Tracking issue for the deprecation and removal of wasm2js (AKA WASM=0)

This would also include the removal of -sWASM=2 (which builds both JS and wasm versions of a program in a single command)>

The process for deprecating and removing features from emscripten is laid out
in https://github.com/emscripten-core/emscripten/blob/main/CONTRIBUTING.md#deprecating-settings-and-features.

At any point in the process we may decide to delay or abort the deprecation and close this issue.

Deprecation checklist:

Open an issue (this)
Add "deprecation" warning to emcc
Send message to emscripten-discuss
Perform global search for usage in public repos
Wait at least 4 releases or 2 months
Remove feature from codebase

Mark wasm2js (WASM=0) as deprecated

Mark wasm2js (WASM=0) as deprecated (#27655)

I confirmed that google internally no more users of emscripten's wasm2js feature.

AI generated report on current usage on the public internet:

Ran global searches across GitHub Code Search (including .github/workflows
and commit history since 2025), Sourcegraph (excluding forks and archived
repos), and Debian Code Search for -sWASM=0, -s WASM=0, -sWASM=2,
-s WASM=2, and -sNO_WASM.

High-Level Summary

Raw GitHub counts:
- "-sWASM=0": ~1,596 files | "-s WASM=0": ~5,528 files
- "-sWASM=2": ~92 files | "-s WASM=2": ~86 files
- "-sNO_WASM" / "-s NO_WASM": 8 files (none active outside emscripten
  tests)
Most hits are inactive/noise: The vast majority of matches are old
generated Emscripten JS files ("Build with -s WASM=0 to target JavaScript instead"), commented-out template flags (e.g., raylib's # -sWASM=0),
unmaintained repos from the 2017–2020 era (often paired with removed flags
like --memory-init-file 0 or --llvm-lto 1), or vendored copies of a few
upstream libraries (libwebp, draco, libsodium, xxHash, sql.js).
Note that WebAssembly/binaryen already dropped its -sWASM=0 build in
Feb 2026 (#7995), and Rust removed the asmjs-unknown-emscripten target.

1. Active Users of `-sWASM=2` / `-s WASM=2`

Usage of WASM=2 is nearly non-existent in active projects; 0 repositories
on GitHub use -sWASM=2 or -s WASM=2 in .github/workflows. The only active
repositories referencing it are:

openframeworks/apothecary (apothecary/apothecary,
modified Oct 2026):
- Passes export LINK_FLAGS="-s WASM=2 -pthread -matomics -mbulk-memory ..."
  in its Emscripten dependency build script.
OpenMPT/openmpt (build/make/config-emscripten.mk,
modified June 2026):
- Supports EMSCRIPTEN_TARGET=all which sets LDFLAGS += -s WASM=2 -s LEGACY_VM_SUPPORT=1 (as well as EMSCRIPTEN_TARGET=js which sets
  -s WASM=0), though wasm (-s WASM=1) is also supported.
krkrsdl2/krkrsdl2 (CMakeLists.txt, modified Sept 2025):
- Has an off-by-default CMake option KRKRSDL2_EMSCRIPTEN_ENABLE_JS_FALLBACK
  that appends -sWASM=2.
qbs/qbs (tests/auto/blackbox/tst_blackbox.cpp, modified Sept
2026; also in Debian Code Search):
- Qt's Qbs build tool has blackbox tests verifying expected output file
  suffixes (.js, .wasm, .wasm.js) when passing -sWASM=0, -sWASM=1,
  and -sWASM=2.

2. Active Users of `-sWASM=0` / `-s WASM=0`

A. Actively Building or Testing `-sWASM=0` in CI / Default Releases

sql-js/sql.js (13.7k stars, Makefile & CI.yml, using
Emscripten 5.0.0):
- Actively builds, tests (npm run test-asm, test-asm-debug,
  test-asm-memory-growth), and publishes sql-asm.js and
  worker.sql-asm.js release artifacts with -s WASM=0 alongside its Wasm
  builds.
jedisct1/libsodium (14k stars,
dist-build/emscripten.sh, modified Sept 2026):
- Always builds an asm.js fallback (emccLibsodium ... -Oz -s WASM=0) and
  concatenates it as a backup module inside libsodium.js alongside the Wasm
  build.
Cyan4973/xxHash (11.3k stars, .github/workflows/ci.yml,
modified Sept 2026; also in Debian Code Search):
- Has an active GitHub Actions CI step Scalar asm.js (-sWASM=0) running
  LDFLAGS="-sWASM=0" make clean check under Node (currently pinned to
  emsdk 3.1.33 in CI).
Yahweasel/libav.js (530 stars, Makefile, modified Aug
2026):
- Builds and ships .asm.js and .asm.mjs variants (-s WASM=0) alongside
  .wasm.js and .wasm.mjs variants for FFmpeg audio/video codecs.
justjake/quickjs-emscripten (1.7k stars,
scripts/prepareVariants.ts, modified Feb 2026):
- Publishes @jitl/quickjs-asmjs-mjs-release-sync using -s WASM=0 -s SINGLE_FILE=1 for constrained JS environments without WebAssembly, though
  it currently pins ASMJS_EMSCRIPTEN_VERSION = "3.1.43" (while using
  5.0.1 for Wasm variants).
alpertron/calculators (254 stars, w.bat / Makefile,
modified Sept 2026):
- Builds an asm.js fallback version with -s WASM=0 alongside its Wasm
  builds.

B. Recent Projects Targeting Wasm-less JS Environments (2025–2026)

Haskell / MicroHs / Mobile JS runtimes (dmjio/miso,
haskell-miso/miso-lynx):
- dmjio/miso (2.4k stars, modified Oct 2026 in PR #1663): Uses
  emcc -sWASM=0 for its browser_js MicroHs target (nix/mhs/default.nix,
  Makefile.mhs) and in sample-app/app.cabal / miso-lynx (targeting
  ByteDance's Lynx mobile engine).
React Native / Bundler single-file JS (poe-platform/poe-code,
KittenML/KittenTTS-react-native):
- poe-platform/poe-code (packages/media-codecs/scripts/build.mjs,
  modified Oct 2026): Uses Emscripten 4.0.13 with -sWASM=0 -sWASM_ASYNC_COMPILATION=0 to bundle media codecs as pure JS.
- KittenML/KittenTTS-react-native
  (scripts/build-cephonemizer.js, modified May 2026): Uses -sWASM=0 so
  React Native's Metro bundler can bundle the runtime as plain JS without
  native/asset glue.
PDF-embedded C/C++ apps (ading2210/linuxpdf, EvanZhouDev/llm.pdf,
nikol4ss/doom-pdf):
- Compile TinyEMU, llama.cpp, and Doom with emcc -sWASM=0 so they can
  execute inside Chrome PDFium / Adobe Acrobat's embedded JS runtime, which
  does not expose WebAssembly.

C. Major Upstream Libraries with Optional / Legacy `-sWASM=0` Targets

These active projects still have -sWASM=0 in their build scripts behind
non-default options (and are the source of most vendored hits in other repos
and Debian Code Search):

opencv/opencv (platforms/js/build_js.py): Passes
-s WASM=0 only when --disable_wasm is explicitly passed on the CLI.
google/draco (cmake/draco_emscripten.cmake): Appends
-sWASM=0 when the CMake option DRACO_WASM is OFF.
webmproject/libwebp (CMakeLists.txt): Defines a webp_js
SDL demo target with -sWASM=0 (alongside webp_wasm with -sWASM=1) when
WEBP_BUILD_WEBP_JS is enabled and WEBP_ENABLE_SIMD is off.
rive-app/rive-runtime (build/setup_compiler.lua): Passes
-sWASM=0 when configured with --emsdk=js instead of --emsdk=wasm.
ton-blockchain/ton (emulator/CMakeLists.txt): Appends
-sWASM=0 only if USE_EMSCRIPTEN_NO_WASM is set.
MiniZinc/libminizinc
(cmake/support/config_emscripten.cmake): Defines a minizinc_asm CMake
target with -s WASM=0 alongside minizinc_wasm.
sciapp/gr (CMakeLists.txt): Sets -sWASM=0 in its EMSCRIPTEN
CMake block (also appears in Debian's gr-framework package).

D. Build Tools & Toolchains

emscripten-core/emsdk:
- bazel/emscripten_toolchain/toolchain.bzl still defines a
  crosstool_cpu_asmjs feature that passes flags = ["-sWASM=0"].
- test/test.py runs do_build(' -s WASM=0', is_expected=False).
conan-io/conan (conan/tools/build/flags.py): Returns
"-sWASM=0" when arch == "asm.js" and compiler == "emscripten".
bkaradzic/bx (scripts/toolchain.lua, vendored into mame,
crown, etc.): Defines an asmjs target config with "-s WASM=0".

Emsdk PR: emscripten-core/emsdk#1806
Conan bug: conan-io/conan#20377
bx bug: bkaradzic/bx#446

[bazel] Remove asmjs / `-sWASM=0` support (#1806)

Remove support wasm2js (`WASM=0` and `WASM=2`)

Remove support for wasm2js (`WASM=0` and `WASM=2`)

Hello, apologies for the late comment, I’ve just seen this issue.

Use case not covered in the survey: Apple Lockdown Mode (iOS/iPadOS/macOS)

Lockdown Mode is Apple’s hardening setting for journalists, activists and others at risk of targeted spyware. In WebKit it disables the JIT and WebAssembly entirely: typeof WebAssembly === 'undefined' in Safari with Lockdown on (verified on iOS 26). The only way to run C/C++/Rust code for those users is plain JavaScript, i.e. wasm2js.

I’m building Lockdown-compatible media playback for a web subtitle editor. HEVC (hevc.js), VP9 (libvpx), AV1 (dav1d) and E-AC-3 (Rust, via binaryen’s wasm2js) all run as plain JS in Lockdown Mode at usable speed in the interpreter. For video codecs there’s no other route: Lockdown also disables WebCodecs video.

I understand the maintenance cost, and I’m not asking to keep -sWASM=0 itself. A supported migration path would be enough. Either of these would do:

Keep binaryen’s standalone wasm2js maintained, and document how to run an Emscripten -sWASM=1 build’s output through it (e.g. a small WebAssembly shim that instantiates the wasm2js module).
Keep the wasm2js runtime piece of the JS glue as an opt-in library, even if emcc no longer drives it.

Our users have no WebAssembly by design, so they’re the audience this removal hits hardest.

Oh no... that is indeed an annoying/worrying use case. I wonder if there are any plans to add a WebAssembly interpreter in that mode? I think this is what MS edge did for their locked down mode.

We do plan on keeping binaryen’s standalone wasm2js around yes. There are other users of that tool.

Funnily enough Safari does have a Wasm interpreter, and therefore can run Wasm in JIT-less mode.. but even that I guess they deem too dangerous for lock down mode.

See https://discussions.apple.com/thread/255669636 .. no answer yet there.

@cohunter

@cohunter, can you confirm that you are not using -sWASM=2 mode? At least perhaps we could remove that subset.

Confirmed, standalone wasm2js is plenty, thank you. Switched to emcc -sWASM=1 + wasm2js with a small WebAssembly shim.

Oh wow, thats great news. If it was easy to switch to standalone wasm2js then we can recommend that to others who want to use that method.

I think that layers of tools is much nicer in any case.

I recommend you do not depreciate WASM=0 builds because:

WASM=0 builds can run locally without emrun you can just execute the html file and it run.
PowerPC like Power9 for example, browsers do not support WASM execution. You lose support for all alternate platform.

I recommend you do not depreciate WASM=0 builds because:

WASM=0 builds can run locally without emrun you can just execute the html file and it run.

Isn't -sSINGLE_FILE enough to make this work?

PowerPC like Power9 for example, browsers do not support WASM execution. You lose support for all alternate platform.

But this doesn't really allow Power9 to use the open web in general, since many sites will be shipping Wasm-only. I guess it solves the problem if for a given app that knows it needs to target Power9? Is that your use case?

Good point -sSINGLE_FILE with WASM=1 does also produce the same effect.

Yes I have a single friend who uses Power9 with Linux/Firefox who appreciates WASM=0 😭

Why do you want to removed this feature btw? is it too much work to up-keep or?

Yes, having to run the whole test sweet in -sWASM=2 and -sWASM=0 modes is kind of costly, both in terms on CI time and maintenance cost.

Also, it complicates the code base a fair bit.

Also, not all Wasm features are supported by wasm2js and that is only likely to increase over time.

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Tracking issue for the deprecation and removal of wasm2js (AKA `WASM=0`) #27608

High-Level Summary

1. Active Users of `-sWASM=2` / `-s WASM=2`

2. Active Users of `-sWASM=0` / `-s WASM=0`

A. Actively Building or Testing `-sWASM=0` in CI / Default Releases

B. Recent Projects Targeting Wasm-less JS Environments (2025–2026)

C. Major Upstream Libraries with Optional / Legacy `-sWASM=0` Targets

D. Build Tools & Toolchains

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Tracking issue for the deprecation and removal of wasm2js (AKA WASM=0) #27608

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sbc100 commented on Oct 6, 2026

sbc100 commented on Oct 6, 2026

High-Level Summary

1. Active Users of -sWASM=2 / -s WASM=2

2. Active Users of -sWASM=0 / -s WASM=0

A. Actively Building or Testing -sWASM=0 in CI / Default Releases

B. Recent Projects Targeting Wasm-less JS Environments (2025–2026)

C. Major Upstream Libraries with Optional / Legacy -sWASM=0 Targets

D. Build Tools & Toolchains

sbc100 commented on Oct 6, 2026

cohunter commented on Oct 9, 2026

sbc100 commented on Oct 9, 2026

sbc100 commented on Oct 12, 2026

sbc100 commented on Oct 12, 2026

cohunter commented on Oct 13, 2026

sbc100 commented on Oct 13, 2026

puckyoulol commented on Oct 16, 2026

sbc100 commented on Oct 16, 2026

puckyoulol commented on Oct 16, 2026

sbc100 commented on Oct 16, 2026

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Issue actions

Tracking issue for the deprecation and removal of wasm2js (AKA `WASM=0`) #27608

1. Active Users of `-sWASM=2` / `-s WASM=2`

2. Active Users of `-sWASM=0` / `-s WASM=0`

A. Actively Building or Testing `-sWASM=0` in CI / Default Releases

C. Major Upstream Libraries with Optional / Legacy `-sWASM=0` Targets