Crate swc

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The main crate of the swc project.


This is documentation for building custom build tools on top of swc.

§Dependency version management

swc has swc_css, which re-exports required modules.


See [testing] and swc_ecma_transforms_testing.

§Custom javascript transforms

§What is JsWord?

It’s basically an interned string. See [swc_atoms].

§Choosing between JsWord vs String

You should prefer JsWord over String if it’s going to be stored in an AST node.

See [swc_atoms] for detailed description.

§Fold vs VisitMut vs Visit

See [swc_visit] for detailed description.

§Variable management (Scoping)

See [swc_ecma_transforms_base::resolver::resolver_with_mark].

§How identifiers work

See the doc on [swc_ecma_ast::Ident] or on [swc_ecma_transforms_base::resolver::resolver_with_mark].

§Comparing two identifiers

See [swc_ecma_utils::Id]. You can use [swc_ecma_utils::IdentLike::to_id] to extract important parts of an [swc_ecma_ast::Ident].

§Creating a unique identifier

See [swc_ecma_utils::private_ident].

§Prepending statements

If you want to prepend statements to the beginning of a file, you can use [swc_ecma_utils::prepend_stmts] or [swc_ecma_utils::prepend] if len == 1.

These methods are aware of the fact that "use strict" directive should be first in a file, and insert statements after directives.

§Improving readability

Each stuffs are documented at itself.

  • If you are creating or binding an [swc_ecma_ast::Expr] with operator, you can use [swc_ecma_ast::op].

  • If you want to create [swc_ecma_ast::CallExpr], you can use [swc_ecma_utils::ExprFactory::as_callee] to create callee.

  • If you want to create [swc_ecma_ast::CallExpr] or [swc_ecma_ast::NewExpr], you can use [swc_ecma_utils::ExprFactory::as_arg] to create arguments.

  • If you want to create [swc_ecma_ast::MemberExpr] where all identifiers are static (e.g. Object.prototype.hasOwnProperty), you can use [swc_ecma_utils::member_expr].

  • If you want to create [swc_ecma_ast::MemberExpr], you can use [swc_ecma_utils::ExprFactory::as_obj] to create object field.

§Reducing binary size

The visitor expands to a lot of code. You can reduce it by using macros like

Note that this will make typescript-related nodes not processed, but it’s typically fine as typescript::strip is invoked at the start and it removes typescript-specific nodes.

§Porting expr.evaluate() of babel

See [swc_ecma_minifier::eval::Evaluator].


  • pub extern crate swc_atoms as atoms;
  • pub use sourcemap;