalFinalizer)
 NAMED_PROP(ExternalMemoryPressure)
+NAMED_PROP(TextEncoderType)
 
 #undef PROP
 #undef NAMED_PROP

include/hermes/VM/NativeFunctions.def

@@ -362,6 +362,7 @@ NATIVE_FUNCTION(symbolPrototypeToString)
 NATIVE_FUNCTION(symbolPrototypeDescriptionGetter)
 NATIVE_FUNCTION(symbolPrototypeValueOf)
 NATIVE_FUNCTION(textEncoderConstructor)
+NATIVE_FUNCTION(textEncoderPrototypeEncoding)
 NATIVE_FUNCTION(throwTypeError)
 NATIVE_FUNCTION(typedArrayBaseConstructor)
 NATIVE_FUNCTION(typedArrayFrom)

include/hermes/VM/PredefinedStrings.def

@@ -485,6 +485,8 @@ STR(squareSymbolReplace, "[Symbol.replace]")
 STR(squareSymbolSplit, "[Symbol.split]")
 
 STR(TextEncoder, "TextEncoder")
+STR(encoding, "encoding")
+STR(utf8, "utf-8")
 
 #ifdef HERMES_ENABLE_INTL
 // TODO T65916424: Consider how we can move these out of the

lib/VM/JSLib/TextEncoder.cpp

@@ -25,6 +25,20 @@ Handle<JSObject> createTextEncoderConstructor(Runtime &runtime) {
       runtime.getPredefinedStringHandle(Predefined::TextEncoder),
       dpf);
 
+  // Based on
+  // Object.getOwnPropertyDescriptor(TextEncoder.prototype, 'encoding'), both
+  // Chrome and Safari have the 'encoding' property as enumerable and
+  // configurable. We set things up to be the same.
+  defineAccessor(
+      runtime,
+      textEncoderPrototype,
+      Predefined::getSymbolID(Predefined::encoding),
+      nullptr,
+      textEncoderPrototypeEncoding,
+      nullptr,
+      /* enumerable */ true,
+      /* configurable */ true);
+
   auto cons = defineSystemConstructor<JSObject>(
       runtime,
       Predefined::getSymbolID(Predefined::TextEncoder),
@@ -52,8 +66,42 @@ textEncoderConstructor(void *, Runtime &runtime, NativeArgs args) {
   }
 
   auto selfHandle = args.vmcastThis<JSObject>();
+
+  auto valueHandle = Runtime::getUndefinedValue();
+  if (LLVM_UNLIKELY(
+          JSObject::defineNewOwnProperty(
+              selfHandle,
+              runtime,
+              Predefined::getSymbolID(
+                  Predefined::InternalPropertyTextEncoderType),
+              PropertyFlags::defaultNewNamedPropertyFlags(),
+              valueHandle) == ExecutionStatus::EXCEPTION)) {
+    return ExecutionStatus::EXCEPTION;
+  }
+
   return selfHandle.getHermesValue();
 }
 
+CallResult<HermesValue>
+textEncoderPrototypeEncoding(void *, Runtime &runtime, NativeArgs args) {
+  GCScope gcScope{runtime};
+
+  auto selfHandle = args.dyncastThis<JSObject>();
+
+  NamedPropertyDescriptor desc;
+  bool exists = JSObject::getOwnNamedDescriptor(
+      selfHandle,
+      runtime,
+      Predefined::getSymbolID(Predefined::InternalPropertyTextEncoderType),
+      desc);
+  if (LLVM_UNLIKELY(!exists)) {
+    return runtime.raiseTypeError(
+        "TextEncoder.prototype.encoding called on non-TextEncoder object");
+  }
+
+  return HermesValue::encodeStringValue(
+      runtime.getPredefinedString(Predefined::utf8));
+}
+
 } // namespace vm
 } // namespace hermes

test/hermes/text-encoder.js

@@ -14,3 +14,12 @@ print('TextEncoder');
 var encoder = new TextEncoder();
 print(Object.prototype.toString.call(encoder));
 // CHECK-NEXT: [object TextEncoder]
+
+const desc = Object.getOwnPropertyDescriptor(TextEncoder.prototype, 'encoding');
+print(desc.enumerable);
+// CHECK-NEXT: true
+print(desc.configurable);
+// CHECK-NEXT: true
+
+print(encoder.encoding);
+// CHECK-NEXT: utf-8