Developer’s Guide

This article provides information that is relevant to people who want to contribute to Emscripten. We welcome contributions from anyone that is interested in helping out!


The information will be less relevant if you’re just using Emscripten, but may still be of interest.

Setting up

For contributing to core Emscripten code, such as, you don’t need to build any binaries as is in Python, and the core JS generation is in JavaScript. You do still need binaries for LLVM and Binaryen, which you can get using the emsdk:

emsdk install tot-upstream
emsdk activate tot-upstream

That gets a “tip-of-tree” build of the very latest binaries. You can use those binaries with a checkout of the core Emscripten repository, simply by calling from that checkout, and it will use the binaries from the emsdk.

If you do want to contribute to LLVM or Binaryen, or to test modifications to them, you can build them from source.

Repositories and branches of interest

The Emscripten main repository is

Aside from the Emscripten repo, the other codebases of interest are LLVM and Binaryen, which Emscripten invokes, and have their own repos.

Submitting patches

Patches should be submitted as pull requests in the normal way on GitHub.


Together with your first patch, add yourself to the AUTHORS file. By doing so, you agree to license your code under the project’s open source licenses (MIT/LLVM).

When submitting patches, please:

  • Add an automatic test if you add any new functionality or fix a bug. Search in tests/*.py for related tests, as often the simplest thing is to add to an existing one. If you’re not sure how to test your code, feel free to ask for help.

  • We normally squash and merge PRs, which means the PR turns into a single commit on the target branch. Because of that, it’s ok to have merge commits in the PR itself, as they get removed. Please put a good description for the final commit in the PR description, and we’ll use it when squashing.

Code reviews

One of the core developers will review a pull request before merging it. If several days pass without any comments on your PR, please comment in the PR which will ping them. (If that happens, sorry! Sometimes things get missed.)

Compiler overview

The Emscripten Compiler Frontend (emcc) is a python script that manages the entire compilation process:

  • emcc calls Clang to compile C++ and wasm-ld to link it. It builds and integrates with the Emscripten system libraries, both the compiled ones and the ones implemented in JS.

  • emcc then calls which performs the final transformation to wasm (including invoking wasm-emscripten-finalize from Binaryen) and calls the JS compiler (see src/compiler.js and related files) which emits the JS.

  • If optimizing wasm, emcc will then call wasm-opt, run meta-dce, and other useful things. It will also run JS optimizations on the JS that is emitted alongside the wasm.

Emscripten Test Suite

Emscripten has a comprehensive test suite, which covers virtually all Emscripten functionality. These tests are run on CI automatically when you create a pull request, and they should all pass. If you run into trouble with a test failure you can’t fix, please let the developers know.


If you find a regression, bisection is often the fastest way to figure out what went wrong. This is true not just for finding an actual regression in Emscripten but also if your project stopped working when you upgrade, and you need to investigate if it’s an Emscripten regression or something else.

The normal git bisect workflow can work if you just need to bisect a single repository. For example, you can bisect only on the emscripten repo if you are on a range that all works with the same versions of LLVM and Binaryen (which was mentioned earlier, are two separate codebases that are depended on).

If you have a large bisection range, you generally can’t bisect a single repo. You can still bisect, though! To do that you need the emsdk and to understand how the release process works for all the repos together. The key “trick” is that:

emsdk install tot

can install an arbitrary build of emscripten: it installs the one identified in emscripten-releases-tot.txt. You can therefore bisect on the releases repo which has a DEPS file that specifies what version of all the various repos are in which release. At each bisection step in this repo the git hash identifies a particular tot release (which when it was built, was tip-of-tree).

For the actual bisection you can use git bisect. Each step in will download a complete build which is not a trivial download. However, at least the number of such steps will be logarithmic!

This bisects down to a single commit in the releases repo. That commit will generally update a single sub-repo from one commit to another. That will often be a very short list or even a single commit. If it’s more than one, you can bisect there while using a fixed build for the other repos.