Developer’s Guide

This article provides information that is relevant to people working on Emscripten itself, as opposed to those that use Emscripten in their own projects.


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

Building Emscripten from source code

As a contributor you will need to build Emscripten from source in order to integrate fixes back into the main repositories.

Repositories and branches of interest

The Emscripten main repository is There are two main branches:

  • master - The “master” branch. This is always safe to pull from and the test suite always passes.
  • incoming - The branch for new code. Code in incoming is merged with the master only after it is code reviewed and has passed all the automated tests.

Emscripten’s compiler core (Fastcomp) is maintained in two other repositories, which use the same master/incoming branch approach:

When building Emscripten from source you should use the same branch (incoming, or master) for building all three repositories. The topic Branches explains how to ensure that the versions are kept in sync.

Submitting patches

Patches should be submitted as pull requests to the incoming branch.


Before submitting 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:

  • Make pull requests to incoming, not master.
  • Do not include merge commits in pull requests; include only commits with the new relevant code.
  • Run all the automatic tests and make sure they pass. Some tests might not be required for very simple patches (for example, when just adding tests for new library functions). If you don’t have time to run all the tests, at least run a random subset of them (say, 100), see that link.
  • “Add an automatic test to tests/ if you add any new functionality or fix a bug.
  • Record the tests that were run in the pull request or issue.

Code reviews

@kripken reviews all pull requests before merging.

Exceptions are sub-projects that are ‘owned’ by other people. These owners can push to incoming directly:

Compiler overview

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

  • emcc calls Clang to convert C++ to bitcode, llvm-opt to optimize it, llvm-link to link it, etc.
  • emcc then calls, which performs the LLVM IR to JavaScript conversion:
    • first runs the Fastcomp core compiler (previously it ran the old core compilersrc/compiler.js).
    • then receives the core compiler output, modifies it slightly (some regexps) and then adds some necessary code around it. This generates the basic emitted JavaScript, which is called emcc-2-original in the intermediate files saved in debug mode.
  • emcc runs tools/ to further process and optimize the generated JavaScript.
    • breaks up the generated JavaScript file into the relevant parts for optimization and calls js-optimizer.js to actually optimize it.
    • js-optimizer.js parses and transforms the JavaScript into better JavaScript using the UglifyJS node.js script.

Emscripten Test Suite

Emscripten has a comprehensive test suite, which covers virtually all Emscripten functionality. These tests must all pass when you are submitting patches.