modify-jsi-features

Adding New JSI Functionality

When adding new functionality (methods) to the JSI Runtime interface, you must modify a specific set of files across multiple layers. This skill describes each file, the patterns to follow, and important conventions.

Architecture Overview

JSI (JavaScript Interface) is an abstraction layer that allows C++ code to interact with JavaScript runtimes. The architecture consists of:

JSI Core (xplat/jsi/jsi/) — The abstract interface definitions
Hermes Implementation (xplat/static_h/API/hermes/) — Hermes-specific implementation
SynthTrace (xplat/static_h/API/hermes/) — Recording and replay infrastructure for debugging

Checklist of Files to Modify

JSI Core Files

xplat/jsi/jsi/jsi.h — Add pure virtual method declaration to IRuntime interface AND override declaration in Runtime class
xplat/jsi/jsi/jsi.cpp — Add default implementation (if providing one)
xplat/jsi/jsi/jsi-inl.h — Add inline helper methods (if needed)
xplat/jsi/jsi/decorator.h — Add method overrides to RuntimeDecorator and WithRuntimeDecorator
xplat/jsi/jsi/test/testlib.cpp — Add tests for the JSI API

Hermes Implementation Files

xplat/static_h/API/hermes/hermes.cpp — Add Hermes-specific implementation in HermesRuntimeImpl
xplat/static_h/unittests/API/APITest.cpp — Add Hermes-specific tests

SynthTrace Files (Required for Hermes)

xplat/static_h/API/hermes/SynthTrace.h — Add new Record types
xplat/static_h/API/hermes/SynthTrace.cpp — Implement Record serialization
xplat/static_h/API/hermes/TracingRuntime.h — Declare tracing method overrides
xplat/static_h/API/hermes/TracingRuntime.cpp — Implement tracing logic
xplat/static_h/API/hermes/SynthTraceParser.cpp — Add parsing for new records
xplat/static_h/API/hermes/TraceInterpreter.cpp — Add replay logic
xplat/static_h/unittests/API/SynthTraceTest.cpp — Add replay tests
xplat/static_h/unittests/API/SynthTraceSerializationTest.cpp — Add serialization tests
xplat/static_h/unittests/API/SynthTraceParserTest.cpp — Add parser tests

Implementation Strategy

When adding new features (methods) to the JSI Runtime, you have two options:

Option A: Default Implementation (Recommended)

Provide a default implementation in jsi::Runtime that works via JavaScript calls. This allows all runtimes (JSC, V8, etc.) to work without modification.

// jsi.h - Add virtual method with default implementation
virtual void myNewMethod(const Object& obj, const Value&

Option B: Pure Virtual (Requires All Runtime Implementations)

Make the method pure virtual, requiring all runtimes (JSCRuntime, V8Runtime, HermesRuntime) to implement it. Only use this when a JavaScript-based default is not possible.

Step-by-Step Guide

1. Add to `jsi.h` (JSI Core Interface)

Add the pure virtual method declaration to the IRuntime interface, and the override declaration to the Runtime class:

Important: The IRuntime interface contains the pure virtual declarations (= 0), while the Runtime class provides the default implementation with override keyword. This separation allows other runtimes to implement the interface differently while Runtime provides a JavaScript-based fallback.

 
If adding convenience methods to JSI types (like `Object`), add them too:
 
```cpp
class JSI_EXPORT Object : public Pointer {
  // ... existing methods ...
 
  /// Sets something on the object.
  void setMyThing(IRuntime& runtime, const Value& val) const {
    return runtime.myNewMethod(*this, val);
  }
 
  /// Gets something from the object.
  inline Value getMyThing(Runtime& runtime) const;
};

2. Add Default Implementation to `jsi.cpp`

Implement the default using JavaScript APIs when possible:

void Runtime::myNewMethod(const Object& obj, const Value& val) {
  // Get the JavaScript function
  auto myMethodFn = global()
                        .getPropertyAsObject(*this, "Object")
                        .getPropertyAsFunction(*this, "myMethod");
  // Call it with the appropriate arguments
  myMethodFn.call(*this, obj, val);
}
 
Value Runtime::myNewGetter(const Object& obj) {
  auto myGetterFn = global()
                        .getPropertyAsObject(*this, "Object")
                        .getPropertyAsFunction(*this, "myGetter");
  return myGetterFn.call(*this, obj);
}

3. Add Inline Methods to `jsi-inl.h` (If Needed)

For methods that return incomplete types at declaration time:

Value Object::getMyThing(Runtime& runtime) const {
  return runtime.myNewGetter(*this);
}

4. Update `decorator.h`

Add method overrides to both RuntimeDecorator and WithRuntimeDecorator:

// In RuntimeDecorator class (around line 282)
void myNewMethod(const Object& obj, const Value& val) override {
  plain_.myNewMethod(obj, val);
}
 
Value myNewGetter(const Object& obj) override {
  return plain_.myNewGetter(obj);
}
 
// In WithRuntimeDecorator class (around line 760)
void myNewMethod(const Object& obj, const Value& val) override {
  Around around{with_};
  RD::myNewMethod(obj, val);
}
 
Value myNewGetter(const Object& obj) override {
  Around around{with_};
  return RD::myNewGetter(obj);
}

5. Add JSI Core Tests in `test/testlib.cpp`

Test the default implementation using a RuntimeDecorator:

TEST_P(JSITest, MyNewFeature) {
  // Use a RuntimeDecorator to test the default implementation
  class RD : public RuntimeDecorator<Runtime, Runtime> {
   public:
    explicit RD(Runtime& rt) : RuntimeDecorator(rt) {}
 
    void myNewMethod(const Object& obj, const Value& val) override {
      return Runtime::myNewMethod(obj, val);
    }
 
    Value myNewGetter(const Object& obj) override {
      return Runtime::myNewGetter(obj);
    }
  };
 
  RD rd = RD(rt);
  Object obj(rd);
 
  // Test the functionality
  obj.setMyThing(rd, Value(123));
  EXPECT_EQ(obj.getMyThing(rd).getNumber(), 123);
}

6. Add Hermes Implementation in `hermes.cpp`

Implement the optimized version using Hermes VM APIs inside HermesRuntimeImpl:

// In HermesRuntimeImpl class declaration (around line 673)
void myNewMethod(const jsi::Object& obj, const jsi::Value& val) override;
jsi::Value myNewGetter(const jsi::Object& obj) override;
 
// Implementation (around line 2074)
void HermesRuntimeImpl::myNewMethod(
    const jsi::Object& obj,
    const jsi::Value& val) {
  // Validate input if needed
  if (!val.isObject() && !val.isNull()) {
    throw jsi::JSError(*this, "Invalid argument type");
  }
 
  // Use Hermes VM APIs directly
  auto cr = vm::JSObject::myVMMethod(
      vm::vmcast<vm::JSObject>(phv(obj)),
      runtime_,
      hvFromValue(val),
      vm::PropOpFlags().plusThrowOnError());
  checkStatus(cr.getStatus());
}
 
jsi::Value HermesRuntimeImpl::myNewGetter(const jsi::Object& obj) {
  vm::CallResult<vm::PseudoHandle<vm::JSObject>> cr =
      vm::JSObject::myVMGetter(handle(obj), runtime_);
  checkStatus(cr.getStatus());
  if (!*cr) {
    return jsi::Value::null();
  }
  return valueFromHermesValue(cr->getHermesValue());
}

7. Add Hermes Tests in `APITest.cpp`

TEST_P(HermesRuntimeTest, MyNewFeature) {
  Object obj(*rt);
 
  // Test normal operation
  Value val(*rt, 123);
  obj.setMyThing(*rt, val);
  EXPECT_EQ(obj.getMyThing(*rt).getNumber(), 123);
 
  // Test edge cases
  obj.setMyThing(*rt, Value::null());
  EXPECT_TRUE(obj.getMyThing(*rt).isNull());
 
  // Test error cases
  EXPECT_THROW(obj.setMyThing(*rt, Value("invalid")), JSError);
}

8. Add SynthTrace Record Types in `SynthTrace.h`

Add new record types for tracing:

// In the RECORD macro list (around line 210)
RECORD(MyNewMethod)                    \
RECORD(MyNewGetter)                    \
 
// Define the record structs (after existing records)
 
/// A MyNewMethodRecord is an event where native code calls myNewMethod
struct MyNewMethodRecord : public Record {
  static constexpr RecordType type{RecordType::MyNewMethod};
  const ObjectID objID_;
  const TraceValue value_;
 
  MyNewMethodRecord(TimeSinceStart time, ObjectID objID, TraceValue value)
      : Record(time), objID_(objID), value_(value) {}
 
  void toJSONInternal(::hermes::JSONEmitter &json) const override;
 
  RecordType getType() const override {
    return type;
  }
 
  std::vector<ObjectID> uses() const override {
    std::vector<ObjectID> uses{objID_};
    pushIfTrackedValue(value_, uses);
    return uses;
  }
};
 
/// A MyNewGetterRecord is an event where native code calls myNewGetter
struct MyNewGetterRecord : public Record {
  static constexpr RecordType type{RecordType::MyNewGetter};
  const ObjectID objID_;
 
  MyNewGetterRecord(TimeSinceStart time, ObjectID objID)
      : Record(time), objID_(objID) {}
 
  void toJSONInternal(::hermes::JSONEmitter &json) const override;
 
  RecordType getType() const override {
    return type;
  }
 
  std::vector<ObjectID> uses() const override {
    return {objID_};
  }
};

9. Implement Record Serialization in `SynthTrace.cpp`

void SynthTrace::MyNewMethodRecord::toJSONInternal(JSONEmitter &json) const {
  Record::toJSONInternal(json);
  json.emitKeyValue("objID", objID_);
  json.emitKeyValue("value", encode(value_));
}
 
void SynthTrace::MyNewGetterRecord::toJSONInternal(JSONEmitter &json) const {
  Record::toJSONInternal(json);
  json.emitKeyValue("objID", objID_);
}

10. Add Tracing Method Declarations in `TracingRuntime.h`

class TracingRuntime : public jsi::RuntimeDecorator<jsi::Runtime> {
  // ... existing methods ...
 
  void myNewMethod(const jsi::Object& obj, const jsi::Value& val) override;
  jsi::Value myNewGetter(const jsi::Object& obj) override;

11. Implement Tracing Logic in `TracingRuntime.cpp`

void TracingRuntime::myNewMethod(
    const jsi::Object& obj,
    const jsi::Value& val) {
  trace_.emplace_back<SynthTrace::MyNewMethodRecord>(
      getTimeSinceStart(), useObjectID(obj), useTraceValue(val));
  RD::myNewMethod(obj, val);
}
 
jsi::Value TracingRuntime::myNewGetter(const jsi::Object& obj) {
  trace_.emplace_back<SynthTrace::MyNewGetterRecord>(
      getTimeSinceStart(), useObjectID(obj));
 
  auto result = RD::myNewGetter(obj);
  trace_.emplace_back<SynthTrace::ReturnToNativeRecord>(
      getTimeSinceStart(), defTraceValue(result));
  return result;
}

12. Add Parsing in `SynthTraceParser.cpp`

case RecordType::MyNewMethod: {
  trace.emplace_back<SynthTrace::MyNewMethodRecord>(
      timeFromStart,
      objID->getValue(),
      SynthTrace::decode(propValue->c_str()));
  break;
}
case RecordType::MyNewGetter:
  trace.emplace_back<SynthTrace::MyNewGetterRecord>(
      timeFromStart, objID->getValue());
  break;

13. Add Replay Logic in `TraceInterpreter.cpp`

case RecordType::MyNewMethod: {
  const auto &record =
      static_cast<const SynthTrace::MyNewMethodRecord &>(*rec);
  auto obj = getJSIValueForUse(record.objID_).getObject(rt_);
  obj.setMyThing(rt_, traceValueToJSIValue(record.value_));
  break;
}
case RecordType::MyNewGetter: {
  const auto &record =
      static_cast<const SynthTrace::MyNewGetterRecord &>(*rec);
  auto obj = getJSIValueForUse(record.objID_).getObject(rt_);
  auto result = obj.getMyThing(rt_);
  retval = std::move(result);
  break;
}

14. Add SynthTrace Tests

In SynthTraceSerializationTest.cpp:

TEST_F(SynthTraceSerializationTest, MyNewMethodTest) {
  EXPECT_EQ(
      R"({"type":"MyNewMethodRecord","time":0,"objID":1,"value":"null:"})",
      to_string(SynthTrace::MyNewMethodRecord(
          dummyTime, 1, SynthTrace::encodeNull())));
}
 
TEST_F(SynthTraceSerializationTest, MyNewGetterTest) {
  EXPECT_EQ(
      R"({"type":"MyNewGetterRecord","time":0,"objID":1})",
      to_string(SynthTrace::MyNewGetterRecord(dummyTime, 1)));
}

In SynthTraceTest.cpp:

TEST_F(SynthTraceReplayTest, MyNewFeatureReplay) {
  {
    auto &rt = *traceRt;
    jsi::Object obj(rt);
    obj.setMyThing(rt, jsi::Value(123));
    rt.global().setProperty(rt, "obj", obj);
 
    auto result = obj.getMyThing(rt);
    rt.global().setProperty(rt, "result", result);
  }
  replay();
  {
    auto &rt = *replayRt;
    auto obj = rt.global().getProperty(rt, "obj").getObject(rt);
    EXPECT_EQ(obj.getMyThing(rt).getNumber(), 123);
 
    auto result = rt.global().getProperty(rt, "result");
    EXPECT_EQ(result.getNumber(), 123);
  }
}

In SynthTraceParserTest.cpp:

TEST_F(SynthTraceParserTest, ParseMyNewMethodRecord) {
  const char *src = R"(
{
  "version": 5,
  "globalObjID": 258,
  "runtimeConfig": {
    "gcConfig": {
      "initHeapSize": 33554432,
      "maxHeapSize": 536870912
    }
  },
  "trace": [
    {
      "type": "MyNewMethodRecord",
      "time": 1234,
      "objID": 1,
      "value": "number:123"
    }
  ]
}
  )";
  auto parseResult = parseSynthTrace(bufFromStr(src));
  SynthTrace &trace = std::get<0>(parseResult);
 
  auto record = dynamic_cast<const SynthTrace::MyNewMethodRecord &>(
      *trace.records().at(0));
  ASSERT_EQ(record.objID_, 1);
}

Key Conventions

Default Implementations: Always provide a default implementation in jsi::Runtime when possible. This allows JSCRuntime and V8Runtime to work without modification.
Hermes Optimization: For Hermes, override the default with an optimized implementation using Hermes VM APIs directly.
SynthTrace is Required: For Hermes, you MUST add SynthTrace support. SynthTrace allows recording and replaying of JSI interactions, which is critical for debugging and testing. Without it, replays will fail.
Consistent Naming: Use consistent naming across all files. If the method is setPrototypeOf in JSI, use SetPrototype for the record type name.
Error Handling: Use jsi::JSError for JavaScript-level errors in JSI code. Use checkStatus() for Hermes VM call results.
Testing: Test at multiple levels:
- JSI core tests verify the abstract interface and default implementation
- Hermes API tests verify the optimized implementation
- SynthTrace tests verify recording, serialization, parsing, and replay

Testing

Running JSI Tests

To run all JSI tests defined in testlib.cpp:

buck test xplat/jsi:jsi

This runs the JSI test suite against multiple runtime implementations (Hermes, StaticHermes, etc.).

Running Hermes API Tests

To run the Hermes-specific API tests:

buck test xplat/static_h:HermesUnitTests

To run a specific test:

buck test xplat/static_h:HermesUnitTests -- MyTestName

Reference Diffs

For a complete example of adding JSI functionality, refer to these diffs:

D66562549: JSI core changes for getPrototypeOf / setPrototypeOf
D66729169: Hermes implementation
D66792338: SynthTrace support

Note: These diffs predate the IRuntime interface, but the patterns remain the same.*_

Adding New JSI Functionality

Architecture Overview

JSI (JavaScript Interface) is an abstraction layer that allows C++ code to interact with JavaScript runtimes. The architecture consists of:

JSI Core (xplat/jsi/jsi/) — The abstract interface definitions
Hermes Implementation (xplat/static_h/API/hermes/) — Hermes-specific implementation
SynthTrace (xplat/static_h/API/hermes/) — Recording and replay infrastructure for debugging

Checklist of Files to Modify

JSI Core Files

xplat/jsi/jsi/jsi.h — Add pure virtual method declaration to IRuntime interface AND override declaration in Runtime class
xplat/jsi/jsi/jsi.cpp — Add default implementation (if providing one)
xplat/jsi/jsi/jsi-inl.h — Add inline helper methods (if needed)
xplat/jsi/jsi/decorator.h — Add method overrides to RuntimeDecorator and WithRuntimeDecorator
xplat/jsi/jsi/test/testlib.cpp — Add tests for the JSI API

Hermes Implementation Files

xplat/static_h/API/hermes/hermes.cpp — Add Hermes-specific implementation in HermesRuntimeImpl
xplat/static_h/unittests/API/APITest.cpp — Add Hermes-specific tests

SynthTrace Files (Required for Hermes)

xplat/static_h/API/hermes/SynthTrace.h — Add new Record types
xplat/static_h/API/hermes/SynthTrace.cpp — Implement Record serialization
xplat/static_h/API/hermes/TracingRuntime.h — Declare tracing method overrides
xplat/static_h/API/hermes/TracingRuntime.cpp — Implement tracing logic
xplat/static_h/API/hermes/SynthTraceParser.cpp — Add parsing for new records
xplat/static_h/API/hermes/TraceInterpreter.cpp — Add replay logic
xplat/static_h/unittests/API/SynthTraceTest.cpp — Add replay tests
xplat/static_h/unittests/API/SynthTraceSerializationTest.cpp — Add serialization tests
xplat/static_h/unittests/API/SynthTraceParserTest.cpp — Add parser tests

Implementation Strategy

When adding new features (methods) to the JSI Runtime, you have two options:

Option A: Default Implementation (Recommended)

Provide a default implementation in jsi::Runtime that works via JavaScript calls. This allows all runtimes (JSC, V8, etc.) to work without modification.

// jsi.h - Add virtual method with default implementation
virtual void myNewMethod(const Object& obj, const Value&

Option B: Pure Virtual (Requires All Runtime Implementations)

Make the method pure virtual, requiring all runtimes (JSCRuntime, V8Runtime, HermesRuntime) to implement it. Only use this when a JavaScript-based default is not possible.

Step-by-Step Guide

1. Add to `jsi.h` (JSI Core Interface)

Add the pure virtual method declaration to the IRuntime interface, and the override declaration to the Runtime class:

 
If adding convenience methods to JSI types (like `Object`), add them too:
 
```cpp
class JSI_EXPORT Object : public Pointer {
  // ... existing methods ...
 
  /// Sets something on the object.
  void setMyThing(IRuntime& runtime, const Value& val) const {
    return runtime.myNewMethod(*this, val);
  }
 
  /// Gets something from the object.
  inline Value getMyThing(Runtime& runtime) const;
};

2. Add Default Implementation to `jsi.cpp`

Implement the default using JavaScript APIs when possible:

void Runtime::myNewMethod(const Object& obj, const Value& val) {
  // Get the JavaScript function
  auto myMethodFn = global()
                        .getPropertyAsObject(*this, "Object")
                        .getPropertyAsFunction(*this, "myMethod");
  // Call it with the appropriate arguments
  myMethodFn.call(*this, obj, val);
}
 
Value Runtime::myNewGetter(const Object& obj) {
  auto myGetterFn = global()
                        .getPropertyAsObject(*this, "Object")
                        .getPropertyAsFunction(*this, "myGetter");
  return myGetterFn.call(*this, obj);
}

3. Add Inline Methods to `jsi-inl.h` (If Needed)

For methods that return incomplete types at declaration time:

Value Object::getMyThing(Runtime& runtime) const {
  return runtime.myNewGetter(*this);
}

4. Update `decorator.h`

Add method overrides to both RuntimeDecorator and WithRuntimeDecorator:

// In RuntimeDecorator class (around line 282)
void myNewMethod(const Object& obj, const Value& val) override {
  plain_.myNewMethod(obj, val);
}
 
Value myNewGetter(const Object& obj) override {
  return plain_.myNewGetter(obj);
}
 
// In WithRuntimeDecorator class (around line 760)
void myNewMethod(const Object& obj, const Value& val) override {
  Around around{with_};
  RD::myNewMethod(obj, val);
}
 
Value myNewGetter(const Object& obj) override {
  Around around{with_};
  return RD::myNewGetter(obj);
}

5. Add JSI Core Tests in `test/testlib.cpp`

Test the default implementation using a RuntimeDecorator:

TEST_P(JSITest, MyNewFeature) {
  // Use a RuntimeDecorator to test the default implementation
  class RD : public RuntimeDecorator<Runtime, Runtime> {
   public:
    explicit RD(Runtime& rt) : RuntimeDecorator(rt) {}
 
    void myNewMethod(const Object& obj, const Value& val) override {
      return Runtime::myNewMethod(obj, val);
    }
 
    Value myNewGetter(const Object& obj) override {
      return Runtime::myNewGetter(obj);
    }
  };
 
  RD rd = RD(rt);
  Object obj(rd);
 
  // Test the functionality
  obj.setMyThing(rd, Value(123));
  EXPECT_EQ(obj.getMyThing(rd).getNumber(), 123);
}

6. Add Hermes Implementation in `hermes.cpp`

Implement the optimized version using Hermes VM APIs inside HermesRuntimeImpl:

// In HermesRuntimeImpl class declaration (around line 673)
void myNewMethod(const jsi::Object& obj, const jsi::Value& val) override;
jsi::Value myNewGetter(const jsi::Object& obj) override;
 
// Implementation (around line 2074)
void HermesRuntimeImpl::myNewMethod(
    const jsi::Object& obj,
    const jsi::Value& val) {
  // Validate input if needed
  if (!val.isObject() && !val.isNull()) {
    throw jsi::JSError(*this, "Invalid argument type");
  }
 
  // Use Hermes VM APIs directly
  auto cr = vm::JSObject::myVMMethod(
      vm::vmcast<vm::JSObject>(phv(obj)),
      runtime_,
      hvFromValue(val),
      vm::PropOpFlags().plusThrowOnError());
  checkStatus(cr.getStatus());
}
 
jsi::Value HermesRuntimeImpl::myNewGetter(const jsi::Object& obj) {
  vm::CallResult<vm::PseudoHandle<vm::JSObject>> cr =
      vm::JSObject::myVMGetter(handle(obj), runtime_);
  checkStatus(cr.getStatus());
  if (!*cr) {
    return jsi::Value::null();
  }
  return valueFromHermesValue(cr->getHermesValue());
}

7. Add Hermes Tests in `APITest.cpp`

TEST_P(HermesRuntimeTest, MyNewFeature) {
  Object obj(*rt);
 
  // Test normal operation
  Value val(*rt, 123);
  obj.setMyThing(*rt, val);
  EXPECT_EQ(obj.getMyThing(*rt).getNumber(), 123);
 
  // Test edge cases
  obj.setMyThing(*rt, Value::null());
  EXPECT_TRUE(obj.getMyThing(*rt).isNull());
 
  // Test error cases
  EXPECT_THROW(obj.setMyThing(*rt, Value("invalid")), JSError);
}

8. Add SynthTrace Record Types in `SynthTrace.h`

Add new record types for tracing:

// In the RECORD macro list (around line 210)
RECORD(MyNewMethod)                    \
RECORD(MyNewGetter)                    \
 
// Define the record structs (after existing records)
 
/// A MyNewMethodRecord is an event where native code calls myNewMethod
struct MyNewMethodRecord : public Record {
  static constexpr RecordType type{RecordType::MyNewMethod};
  const ObjectID objID_;
  const TraceValue value_;
 
  MyNewMethodRecord(TimeSinceStart time, ObjectID objID, TraceValue value)
      : Record(time), objID_(objID), value_(value) {}
 
  void toJSONInternal(::hermes::JSONEmitter &json) const override;
 
  RecordType getType() const override {
    return type;
  }
 
  std::vector<ObjectID> uses() const override {
    std::vector<ObjectID> uses{objID_};
    pushIfTrackedValue(value_, uses);
    return uses;
  }
};
 
/// A MyNewGetterRecord is an event where native code calls myNewGetter
struct MyNewGetterRecord : public Record {
  static constexpr RecordType type{RecordType::MyNewGetter};
  const ObjectID objID_;
 
  MyNewGetterRecord(TimeSinceStart time, ObjectID objID)
      : Record(time), objID_(objID) {}
 
  void toJSONInternal(::hermes::JSONEmitter &json) const override;
 
  RecordType getType() const override {
    return type;
  }
 
  std::vector<ObjectID> uses() const override {
    return {objID_};
  }
};

9. Implement Record Serialization in `SynthTrace.cpp`

void SynthTrace::MyNewMethodRecord::toJSONInternal(JSONEmitter &json) const {
  Record::toJSONInternal(json);
  json.emitKeyValue("objID", objID_);
  json.emitKeyValue("value", encode(value_));
}
 
void SynthTrace::MyNewGetterRecord::toJSONInternal(JSONEmitter &json) const {
  Record::toJSONInternal(json);
  json.emitKeyValue("objID", objID_);
}

10. Add Tracing Method Declarations in `TracingRuntime.h`

class TracingRuntime : public jsi::RuntimeDecorator<jsi::Runtime> {
  // ... existing methods ...
 
  void myNewMethod(const jsi::Object& obj, const jsi::Value& val) override;
  jsi::Value myNewGetter(const jsi::Object& obj) override;

11. Implement Tracing Logic in `TracingRuntime.cpp`

void TracingRuntime::myNewMethod(
    const jsi::Object& obj,
    const jsi::Value& val) {
  trace_.emplace_back<SynthTrace::MyNewMethodRecord>(
      getTimeSinceStart(), useObjectID(obj), useTraceValue(val));
  RD::myNewMethod(obj, val);
}
 
jsi::Value TracingRuntime::myNewGetter(const jsi::Object& obj) {
  trace_.emplace_back<SynthTrace::MyNewGetterRecord>(
      getTimeSinceStart(), useObjectID(obj));
 
  auto result = RD::myNewGetter(obj);
  trace_.emplace_back<SynthTrace::ReturnToNativeRecord>(
      getTimeSinceStart(), defTraceValue(result));
  return result;
}

12. Add Parsing in `SynthTraceParser.cpp`

case RecordType::MyNewMethod: {
  trace.emplace_back<SynthTrace::MyNewMethodRecord>(
      timeFromStart,
      objID->getValue(),
      SynthTrace::decode(propValue->c_str()));
  break;
}
case RecordType::MyNewGetter:
  trace.emplace_back<SynthTrace::MyNewGetterRecord>(
      timeFromStart, objID->getValue());
  break;

13. Add Replay Logic in `TraceInterpreter.cpp`

case RecordType::MyNewMethod: {
  const auto &record =
      static_cast<const SynthTrace::MyNewMethodRecord &>(*rec);
  auto obj = getJSIValueForUse(record.objID_).getObject(rt_);
  obj.setMyThing(rt_, traceValueToJSIValue(record.value_));
  break;
}
case RecordType::MyNewGetter: {
  const auto &record =
      static_cast<const SynthTrace::MyNewGetterRecord &>(*rec);
  auto obj = getJSIValueForUse(record.objID_).getObject(rt_);
  auto result = obj.getMyThing(rt_);
  retval = std::move(result);
  break;
}

14. Add SynthTrace Tests

In SynthTraceSerializationTest.cpp:

TEST_F(SynthTraceSerializationTest, MyNewMethodTest) {
  EXPECT_EQ(
      R"({"type":"MyNewMethodRecord","time":0,"objID":1,"value":"null:"})",
      to_string(SynthTrace::MyNewMethodRecord(
          dummyTime, 1, SynthTrace::encodeNull())));
}
 
TEST_F(SynthTraceSerializationTest, MyNewGetterTest) {
  EXPECT_EQ(
      R"({"type":"MyNewGetterRecord","time":0,"objID":1})",
      to_string(SynthTrace::MyNewGetterRecord(dummyTime, 1)));
}

In SynthTraceTest.cpp:

TEST_F(SynthTraceReplayTest, MyNewFeatureReplay) {
  {
    auto &rt = *traceRt;
    jsi::Object obj(rt);
    obj.setMyThing(rt, jsi::Value(123));
    rt.global().setProperty(rt, "obj", obj);
 
    auto result = obj.getMyThing(rt);
    rt.global().setProperty(rt, "result", result);
  }
  replay();
  {
    auto &rt = *replayRt;
    auto obj = rt.global().getProperty(rt, "obj").getObject(rt);
    EXPECT_EQ(obj.getMyThing(rt).getNumber(), 123);
 
    auto result = rt.global().getProperty(rt, "result");
    EXPECT_EQ(result.getNumber(), 123);
  }
}

In SynthTraceParserTest.cpp:

TEST_F(SynthTraceParserTest, ParseMyNewMethodRecord) {
  const char *src = R"(
{
  "version": 5,
  "globalObjID": 258,
  "runtimeConfig": {
    "gcConfig": {
      "initHeapSize": 33554432,
      "maxHeapSize": 536870912
    }
  },
  "trace": [
    {
      "type": "MyNewMethodRecord",
      "time": 1234,
      "objID": 1,
      "value": "number:123"
    }
  ]
}
  )";
  auto parseResult = parseSynthTrace(bufFromStr(src));
  SynthTrace &trace = std::get<0>(parseResult);
 
  auto record = dynamic_cast<const SynthTrace::MyNewMethodRecord &>(
      *trace.records().at(0));
  ASSERT_EQ(record.objID_, 1);
}

Key Conventions

Default Implementations: Always provide a default implementation in jsi::Runtime when possible. This allows JSCRuntime and V8Runtime to work without modification.
Hermes Optimization: For Hermes, override the default with an optimized implementation using Hermes VM APIs directly.
SynthTrace is Required: For Hermes, you MUST add SynthTrace support. SynthTrace allows recording and replaying of JSI interactions, which is critical for debugging and testing. Without it, replays will fail.
Consistent Naming: Use consistent naming across all files. If the method is setPrototypeOf in JSI, use SetPrototype for the record type name.
Error Handling: Use jsi::JSError for JavaScript-level errors in JSI code. Use checkStatus() for Hermes VM call results.
Testing: Test at multiple levels:
- JSI core tests verify the abstract interface and default implementation
- Hermes API tests verify the optimized implementation
- SynthTrace tests verify recording, serialization, parsing, and replay

Testing

Running JSI Tests

To run all JSI tests defined in testlib.cpp:

buck test xplat/jsi:jsi

This runs the JSI test suite against multiple runtime implementations (Hermes, StaticHermes, etc.).

Running Hermes API Tests

To run the Hermes-specific API tests:

buck test xplat/static_h:HermesUnitTests

To run a specific test:

buck test xplat/static_h:HermesUnitTests -- MyTestName

Reference Diffs

For a complete example of adding JSI functionality, refer to these diffs:

D66562549: JSI core changes for getPrototypeOf / setPrototypeOf
D66729169: Hermes implementation
D66792338: SynthTrace support

Note: These diffs predate the IRuntime interface, but the patterns remain the same.*_

Documentation

Documentation

Adding New JSI Functionality

Architecture Overview

Checklist of Files to Modify

JSI Core Files

Hermes Implementation Files

SynthTrace Files (Required for Hermes)

Implementation Strategy

Option A: Default Implementation (Recommended)

Option B: Pure Virtual (Requires All Runtime Implementations)

Step-by-Step Guide

1. Add to jsi.h (JSI Core Interface)

2. Add Default Implementation to jsi.cpp

3. Add Inline Methods to jsi-inl.h (If Needed)

4. Update decorator.h

5. Add JSI Core Tests in test/testlib.cpp

6. Add Hermes Implementation in hermes.cpp

7. Add Hermes Tests in APITest.cpp

8. Add SynthTrace Record Types in SynthTrace.h

9. Implement Record Serialization in SynthTrace.cpp

10. Add Tracing Method Declarations in TracingRuntime.h

11. Implement Tracing Logic in TracingRuntime.cpp

12. Add Parsing in SynthTraceParser.cpp

13. Add Replay Logic in TraceInterpreter.cpp

14. Add SynthTrace Tests

Key Conventions

Testing

Running JSI Tests

Running Hermes API Tests

Reference Diffs

Documentation

modify-jsi-features

Documentation

Adding New JSI Functionality

Architecture Overview

Checklist of Files to Modify

JSI Core Files

Hermes Implementation Files

SynthTrace Files (Required for Hermes)

Implementation Strategy

Option A: Default Implementation (Recommended)

Option B: Pure Virtual (Requires All Runtime Implementations)

Step-by-Step Guide

1. Add to jsi.h (JSI Core Interface)

2. Add Default Implementation to jsi.cpp

3. Add Inline Methods to jsi-inl.h (If Needed)

4. Update decorator.h

5. Add JSI Core Tests in test/testlib.cpp

6. Add Hermes Implementation in hermes.cpp

7. Add Hermes Tests in APITest.cpp

8. Add SynthTrace Record Types in SynthTrace.h

9. Implement Record Serialization in SynthTrace.cpp

10. Add Tracing Method Declarations in TracingRuntime.h

11. Implement Tracing Logic in TracingRuntime.cpp

12. Add Parsing in SynthTraceParser.cpp

13. Add Replay Logic in TraceInterpreter.cpp

14. Add SynthTrace Tests

Key Conventions

Testing

Running JSI Tests

Running Hermes API Tests

Reference Diffs

1. Add to `jsi.h` (JSI Core Interface)

2. Add Default Implementation to `jsi.cpp`

3. Add Inline Methods to `jsi-inl.h` (If Needed)

4. Update `decorator.h`

5. Add JSI Core Tests in `test/testlib.cpp`

6. Add Hermes Implementation in `hermes.cpp`

7. Add Hermes Tests in `APITest.cpp`

8. Add SynthTrace Record Types in `SynthTrace.h`

9. Implement Record Serialization in `SynthTrace.cpp`

10. Add Tracing Method Declarations in `TracingRuntime.h`

11. Implement Tracing Logic in `TracingRuntime.cpp`

12. Add Parsing in `SynthTraceParser.cpp`

13. Add Replay Logic in `TraceInterpreter.cpp`

1. Add to `jsi.h` (JSI Core Interface)

2. Add Default Implementation to `jsi.cpp`

3. Add Inline Methods to `jsi-inl.h` (If Needed)

4. Update `decorator.h`

5. Add JSI Core Tests in `test/testlib.cpp`

6. Add Hermes Implementation in `hermes.cpp`

7. Add Hermes Tests in `APITest.cpp`

8. Add SynthTrace Record Types in `SynthTrace.h`

9. Implement Record Serialization in `SynthTrace.cpp`

10. Add Tracing Method Declarations in `TracingRuntime.h`

11. Implement Tracing Logic in `TracingRuntime.cpp`

12. Add Parsing in `SynthTraceParser.cpp`

13. Add Replay Logic in `TraceInterpreter.cpp`