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

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#27933
Open
Tracking issue for the deprecation and removal of wasm2js (AKA WASM=0)#27608
#27933
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deprecation

Description

@sbc100
sbc100
opened on Aug 25, 2026

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

Activity

  1. sbc100
    changed the title [-]Tracking issue for the deprecation and removal of wasm2js (-sWASM=0)[/-] [+]Tracking issue for the deprecation and removal of wasm2js (`WASM=0`)[/+] on Aug 25, 2026
  2. sbc100
    changed the title [-]Tracking issue for the deprecation and removal of wasm2js (`WASM=0`)[/-] [+]Tracking issue for the deprecation and removal of wasm2js (AKA `WASM=0`)[/+] on Aug 25, 2026
  3. sbc100
    added 3 commits that reference this issue on Sep 2, 2026

    Mark wasm2js (WASM=0) as deprecated

    9b759a0

    Mark wasm2js (WASM=0) as deprecated

    712e0c8

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

    dc0f21d
  4. sbc100
    added
    deprecation
    on Oct 1, 2026

  5. sbc100 commented on Oct 6, 2026

    @sbc100
    sbc100
    on Oct 6, 2026
    CollaboratorAuthor

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

  6. sbc100 commented on Oct 6, 2026

    @sbc100
    sbc100
    on Oct 6, 2026
    CollaboratorAuthor

    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:

    1. openframeworks/apothecary (apothecary/apothecary,
      modified Oct 2026):
      • Passes export LINK_FLAGS="-s WASM=2 -pthread -matomics -mbulk-memory ..."
        in its Emscripten dependency build script.
    2. 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.
    3. krkrsdl2/krkrsdl2 (CMakeLists.txt, modified Sept 2025):
      • Has an off-by-default CMake option KRKRSDL2_EMSCRIPTEN_ENABLE_JS_FALLBACK
        that appends -sWASM=2.
    4. 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

    1. 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.
    2. 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.
    3. 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).
    4. 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.
    5. 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).
    6. 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)

    1. 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).
    2. 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.
    3. 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".

  7. sbc100 commented on Oct 6, 2026

    @sbc100
    sbc100
    on Oct 6, 2026
    CollaboratorAuthor

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

  8. sbc100
    added 3 commits that reference this issue on Oct 6, 2026

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

    b441542

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

    876a79e

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

    a4ebc30

  9. cohunter commented on Oct 9, 2026

    @cohunter
    cohunter
    on Oct 9, 2026

    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:

    1. 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).
    2. 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.

  10. sbc100 commented on Oct 9, 2026

    @sbc100
    sbc100
    on Oct 9, 2026
    CollaboratorAuthor

    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.

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