Add native fetch() polyfill by bkaradzic-microsoft · Pull Request #188 · BabylonJS/JsRuntimeHost

bkaradzic-microsoft · 2026-06-06T00:02:40Z

Implements the WHATWG etch() API as a native polyfill under Polyfills/Fetch/, mirroring the XMLHttpRequest polyfill layout. Like XMLHttpRequest, it is built on the same platform-specific UrlLib transports, so libcurl/WinHTTP/etc. behavior is identical between the two.

Closes #98.

API

fetch(input, init) returns a Promise that resolves to a Response-like object exposing:

ok, status, statusText, url, redirected, type, bodyUsed
headers with get(name) / has(name) / forEach(cb) (case-insensitive)
text(), arrayBuffer(), json(), blob() (each returns a Promise)
clone()

Behavior / deliberate limitations

Per the fetch spec, the promise rejects only on transport-level failures; a completed request with a non-2xx status (e.g. 404) still resolves with ok === false.
The body is fully buffered before the promise resolves, so the body accessors may be called more than once (bodyUsed stays false) — a lenient deviation from the spec's single-use semantics.
GET/POST only and string request bodies only, matching the underlying UrlLib transport (same constraints as XMLHttpRequest).
blob() requires the Blob polyfill to be initialized.

Wiring

New JSRUNTIMEHOST_POLYFILL_FETCH option (default ON). UrlLib is now fetched when either XMLHttpRequest or Fetch is enabled.
Initialized in the test harness and covered by a new describe("fetch") block (12 cases: status/ok, 404-resolves, statusText, text/arrayBuffer/json/blob, case-insensitive headers, clone, init method, no-arg rejection). A small Assets/sample.json backs the deterministic json() test.

Verification

Built the Fetch target under warnings_as_errors and ran the full UnitTests suite on Win32/Chakra: 192 passing, including all 12 new fetch cases.

Context

This is step 2 of the fetch native-polyfill pivot discussed in BabylonNative#1707 (reviewers asked fetch to follow the same native pattern as File/FileReader (#169) and AbortController). Once this lands, BabylonNative#1707 becomes a small bump that initializes Polyfills::Fetch.

Copilot

Pull request overview

Adds a native fetch() polyfill implemented in C++ on top of the existing UrlLib transports (matching the XMLHttpRequest approach), wires it behind a new CMake option, and extends the unit test suite to validate core fetch/Response behaviors.

Changes:

Introduce Polyfills/Fetch library implementing fetch(input, init) and a minimal Response/Headers surface.
Wire JSRUNTIMEHOST_POLYFILL_FETCH (default ON) and ensure UrlLib is fetched when either XHR or Fetch is enabled.
Add Mocha unit tests and a JSON asset to validate response properties and body readers (text/arrayBuffer/json/blob) plus cloning and header behavior.

Reviewed changes

Copilot reviewed 11 out of 11 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 Fetch polyfill in the UnitTests runtime.
Tests/UnitTests/Scripts/tests.ts	Adds a new `describe("fetch")` suite covering status/ok, statusText, body readers, headers, clone, init.method, and no-arg rejection.
Tests/UnitTests/CMakeLists.txt	Links the new `Fetch` target into the UnitTests executable; Assets are globbed/copied so `sample.json` is picked up.
Tests/UnitTests/Assets/sample.json	Adds deterministic JSON payload for the `response.json()` test.
Polyfills/Fetch/Source/Fetch.h	Declares internal Fetch initializer.
Polyfills/Fetch/Source/Fetch.cpp	Implements `fetch()`, `Response`-like object, and `headers` helpers over `UrlLib`.
Polyfills/Fetch/Readme.md	Documents supported surface area and known limitations.
Polyfills/Fetch/Include/Babylon/Polyfills/Fetch.h	Adds public API header for initializing the polyfill.
Polyfills/Fetch/CMakeLists.txt	Defines the `Fetch` library target and links against `JsRuntime`, `arcana`, and `UrlLib`.
Polyfills/CMakeLists.txt	Adds conditional inclusion of the Fetch subdirectory.
CMakeLists.txt	Adds `JSRUNTIMEHOST_POLYFILL_FETCH` option and updates `UrlLib` dependency gating.

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bkaradzic-microsoft · 2026-06-06T00:16:03Z

Addressed both review comments in ebe6a80:

blob() on JSC/JSI: switched the Blob-availability check from IsFunction() to IsUndefined()/IsNull(), matching the existing File polyfill workaround. Some JavaScriptCore/JSI builds report constructor functions as typeof 'object', which was causing the \�lob()\ test to fail across all JSC/JSI CI jobs.
: added the explicit include for \std::tolower/\std::toupper.

This should turn the failing JSC/JSI/Android/Ubuntu/macOS/iOS builds green (they were all failing solely on the blob() test).

bkaradzic-microsoft · 2026-06-06T01:05:25Z

CI is now fully green (23/23 ✅). Pushed four follow-up fixes after the initial submission, all surfaced by platform-specific CI:

*\40f5ff7* — Fixed a stack-use-after-return crash (\std::system_error: Invalid argument\ on clang/libc++, JSC, JSI, Android). arcana's \ ask::then()\ captures the scheduler by reference and runs it on the UrlLib worker thread after \etch()\ has returned, so the stack-local \JsRuntimeScheduler\ dangled. Now heap-allocated and co-owned by the continuation (mirrors how XMLHttpRequest keeps its scheduler alive as a member). Caught by ASAN.
*\�be6a80* — \�lob()\ now detects the Blob polyfill via \IsUndefined()/IsNull()\ instead of \IsFunction()\ (JSC/JSI report constructors as typeof 'object'), plus added <cctype>. (addresses both Copilot review comments)
*\44bc77a* — Made the \�lob()\ \parts/\options\ Napi objects non-const (Node-API-JSI declares \Set()\ non-const → C2663).
*\1118799* — Linked \Fetch\ into the Android \UnitTestsJNI\ target, which has its own polyfill link list.

Ready for re-review.

Implements the WHATWG fetch() API as a native polyfill under Polyfills/Fetch/, mirroring the XMLHttpRequest polyfill layout. Like XMLHttpRequest, it is built on top of the platform-specific transports in UrlLib, so libcurl/WinHTTP/etc. behavior is identical between the two. fetch(input, init) returns a Promise that resolves to a Response-like object exposing ok/status/statusText/url/redirected/type/bodyUsed, a case-insensitive headers accessor (get/has/forEach), the body accessors text()/arrayBuffer()/json()/blob(), and clone(). Per the fetch spec, the promise only rejects on transport-level failures; a completed request with a non-2xx status (e.g. 404) still resolves with ok === false. The body is fully buffered before the promise resolves, so the body accessors may be called more than once (a deliberate lenient deviation from the spec's single-use semantics). Methods are limited to GET/POST and request bodies to strings, matching the underlying UrlLib transport. Wired behind the JSRUNTIMEHOST_POLYFILL_FETCH option (UrlLib is now fetched when either XMLHttpRequest or Fetch is enabled) and covered by new unit tests. Closes BabylonJS#98. Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

- blob() now detects the Blob polyfill via IsUndefined()/IsNull() instead of IsFunction(). Some JavaScriptCore/JSI builds classify constructor functions as typeof 'object', so IsFunction() incorrectly rejected even when the Blob polyfill was installed, failing the blob() test across all JSC/JSI CI jobs (matches the existing File polyfill workaround). - Add <cctype> for std::tolower/std::toupper used by ToLower/StatusText, which were relying on transitive includes. Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

arcana's task::then() captures the scheduler by reference and invokes it on the UrlLib worker thread when the request completes, which happens after the synchronous fetch() call has already returned. The previous stack-local JsRuntimeScheduler was therefore destroyed before the continuation ran, leaving arcana with a dangling reference. On Windows/Chakra this happened to survive, but on clang/libc++, JSC, JSI and Android it dereferenced freed memory and aborted with 'std::system_error: Invalid argument' inside JsRuntime::Dispatch (caught by ASAN as a stack-use-after-return). Heap-allocate the scheduler in a shared_ptr and co-own it from the continuation callable so it stays alive until the request completes, mirroring how XMLHttpRequest keeps its scheduler alive as a member. Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Node-API-JSI declares Napi::Object::Set / Napi::Array::Set as non-const member functions, so calling Set() on 'const auto' instances failed to compile (C2663) on the *_JSI and Android targets while compiling fine under Chakra/V8. Declare the blob() 'parts' array and 'options' object as non-const, matching the other builder objects in this file. Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

The Android UnitTests app uses its own CMakeLists with an explicit polyfill link list, separate from the desktop Tests/UnitTests target. Without Fetch there, Shared.cpp failed to find <Babylon/Polyfills/Fetch.h> on Android_JSC and Android_V8. Add 'PRIVATE Fetch' to mirror the desktop test target. Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

bghgary

[Reviewed by Copilot on behalf of @bghgary]

Clean polyfill. One correctness fix and one optional cleanup inline.

…thod compares Address review feedback on the native fetch() polyfill: - Fix a hang: a throw inside the SendAsync continuation (e.g. from BuildResponse) was captured into the discarded task result and dropped, so the promise never settled and await fetch(...) hung. Split into two chained .then() continuations -- the first does the work and resolves, the second surfaces any error as a promise rejection. The scheduler is co-owned by the second continuation so it outlives the request. - Replace ToLower with a non-allocating EqualsIgnoreCase comparator used by both ParseMethod and FindHeader (removes per-header allocations). - Make ResponseData::body a plain std::vector<std::byte> instead of a shared_ptr; the struct is always held by shared_ptr, so the inner pointer only added an allocation and falsely implied clones get independent bodies. - Remove the unnecessary BuildResponse forward declaration. - Collapse the two fetch() catch blocks into a single catch (...) via the exception_ptr Error overload, removing the message-prefix asymmetry. Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Follow-up to BabylonJS#188. UrlLib now exposes UrlRequest::StatusText() (the wire reason phrase where available, else a canonical code->text table), so: - Fetch: drop the private status-code->text table and read request.StatusText() into Response.statusText. - XMLHttpRequest: expose statusText (previously absent) for free. Adds fetch + XMLHttpRequest statusText tests (OK / Not Found). Bumps the UrlLib pin to BabylonJS/UrlLib 74985214, which adds UrlRequest::StatusText() (BabylonJS/UrlLib#30). Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

bkaradzic-microsoft · 2026-06-10T16:33:14Z

Thanks for the review, @bghgary — all comments are addressed in ac2019f, and the follow-ups you flagged are now resolved/tracked.

Correctness fix (the blocker): the SendAsync() continuation is split into two chained .then()s — the first does the work and resolves, the second inspects the expected result and rejects on error. So a throw inside the continuation (e.g. from BuildResponse) now settles the promise as a rejection instead of being captured into a discarded result and dropped, which previously hung await fetch(...). The scheduler is co-owned by the second continuation so it outlives the in-flight request.

Cleanups: single catch (...) via the exception_ptr Napi::Error overload (removes the Napi::Error-vs-std::exception prefix asymmetry); non-allocating EqualsIgnoreCase(string_view, string_view) replacing ToLower, used by both ParseMethod and FindHeader; ResponseData::body is now a plain std::vector<std::byte>; removed the unnecessary BuildResponse forward declaration.

statusText "belongs in UrlLib" — done. BabylonJS/UrlLib#30 is merged and exposes UrlRequest::StatusText() (wire reason phrase where available, else a canonical code→text fallback table). The stacked follow-up #195 drops the private table here and reads request.StatusText() into Response.statusText (and gives XMLHttpRequest.statusText for free). Tracked + closed via #193.

Node-API-JSC napi_typeof bug (the blob() IsFunction() workaround): root cause is napi_typeof deciding function-ness solely via JSObjectIsFunction, which returns false for JSObjectMakeConstructor constructors on some JSC builds, so it reports napi_object where V8 returns napi_function. Tracked in #194.

CI is green. Re-requested your review — PTAL.

bghgary

[Reviewed by Copilot on behalf of @bghgary]

LGTM

Follow-up to BabylonJS#188. UrlLib now exposes UrlRequest::StatusText() (the wire reason phrase where available, else a canonical code->text table), so: - Fetch: drop the private status-code->text table and read request.StatusText() into Response.statusText. - XMLHttpRequest: expose statusText (previously absent) for free. Adds fetch + XMLHttpRequest statusText tests (OK / Not Found). Bumps the UrlLib pin to BabylonJS/UrlLib 74985214, which adds UrlRequest::StatusText() (BabylonJS/UrlLib#30). Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Copilot AI review requested due to automatic review settings June 6, 2026 00:02

Copilot started reviewing on behalf of bkaradzic-microsoft June 6, 2026 00:02 View session

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Comment thread Polyfills/Fetch/Source/Fetch.cpp

Comment thread Polyfills/Fetch/Source/Fetch.cpp

bkaradzic-microsoft requested a review from bghgary June 6, 2026 00:12

bkaradzic-microsoft mentioned this pull request Jun 6, 2026

Use native fetch() polyfill from JsRuntimeHost in Playground BabylonJS/BabylonNative#1707

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bkaradzic and others added 5 commits June 9, 2026 09:29

bkaradzic-microsoft force-pushed the weekend/tpc-1581-fetch-native branch from 1118799 to fbe94db Compare June 9, 2026 16:29

bghgary requested changes Jun 9, 2026

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This was referenced Jun 9, 2026

UrlLib: expose HTTP status reason phrase (statusText) #193

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Node-API-JSC: napi_typeof reports napi_object for JSObjectMakeConstructor constructors #194

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bkaradzic-microsoft requested a review from bghgary June 9, 2026 21:30

bkaradzic-microsoft mentioned this pull request Jun 9, 2026

UrlLib StatusText for fetch + XMLHttpRequest.statusText (closes #193) #195

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bghgary approved these changes Jun 10, 2026

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bkaradzic-microsoft merged commit a407a33 into BabylonJS:main Jun 10, 2026
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matthargett mentioned this pull request Jun 10, 2026

Expose normalized transport-error detail (ErrorString/ErrorSymbol/ErrorCode) BabylonJS/UrlLib#31

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matthargett mentioned this pull request Jun 10, 2026

fetch/XHR transport failures and unhandled rejections lose all diagnostic fidelity before reaching the embedding app #196

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Add native fetch() polyfill#188

Add native fetch() polyfill#188
bkaradzic-microsoft merged 6 commits into
BabylonJS:mainfrom
bkaradzic-microsoft:weekend/tpc-1581-fetch-native

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bkaradzic-microsoft commented Jun 6, 2026

API

Behavior / deliberate limitations

Wiring

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Pull request overview

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