June 2026 delivered major feature delivery and reliability improvements across Hermes and React Native integration. Key features: ES2025-compliant Array.prototype.toSorted with DirectSortModel optimization, enabling efficient sorting for fresh arrays; Math builtins extension and cross-platform rounding stability to ensure consistent tracing and performance; FinalizationRegistry cleanup errors surfaced to test runners for correctness; NAPI TSFN thread detachment race fix improving async work stability; Event loop control interface stabilization to a stable public API for better integration; Windows CI/build stability through environment pinning and build config adjustments. Also synchronized with React Native by promoting event loop control interfaces in Hermes to maintain consistency across platforms. Result: improved performance, cross-platform reliability, and a stronger foundation for RN integration.

In May 2026, Hermes delivered a broad package of performance, stability, and test-engine enhancements aimed at faster feedback, stronger ES compliance, and cross-plat CI reliability. Notable performance gains include SIMD-accelerated JSON string scanning in JSONLexer (ASCII/UTF-16 paths) which reduces escape-heavy string processing time in JSON handling. On the Promise stack, a comprehensive set of ES2025-compliant changes were implemented: Promise.try, Promise.prototype.finally rewritten per spec, enhanced Promise.prototype.then with IsPromise checks and NewPromiseCapability, internal resolve/reject receiver handling, proper property descriptors, and a cohesive all/any/allSettled/race rewrites; together they improve correctness, error propagation, and interop with subclassing and consistency with the spec. Additional refactors standardized toStringTag and symbol-based tagging to improve debugging and object identification. A major focus was building a robust in-process test harness to accelerate feedback. This included ExecutionScope management via MutatorScope and ExecutionScopeRAII (with NoMutatorScope for non-JS-interacting methods), enabling safer JS execution and GC interaction within HermesRuntimeImpl. Phase-driven work delivered a modern C++ test runner: frontmatter parsing and source preprocessing, harness caching, test discovery, skiplist filtering, in-process execution, and structured result reporting. Cross-platform CI readiness and test orchestration were improved with exposure of test262 mode via HermesInternal.test262Enabled, QEMU_RUN_PREFIX propagation to test runners for cross-arch CI, longer per-test timeouts to accommodate slower emulation, and flags for optimization and JIT to mirror Python-based runners. Lock-free result collection, crash isolation improvements, and resource handling fixes contributed to more reliable, scalable test runs and faster bug discovery. Business value: these changes collectively increase runtime performance, improve ES spec compliance for core async primitives, enhance test coverage and stability, and accelerate developer feedback through faster, more deterministic in-process testing and CI across architectures.

April 2026 monthly review focusing on performance, reliability, and cross-repo impact across Hermes and React Native. Key features delivered and major bug fixes: - SIMD-accelerated search paths for Hermes Array and TypedArray APIs, including fast paths for Array.prototype.includes and TypedArray indexOf/lastIndexOf/includes, complemented by large-array microbenchmarks to quantify gains on large datasets and raw-bit-equality cases. This includes per-type handling (Int32/Uint32, Int16/Uint16, Int8/Uint8, Float32/64) with SIMD-backed scans and preserved scalar paths for boxed values. - FinalizationRegistry cleanup memory safety fix: ensure lv.cells is re-read after every callback to prevent reads from stale buffers and double-free scenarios during user-driven mutations. - Build stability and portability improvements: introduced/mobile build defaults (Hermes mobile builds ON for iOS/tvOS), hardened async test result collection, date/timezone resilience, and cross-arch fixes (aarch64 PIC, MSVC edge cases, and related cleanups), enhancing reliability across targets. - React Native API surface stabilization: excluded metainternal headers from the public C++ API snapshots to prevent internal leakage and stabilize consumer-facing API surface. - Overall impact: higher runtime performance for array/typed array searches, improved memory safety, and stronger cross-platform build reliability, contributing to better app performance, fewer runtime issues, and more predictable CI/CD outcomes. Technologies/skills demonstrated: - SIMD intrinsics and fast-path design in a production JS engine (Hermes) - C++ engineering for memory safety and cross-arch build fixes - Microbenchmarking and performance validation - TZ data handling resilience and cross-platform testing - API surface governance and tooling improvements across Hermes and React Native.

March 2026 (2026-03) delivered substantial ES2025/ES2021 conformance, new runtime APIs, and memory/performance improvements across the Hermes codebase. Key features shipped include the JSFinalizationRegistry class with constructor, registration/unregistration semantics, FinalizationRecord support, and runtime wiring with tests; and the TC39 Uint8Array Base64/Hex methods (toBase64, toHex, fromBase64, fromHex, setFromBase64, setFromHex) with a documented binary-size impact. ES2025 correctness received focused attention via Proxy invariant checks for [[GetOwnProperty]] and [[DefineOwnProperty]] and additional DataView IsDetachedBuffer checks. JSON parser robustness and Array.prototype fixes address correctness and side-effect safety (avoid GC handle accumulation; enforce minus-digit semantics; remove unneeded getIndexed_RJS call and fix toSpliced overflow). Memory and performance improvements include heap snapshot support, free-index reuse, and cell shrinking, complemented by test-speed optimizations (ASAN) and better visibility into the slowest tests. These changes collectively improve reliability, standards conformance, runtime efficiency, and developer productivity, while reducing CI churn and enabling broader test coverage.

February 2026 highlights across facebook/hermes and facebook/react-native: strengthened memory management and encoding for Hermes to boost GenGC reliability under memory pressure, modernized allocator with scope management, and introduced direct SHV/HV encoding paths. Added SIMD-accelerated linear search utilities for JSLib paths, fixed critical stability issues in data structures and proxy invariants, and expanded CI coverage across Windows/ARM64 with ASAN tests. These changes improve performance, memory efficiency, reliability, and developer velocity in production builds.

January 2026 monthly summary for the facebook/hermes repository, focusing on delivering cross-cutting performance improvements, memory scalability, build robustness, code safety, and test stability. The team shipped architectural improvements to runtime configuration, expanded heap capacity on 64-bit platforms, and reinforced platform-specific build and test reliability. The work emphasizes business value by delivering more consistent JIT behavior, enabling larger memory workloads, and reducing CI/test flakiness, enabling faster feature shipping with fewer regressions.

Concise month-end summary for 2025-12 highlighting key features delivered, major bugs fixed, overall impact, and technologies demonstrated across React Native and Hermes. This period focused on header hygiene, runtime introspection, safer memory accounting, and test reliability improvements to accelerate development cycles and reduce regression risk.

November 2025 performance summary: Hermes memory/GC instrumentation improvements, native backend support for TraceInterpreter, AST error handling refinements, and cross-platform memory layout cleanup, complemented by React Native Hermes variant configuration stabilization and HV32 test alignment. These changes deliver more accurate memory diagnostics, more reliable builds/tests, and stronger cross‑platform stability across Hermes-based projects.

Month: 2025-10 — Consolidated CDP debugging infrastructure across Hermes and React Native, delivering removal of legacy CDPHandler, restoration where needed, and stabilization to ensure Hermes debugging support for non-V1 builds while simplifying CI/configuration.

September 2025 highlights: Focused on Hermes API consolidation, runtime exposure, and SHUnit integration to streamline Hermes usage across Facebook Hermes and React Native. Delivered API surface consolidation and backend enhancements, unified instrumentation for opcode/stats, and introduced SHUnit support enabling flexible JavaScript evaluation. Expanded Hermes runtime exposure via hermes.h and centralized profiling under JSI Instrumentation. Refactors migrated IHermesExtra into IHermes and relocated related methods, reducing maintenance burden and improving cross-repo consistency. No major bugs fixed this month; the work primarily established a stronger foundation for RN integration and performance diagnostics.

July 2025 monthly summary: Focused on stabilizing test suites and improving error diagnostics in Hermes repos, delivering measurable business value through fewer flaky tests and quicker debugging.

Month: 2025-05. Focused on hardening runtime configuration to improve reliability and cross-environment compatibility. Delivered a feature that enables ArrayBuffer by default in the hardened Hermes runtime, reducing manual configuration and potential runtime errors. No major bugs fixed this month; primary effort was feature delivery and stabilizing the initialization path.

February 2025 monthly summary for repository mrousavy/hermes. Focus was on strengthening CI testing for iOS by updating the iPhone simulator version in the GitHub Actions workflow to maintain compatibility with the latest Xcode and macOS tooling. This change reduces environment drift and accelerates feedback for iOS builds.

January 2025 monthly summary focusing on the Hermes repository with emphasis on robustness and test coverage. Delivered a critical bug fix to the NaN hashing path and added regression tests to ensure stability across NaN representations, improving hashing consistency for data structures and memoization.