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# Contributing to Bun
> **Important:** All contributions need test coverage. If you are adding a new feature, please add a test. If you are fixing a bug, please add a test that fails before your fix and passes after your fix.
## Bun's codebase
Bun is written mostly in Zig, but WebKit & JavaScriptCore (the JavaScript engine) is written in C++.
Today (February 2023), Bun's codebase has five distinct parts:
- JavaScript, JSX, & TypeScript transpiler, module resolver, and related code
- JavaScript runtime ([`src/bun.js/`](src/bun.js/))
- JavaScript runtime bindings ([`src/bun.zig/bindings/**/*.cpp`](src/bun.zig/bindings/))
- Package manager ([`src/install/`](src/install/))
- Shared utilities ([`src/string_immutable.zig`](src/string_immutable.zig))
The JavaScript transpiler & module resolver is mostly independent from the runtime. It predates the runtime and is entirely in Zig. The JavaScript parser is mostly in [`src/js_parser.zig`](src/js_parser.zig). The JavaScript AST data structures are mostly in [`src/js_ast.zig`](src/js_ast.zig). The JavaScript lexer is in [`src/js_lexer.zig`](src/js_lexer.zig). A lot of this code started as a port of esbuild's equivalent code from Go to Zig, but has had many small changes since then.
## Getting started
Please refer to [Bun's Development Guide](https://bun.sh/docs/project/development) to get your dev environment setup!
## Memory management in Bun
For the Zig code, please:
1. Do your best to avoid dynamically allocating memory.
2. If we need to allocate memory, carefully consider the owner of that memory. If it's a JavaScript object, it will need a finalizer. If it's in Zig, it will need to be freed either via an arena or manually.
3. Prefer arenas over manual memory management. Manually freeing memory is leak & crash prone.
4. If the memory needs to be accessed across threads, use `bun.default_allocator`. Mimalloc threadlocal heaps are not safe to free across threads.
The JavaScript transpiler has special-handling for memory management. The parser allocates into a single arena and the memory is recycled after each parse.
## JavaScript runtime
Most of Bun's JavaScript runtime code lives in [`src/bun.js`](src/bun.js).
### Calling C++ from Zig & Zig from C++
TODO: document this (see [`bindings.zig`](src/bun.js/bindings/bindings.zig) and [`bindings.cpp`](src/bun.js/bindings/bindings.cpp) for now)
### Adding a new JavaScript class
1. Add a new file in [`src/bun.js/*.classes.ts`](src/bun.js) to define the instance and static methods for the class.
2. Add a new file in [`src/bun.js/**/*.zig`](src/bun.js) and expose the struct in [`src/bun.js/generated_classes_list.zig`](src/bun.js/generated_classes_list.zig)
3. Run `make codegen`
Copy from examples like `Subprocess` or `Response`.
### ESM Modules and Builtins JS
Bun implements ESM modules in a mix of native code and JavaScript.
Several Node.js modules are implemented in JavaScript and loosely based on browserify polyfills.
Builtin modules in Bun are located in [`src/js`](src/js/). These files are transpiled and support a JavaScriptCore-only syntax for internal slots, which is explained further in [`src/js/README.md`](src/js/README.md).
Native C++ modules are in `src/bun.js/modules/`.
The module loader is in [`src/bun.js/module_loader.zig`](src/bun.js/module_loader.zig).
### Memory management in Bun's JavaScript runtime
TODO: fill this out (for now, use `JSC.Strong` in most cases)
### Strings
TODO: fill this out (for now, use `JSValue.toSlice()` in most cases)
#### JavaScriptCore C API
Do not copy from examples leveraging the JavaScriptCore C API. Please do not use this in new code. We will not accept PRs that add new code that uses the JavaScriptCore C API.
## Testing
See [`test/README.md`](test/README.md) for information on how to run tests.
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