Implement Error cause.

Summary:
https://tc39.es/proposal-error-cause/

This is a stage 3 proposal to add a `cause` property to JSErrors
when the option is provided in the constructor.

Add a check in the Error constructor which checks for the property
and defines it if it exists.

Reviewed By: dulinriley

Differential Revision: D27102691

fbshipit-source-id: 45c02c9ed009961a0442bd5c7534fc509310d26c

include/hermes/VM/PredefinedStrings.def

@@ -138,6 +138,7 @@ STR(gc, "gc")
 STR(name, "name")
 STR(displayName, "displayName")
 STR(message, "message")
+STR(cause, "cause")
 STR(stack, "stack")
 STR(stacktraceTooLong, "Stacktrace too long")
 #define ALL_ERROR_TYPE(name) STR(name, "" #name)

lib/VM/JSLib/Error.cpp

@@ -118,6 +118,38 @@ static CallResult<HermesValue> constructErrorObject(
     }
   }
 
+  // https://tc39.es/proposal-error-cause/
+  // InstallErrorCause(O, options).
+  // If Type(options) is Object and ? HasProperty(options, "cause") is true
+  if (Handle<JSObject> options = args.dyncastArg<JSObject>(1)) {
+    GCScopeMarkerRAII marker{runtime};
+    NamedPropertyDescriptor desc;
+    Handle<JSObject> propObj =
+        runtime->makeHandle(JSObject::getNamedDescriptorPredefined(
+            options, runtime, Predefined::cause, desc));
+    if (propObj) {
+      // a. Let cause be ? Get(options, "cause").
+      auto causeRes = JSObject::getNamedPropertyValue_RJS(
+          selfHandle, runtime, std::move(propObj), desc);
+      if (LLVM_UNLIKELY(causeRes == ExecutionStatus::EXCEPTION)) {
+        return ExecutionStatus::EXCEPTION;
+      }
+      Handle<> cause = runtime->makeHandle(std::move(*causeRes));
+      // b. Perform ! CreateNonEnumerableDataPropertyOrThrow(O, "cause", cause).
+      if (LLVM_UNLIKELY(
+              JSObject::defineOwnProperty(
+                  selfHandle,
+                  runtime,
+                  Predefined::getSymbolID(Predefined::cause),
+                  DefinePropertyFlags::getNewNonEnumerableFlags(),
+                  cause,
+                  PropOpFlags().plusThrowOnError()) ==
+              ExecutionStatus::EXCEPTION)) {
+        return ExecutionStatus::EXCEPTION;
+      }
+    }
+  }
+
   return selfHandle.getHermesValue();
 }
 

test/hermes/error-cause.js

@@ -0,0 +1,47 @@
+/**
+ * Copyright (c) Facebook, Inc. and its affiliates.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ */
+
+// RUN: %hermes -O %s | %FileCheck --match-full-lines %s
+// RUN: %hermes -O -emit-binary -out %t.hbc %s && %hermes %t.hbc | %FileCheck --match-full-lines %s
+
+print('error cause');
+// CHECK-LABEL: error cause
+
+try {
+  try {
+    throw Error("err1");
+  } catch (e1) {
+    throw Error("err2", { cause: e1 });
+  }
+} catch (e2) {
+  print(e2.message);
+// CHECK-NEXT: err2
+  print(e2.cause.message);
+// CHECK-NEXT: err1
+}
+
+try {
+  throw Error("err", {get cause() {
+    print('getter');
+    return 'foo';
+  }});
+// CHECK-NEXT: getter
+} catch (e) {
+  print(e.message);
+// CHECK-NEXT: err
+  print(e.cause);
+// CHECK-NEXT: foo
+}
+
+try {
+  throw Error("err", {});
+} catch (e) {
+  print(e.message);
+// CHECK-NEXT: err
+  print(Object.hasOwnProperty(e, 'cause'));
+// CHECK-NEXT: false
+}