Architecture Design

Overall Architecture

Core Process

1. Code Parsing

   • Convert source code to token stream using SWC's lexer
   • Build AST from token stream using parser

2. AST Transformation

   • Implement VarTransformer visitor
   • Traverse AST to locate const and let declarations
   • Modify declaration types to var

3. Code Generation

   • Use SWC's code generator
   • Convert modified AST back to source code

Tech Stack

Code Structure

rsdown/
├── src/
│   ├── lib.rs      # Core transformation logic
│   └── debug.rs    # Debug utilities
├── test/
│   └── index.test.ts   # Test cases
└── docs/
    └── .vitepress/     # Documentation config

Implementation Details

1. Lexical and Syntax Analysis

// Create lexer
let lexer = Lexer::new(
    Syntax::Es(Default::default()),
    Default::default(),
    StringInput::from(&*fm),
    None,
);

// Create parser and parse
let mut parser = Parser::new_from(lexer);
let mut module = parser.parse_module().unwrap();

2. AST Transformation

impl VisitMut for VarTransformer {
    fn visit_mut_var_decl(&mut self, var_decl: &mut VarDecl) {
        // Transform const and let to var
        var_decl.kind = VarDeclKind::Var;
    }
}

3. Code Generation

let mut buf = vec![];
let writer = JsWriter::new(cm.clone(), "\n", &mut buf, None);
let mut emitter = Emitter {
    cfg: swc_ecma_codegen::Config::default(),
    cm: cm.clone(),
    comments: None,
    wr: writer,
};

emitter.emit_module(&module).unwrap();