Add TextEncoder polyfill (WHATWG Encoding Standard)

[bkaradzic-microsoft]

Adds a TextEncoder polyfill that mirrors the existing TextDecoder polyfill in this repository, so non-Babylon-Native consumers can get both halves of the WHATWG Encoding Standard from JsRuntimeHost without having to pull in or duplicate a separate implementation.

TextEncoder is needed by older Chakra-based runtimes where the global is not built in. Modern V8 / JSC / Hermes runtimes already expose it natively, in which case Initialize() is a no-op.

Surface

• TextEncoder() constructor — UTF-8 only, per spec
• encoding accessor — always "utf-8"
• encode(input) → Uint8Array
• encodeInto(source, destination) → { read, written }
  • read is in UTF-16 code units, so a 4-byte UTF-8 sequence (code point outside the BMP) reports 2
  • Multi-byte UTF-8 sequences are never split across the destination boundary

Build / wiring

• New gated option JSRUNTIMEHOST_POLYFILL_TEXTENCODER (default ON), matching the pattern used by every other polyfill in this repo.
• New Polyfills/TextEncoder/ library with the same layout as Polyfills/TextDecoder/ (CMakeLists.txt, Include/Babylon/Polyfills/TextEncoder.h, Source/TextEncoder.cpp, README.md).
• Linked into the unit-test executable (Windows + Android) and initialized in Tests/UnitTests/Shared/Shared.cpp next to TextDecoder::Initialize.

Tests

Adds a new describe("TextEncoder", ...) block in Tests/UnitTests/Scripts/tests.ts covering:

• encoding === "utf-8"
• encode() on ASCII, undefined / no-arg, multi-byte UTF-8 (e.g. "é" → [0xC3, 0xA9]), and embedded null bytes
• encodeInto() happy path, refusal to split a multi-byte sequence when the destination is too small, and the surrogate-pair read semantics for U+1F600
• encodeInto() TypeError when the destination is not a Uint8Array

Motivation

Surfaced during the BabylonNative review of BabylonJS/BabylonNative#1708, where this polyfill originally lived. Per @bghgary's review feedback, TextEncoder belongs alongside TextDecoder in JsRuntimeHost so it's available to any consumer of this runtime host, not just BN.

[Copilot]

Pull request overview

Adds a WHATWG-compatible TextEncoder polyfill (UTF-8 only) to complement the existing TextDecoder polyfill, with build-time gating and unit-test wiring so older Chakra-based runtimes can rely on JsRuntimeHost for both APIs.

Changes:

• Introduces new Polyfills/TextEncoder library (C++ implementation + public header + README) and wires it into the polyfills CMake.
• Adds JSRUNTIMEHOST_POLYFILL_TEXTENCODER CMake option (default ON) and includes the new polyfill when enabled.
• Updates unit tests to link/initialize TextEncoder and adds JS test coverage for encoding, encode, and encodeInto.

Reviewed changes

Copilot reviewed 10 out of 10 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
Tests/UnitTests/Shared/Shared.cpp	Initializes the new TextEncoder polyfill in the unit test runtime.
Tests/UnitTests/Scripts/tests.ts	Adds a TextEncoder test suite covering encoding, encode, and encodeInto.
Tests/UnitTests/CMakeLists.txt	Links the new TextEncoder library into the unit test executable.
Tests/UnitTests/Android/app/src/main/cpp/CMakeLists.txt	Links TextEncoder into the Android unit test JNI target.
Polyfills/TextEncoder/Source/TextEncoder.cpp	Implements the TextEncoder polyfill (encoding, encode, encodeInto).
Polyfills/TextEncoder/README.md	Documents supported surface area and usage.
Polyfills/TextEncoder/Include/Babylon/Polyfills/TextEncoder.h	Adds the public Initialize API for the polyfill.
Polyfills/TextEncoder/CMakeLists.txt	Defines the TextEncoder library target and include paths.
Polyfills/CMakeLists.txt	Adds conditional inclusion of the TextEncoder subdirectory.
CMakeLists.txt	Adds JSRUNTIMEHOST_POLYFILL_TEXTENCODER option.

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.