Contributing

Configuring a development environment for Bun can take 10-30 minutes depending on your internet connection and computer speed. You will need ~10GB of free disk space for the repository and build artifacts.

If you are using Windows, please refer to this guide

Install Dependencies

Using your system's package manager, install Bun's dependencies:

brew install automake ccache cmake coreutils gnu-sed go icu4c libiconv libtool ninja pkg-config rust ruby

sudo apt install curl wget lsb-release software-properties-common cargo ccache cmake git golang libtool ninja-build pkg-config rustc ruby-full xz-utils

sudo pacman -S base-devel ccache cmake git go libiconv libtool make ninja pkg-config python rust sed unzip ruby

sudo dnf install cargo ccache cmake git golang libtool ninja-build pkg-config rustc ruby libatomic-static libstdc++-static sed unzip which libicu-devel 'perl(Math::BigInt)'

sudo zypper install go cmake ninja automake git rustup && rustup toolchain install stable

Before starting, you will need to already have a release build of Bun installed, as we use our bundler to transpile and minify our code, as well as for code generation scripts.

curl -fsSL https://bun.sh/install | bash

npm install -g bun

brew tap oven-sh/bun

brew install bun

Install LLVM

Bun requires LLVM 16 and Clang 16 (clang is part of LLVM). This version requirement is to match WebKit (precompiled), as mismatching versions will cause memory allocation failures at runtime. In most cases, you can install LLVM through your system package manager:

brew install llvm@16

# LLVM has an automatic installation script that is compatible with all versions of Ubuntu

wget https://apt.llvm.org/llvm.sh -O - | sudo bash -s -- 16 all

sudo pacman -S llvm clang lld

sudo dnf install 'dnf-command(copr)'

sudo dnf copr enable -y @fedora-llvm-team/llvm-snapshots

sudo dnf install llvm clang lld

sudo zypper install clang16 lld16 llvm16

If none of the above solutions apply, you will have to install it manually.

Make sure LLVM 16 is in your path:

which clang-16

If not, run this to manually link it:

# use fish_add_path if you're using fish

export PATH="$(brew --prefix llvm@16)/bin:$PATH"

# use fish_add_path if you're using fish

export PATH="$PATH:/usr/lib/llvm16/bin"

Building Bun

After cloning the repository, run the following command to run the first build. This may take a while as it will clone submodules and build dependencies.

bun setup

The binary will be located at ./build/bun-debug. It is recommended to add this to your $PATH. To verify the build worked, let's print the version number on the development build of Bun.

build/bun-debug --version

x.y.z_debug

To rebuild, you can invoke bun run build

bun run build

These two scripts, setup and build, are aliases to do roughly the following:

./scripts/setup.sh

cmake -S . -B build -G Ninja -DCMAKE_BUILD_TYPE=Debug

ninja -C build # 'bun run build' runs just this

Advanced users can pass CMake flags to customize the build.

VSCode

VSCode is the recommended IDE for working on Bun, as it has been configured. Once opening, you can run Extensions: Show Recommended Extensions to install the recommended extensions for Zig and C++. ZLS is automatically configured.

If you use a different editor, make sure that you tell ZLS to use the automatically installed Zig compiler, which is located at ./.cache/zig/zig (zig.exe on Windows).

Code generation scripts

Bun leverages a lot of code generation scripts.

The ./src/bun.js/bindings/headers.h file has bindings to & from Zig <> C++ code. This file is generated by running the following:

make headers

This ensures that the types for Zig and the types for C++ match up correctly, by using comptime reflection over functions exported/imported.

TypeScript files that end with *.classes.ts are another code generation script. They generate C++ boilerplate for classes implemented in Zig. The generated code lives in:

• src/bun.js/bindings/ZigGeneratedClasses.cpp
• src/bun.js/bindings/ZigGeneratedClasses.h
• src/bun.js/bindings/generated_classes.zig To generate the code, run:

make codegen

Lastly, we also have a code generation script for our native stream implementations. To run that, run:

make generate-sink

You probably won't need to run that one much.

Modifying ESM modules

Certain modules like node:fs, node:stream, bun:sqlite, and ws are implemented in JavaScript. These live in src/js/{node,bun,thirdparty} files and are pre-bundled using Bun. In debug builds, Bun automatically loads these from the filesystem, wherever it was compiled, so no need to re-run make dev.

Release build

To build a release build of Bun, run:

bun run build:release

The binary will be located at ./build-release/bun and ./build-release/bun-profile.

Valgrind

On Linux, valgrind can help find memory issues.

Keep in mind:

• JavaScriptCore doesn't support valgrind. It will report spurious errors.
• Valgrind is slow
• Mimalloc will sometimes cause spurious errors when debug build is enabled

You'll need a very recent version of Valgrind due to DWARF 5 debug symbols. You may need to manually compile Valgrind instead of using it from your Linux package manager.

--fair-sched=try is necessary if running multithreaded code in Bun (such as the bundler). Otherwise it will hang.

valgrind --fair-sched=try --track-origins=yes bun-debug <args>

Building WebKit locally + Debug mode of JSC

WebKit is not cloned by default (to save time and disk space). To clone and build WebKit locally, run:

# once you run this, `make submodule` can be used to automatically
# update WebKit and the other submodules

git submodule update --init --depth 1 --checkout src/bun.js/WebKit

# to make a jsc release build

make jsc

# JSC debug build does not work perfectly with Bun yet, this is actively being
# worked on and will eventually become the default.

make jsc-build-linux-compile-debug cpp

make jsc-build-mac-compile-debug cpp

Note that the WebKit folder, including build artifacts, is 8GB+ in size.

If you are using a JSC debug build and using VScode, make sure to run the C/C++: Select a Configuration command to configure intellisense to find the debug headers.

Troubleshooting

'span' file not found on Ubuntu

The Clang compiler typically uses the libstdc++ C++ standard library by default. libstdc++ is the default C++ Standard Library implementation provided by the GNU Compiler Collection (GCC). While Clang may link against the libc++ library, this requires explicitly providing the -stdlib flag when running Clang.

Bun relies on C++20 features like std::span, which are not available in GCC versions lower than 11. GCC 10 doesn't have all of the C++20 features implemented. As a result, running make setup may fail with the following error:

fatal error: 'span' file not found
#include <span>
         ^~~~~~

The issue may manifest when initially running bun setup as Clang being unable to compile a simple program:

The C++ compiler

  "/usr/bin/clang++-16"

is not able to compile a simple test program.

To fix the error, we need to update the GCC version to 11. To do this, we'll need to check if the latest version is available in the distribution's official repositories or use a third-party repository that provides GCC 11 packages. Here are general steps:

sudo apt update

sudo apt install gcc-11 g++-11

# If the above command fails with `Unable to locate package gcc-11` we need
# to add the APT repository

sudo add-apt-repository -y ppa:ubuntu-toolchain-r/test

# Now run `apt install` again

sudo apt install gcc-11 g++-11

Now, we need to set GCC 11 as the default compiler:

sudo update-alternatives --install /usr/bin/gcc gcc /usr/bin/gcc-11 100

sudo update-alternatives --install /usr/bin/g++ g++ /usr/bin/g++-11 100

libarchive

If you see an error when compiling libarchive, run this:

brew install pkg-config

macOS library not found for -lSystem

If you see this error when compiling, run:

xcode-select --install

Cannot find libatomic.a

Bun defaults to linking libatomic statically, as not all systems have it. If you are building on a distro that does not have a static libatomic available, you can run the following command to enable dynamic linking:

bun setup -DUSE_STATIC_LIBATOMIC=OFF

The built version of Bun may not work on other systems if compiled this way.