Systems

Internal building blocks that aren't user-facing subcommands. Most apps reuse multiple systems on this list.

What's here

System	Source root	Page
Parser, lexer, printer, transpiler	src/js_parser.zig, src/js_lexer.zig, src/js_printer.zig, src/transpiler.zig	Parser & transpiler
Module resolver	src/resolver/	Module resolution
HTTP client and server stack	src/http/, src/bun.js/api/server.zig	HTTP stack
Event loop	src/bun.js/event_loop.zig	Event loop
C++ JSC bindings	src/bun.js/bindings/	C++ bindings
Built-in JS modules	src/js/	Built-in JS modules
Code generation	src/codegen/	Code generation
CSS parser	src/css/	CSS parser
SQL clients	src/sql/	SQL clients
File watcher and hot reload	src/Watcher.zig, src/bun.js/hot_reloader.zig	Watcher & hot reload
Crash handler	src/crash_handler.zig	Crash handler
Allocators and bun.sys	src/allocators/, src/sys.zig, src/sys/	Allocators & sys
Other subsystems	various	Other subsystems

How they fit together

Most subcommands reach for the same handful of systems:

• bun run, bun test, bun repl → Parser, Module resolver, JSC bindings, Event loop, Built-in JS modules
• bun build → Parser, Module resolver, CSS parser, JSC bindings (only for plugins), Code generation (build time)
• bun install, bunx, bun upgrade → HTTP stack, allocators, sys
• bun.serve → HTTP stack, Event loop, JSC bindings
• Bake → Watcher, Bundler, HTTP stack, Hot reload

The crash_handler is implicitly active for every command. The code-generation subsystem only runs at build time.