Variable Management

SWC uses a very unique way to manage variables.

Syntax context

SWC uses a syntax context to manage variables. Internals are documented at the rustdoc of the resolver pass (opens in a new tab).

Basically the order of processing is like this. This processing should be wrapped by GLOBALS.set(&Default::default(), || { ... }).

resolver
your_pass (can be multiple passes)
hygiene
fixer (Optional, but if your transform may produce invalid syntax, this would add necessary parentheses)

Terms

A private identifier is a identifier that is guaranteed to be unique, and it's renamed by the hygiene pass if there's a conflict.

Creating a private identifier

You can use Ident::new_private to create a private identifier. Note that this requires an access to GLOBALS, which is a scoped thread-local storage.

let id = Ident::new_private("my_var");

Full example

Full example:

GLOBALS.set(&Default::default(), || {
  let unresolved_mark = Mark::new();
  let top_level_mark = Mark::new();
 
  // https://rustdoc.swc.rs/swc_core/ecma/transforms/base/fn.resolver.html
  program.mutate(resolver(unresolved_mark, top_level_mark, is_your_input_file_typescript));
 
  // Apply your transforms and use Ident::new_private from there
  program.mutate(your_pass);
  // or
  program.visit_mut_with(your_pass);
  // or
  program = program.fold(your_pass);
 
 
 
  // https://rustdoc.swc.rs/swc_core/ecma/transforms/base/hygiene/fn.hygiene.html
  program.mutate(hygiene());
 
})

String Management Profiling