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Build ONNX Runtime for Web

There are 2 steps to build ONNX Runtime Web:

  • build ONNX Runtime for WebAssembly
    • or skip and download a pre-built artifacts
  • build onnxruntime-web (NPM package)


Build ONNX Runtime for WebAssembly


  • Checkout the source tree:
    git clone --recursive
    cd onnxruntime
  • Install cmake-3.18 or higher.

  • Install Node.js (16.0+)

  • Python (3.8+):
    • python should be added to the PATH environment variable
  • Prepare emsdk: emsdk should be automatically installed at <ORT_ROOT>/cmake/external/emsdk/emsdk. If the folder structure does not exist, run the following commands in <ORT_ROOT>/ to install git submodules:
    git submodule sync --recursive
    git submodule update --init --recursive

    (If you are using Windows, you can skip this step) in <ORT_ROOT>/cmake/external/emsdk/, run the following commands to setup emsdk:

    ./emsdk install latest
    ./emsdk activate latest
    source ./

Build Instructions

in <ORT_ROOT>/, run one of the following commands to build WebAssembly:

# In windows, use 'build' to replace './'

# The following command build debug.
./ --build_wasm

# The following command build debug with debug info.
./ --build_wasm --skip_tests --enable_wasm_debug_info

# The following command build release.
./ --config Release --build_wasm --skip_tests --disable_wasm_exception_catching --disable_rtti

ONNX Runtime WebAssembly can be built with or without multi-thread/SIMD support, specified by appending the following flags.

build flag usage
--enable_wasm_threads build with multi-thread support
--enable_wasm_simd build with SIMD support

To get all build artifacts of ONNX Runtime WebAssembly, it needs 4 times of build with the combinations of ON/OFF of the 2 flags. A full list of build artifacts are as below:

file name --enable_wasm_threads --enable_wasm_simd
ort-wasm.js X X
ort-wasm.wasm X X
ort-wasm-threaded.js O X
ort-wasm-threaded.wasm O X
ort-wasm-threaded.worker.js O X
ort-wasm-simd.wasm X O
ort-wasm-simd-threaded.wasm O O

Minimal Build Support

ONNX Runtime WebAssembly can be built with flag --minimal_build. This will generate smaller artifacts and also have a less runtime memory usage. An ORT format model is required. A detailed instruction will come soon. See also ORT format Conversion.


Q: unittest fails on Release build.

A: unittest requires C++ exceptions to work properly. However, for performance concern, we disabled exception catching in emscripten. So please specify --skip_tests in Release build.

Q: unittest fails on Debug build with debug info.

A: building with debug info will generate very huge artifacts (>1GB for unittest) and failed to load in Node.js. So please specify --skip_tests in build with debug info.

Q: I have a C++ project for web scenario, which runs a ML model using ONNX Runtime and generates WebAssembly as an output. Does ONNX Runtime Web support a static WebAssembly library, so that my application can link with it and make all pre/post processors to be compiled together into WebAssembly?

A: With --build_wasm, a build script generates .wasm and .js files for web scenarios and intermediate libraries are not linked properly with other C/C++ projects. When you build ONNX Runtime Web using --build_was_static_lib instead of --build_wasm, a build script generates a static library of ONNX Runtime Web named libonnxruntime_webassembly.a in output directory. To run a simple inferencing like an unit test, what you need is three header files as follows and libonnxruntime_webassembly.a.

  • include/onnxruntime/core/session/onnxruntime_c_api.h
  • include/onnxruntime/core/session/onnxruntime_cxx_api.h
  • include/onnxruntime/core/session/onnxruntime_cxx_inline.h

One important note is that ONNX Runtime has dependencies on lots of 3rd party libraries such as protobuf, onnx, and others. You may need copy necessary header files to your project. You also take care of cases of library version conflicts or emsdk version conflicts between ONNX Runtime and your project.

Build onnxruntime-web (NPM package)


  • Install Node.js (16.0+)

  • Chrome or Edge browser for running tests.

Build Instructions

  1. Install NPM packages

    1. in <ORT_ROOT>/js/, run npm ci.
    2. in <ORT_ROOT>/js/common/, run npm ci.
    3. in <ORT_ROOT>/js/web/, run npm ci.
  2. Prepare ONNX Runtime WebAssembly artifacts.

    You can either use the prebuilt artifacts or build it by yourself.

    • Setup by script.

      In <ORT_ROOT>/js/web/, run npm run pull:wasm to pull WebAssembly artifacts for latest master branch from CI pipeline.

    • Download artifacts from pipeline manually.

      you can download prebuilt WebAssembly artifacts from Windows WebAssembly CI Pipeline. Select a build, download artifact “Release_wasm” and unzip. See instructions below to put files into destination folders.

    • Build WebAssembly artifacts.

      1. Build ONNX Runtime WebAssembly

        Follow instructions above for building ONNX Runtime WebAssembly.

      2. Copy following files from build output folder to <ORT_ROOT>/js/web/dist/ (create the folder if it does not exist):

        • ort-wasm.wasm
        • ort-wasm-threaded.wasm (build with flag ‘–enable_wasm_threads’)
        • ort-wasm-simd.wasm (build with flag ‘–enable_wasm_simd’)
        • ort-wasm-simd-threaded.wasm (build with flags ‘–enable_wasm_threads –enable_wasm_simd’)
      3. Copy following files from build output folder to <ORT_ROOT>/js/web/lib/wasm/binding/:

        • ort-wasm.js
        • ort-wasm-threaded.js (build with flag ‘–enable_wasm_threads’)
        • ort-wasm-threaded.worker.js (build with flag ‘–enable_wasm_threads’)
  3. Use following command in folder <ORT_ROOT>/js/web to build:

    npm run build