Plugin cheatsheet
This page describes the known hard points for implementing plugins for ecmascript.
You may find documentation athttps://rustdoc.swc.rs/swc useful, especially if you are dealing with a visitor orId issue.
Understanding types
JsWord
String allocates, and ‘text’-s of source code has a special trait.Those are lots of duplicates. Obviously, if your variable is namedfoo, you need to usefoo multiple times.So SWC interns the string to reduce the number of allocations.
JsWord is a string type that is interned.You can create aJsWord from&str, or from aString.Use.into() to convert toJsWord.
Ident,Id,Mark,SyntaxContext
SWC uses a special system for managing variables.Seethe rustdoc forIdent for details.
Common issues
Getting AST representation of input
SWC Playground supports getting AST from the input code.
Variable management of SWC
Error reporting
Seerustdoc forswc_common::errors::Handler.
ComparingJsWord with&str
If you don’t know whatJsWord is, seethe rustdoc for swc_atoms .
You can create&str by doing&val whereval is a variable of typeJsWord.
MatchingBox<T>
You will need to usematch to match on various nodes, includingBox<T>.For performance reason, all expressions are stored in a boxed form. (Box<Expr>)
SWC stores callee of call expressions as aCallee enum, and it hasBox<Expr>.
use swc_core::ast::*;use swc_core::visit::{VisitMut,VisitMutWith};struct MatchExample;impl VisitMut for MatchExample { fn visit_mut_callee(&mut self, callee: &mut Callee) { callee.visit_mut_children_with(self); if let Callee::Expr(expr)= callee { // expr is `Box<Expr>` if let Expr::Ident(i)= &mut **expr { i.sym= "foo".into(); } } }}Changing AST type
If you want to changeExportDefaultDecl toExportDefaultExpr, you should do it fromvisit_mut_module_decl.
Inserting new nodes
If you want to inject a newStmt, you need to store the value in the struct, and inject it fromvisit_mut_stmts orvisit_mut_module_items.Seea destructuring core transform .
struct MyPlugin { stmts: Vec<Stmt>,}Tips
Decorators and TypeScript types
Those are handled before your plugin is called. So you can’t access them from the Wasm plugin.This design decision is made to make Wasm plugins easier to write and the Wasm binary smaller.
- Tracking issue:https://github.com/swc-project/swc/issues/9132
Comments while testing
You can make your pass generic overC: Comments.test_fixture provides&mut Tester, which hascomments field.
Applyresolver while testing
SWC applies plugin after applyingresolver, so it’s better to test your transform with it.As written in the rustdoc for theresolver, you have to use correctSyntaxContext if you need to reference global variable (e.g.__dirname,require) or top-level bindings written by the user.
fn tr()-> impl Pass { ( resolver(Mark::new(),Mark::new(),false), // Most of transform does not care about globals so it does not need `SyntaxContext` your_transform() )}test!( Syntax::default(), |_| tr(), basic, // input "(function a ([a]) { a });", // output "(function a([_a]) { _a; });");Make your handlers stateless
Let’s say we are going to handle all array expressions in a function expression.You can add a flag to the visitor to check if we are in a function expression.You will be tempted to do
struct Transform { in_fn_expr: bool}impl VisitMut for Transform { noop_visit_mut_type!(); fn visit_mut_fn_expr(&mut self, n: &mut FnExpr) { self.in_fn_expr= true; n.visit_mut_children_with(self); self.in_fn_expr= false; } fn visit_mut_array_lit(&mut self, n: &mut ArrayLit) { if self.in_fn_expr { // Do something } }}but this cannot handle
const foo = function () { const arr = [1,2,3]; const bar = function () {}; const arr2 = [2,4,6];}After visitingbar,in_fn_expr isfalse.You have to do
struct Transform { in_fn_expr: bool}impl VisitMut for Transform { noop_visit_mut_type!(); fn visit_mut_fn_expr(&mut self, n: &mut FnExpr) { let old_in_fn_expr= self.in_fn_expr; self.in_fn_expr= true; n.visit_mut_children_with(self); self.in_fn_expr= old_in_fn_expr; } fn visit_mut_array_lit(&mut self, n: &mut ArrayLit) { if self.in_fn_expr { // Do something } }}instead.
Test with@swc/jest
You can test your transform with@swc/jest by adding your plugin to yourjest.config.js.
module.exports = { rootDir: __dirname, moduleNameMapper: { "css-variable$":"../../dist", }, transform: { "^.+\\.(t|j)sx?$": [ "@swc/jest", { jsc: { experimental: { plugins: [ [ require.resolve( "../../swc/target/wasm32-wasi/release/swc_plugin_css_variable.wasm" ), { basePath: __dirname, displayName:true, }, ], ], }, }, }, ], },};Seehttps://github.com/jantimon/css-variable/blob/main/test/swc/jest.config.js
Path is one of unix, while FileName can be one of host OS
This is because linux version ofPath code is used while compiling to wasm.So you may need to replace\\ with/ in your plugin.As/ is a valid path separator in windows, it’s valid thing to do.
Comments
A comment belongs to a span of the node. (lo for leading comments, hi for trailing comments)If you want to add a leading comment to a node, you can doPluginCommentsProxy.add_leading(span.lo, comment);.
SeePluginCommentsProxy .
Ownership model (of rust)
This section is not about
swcitself. But this is described at here because it’s the cause of almost all trickyness of APIs.
In rust, only one variable canown a data, and there’s at most one mutable reference to it.Also, you need toown the value or have a mutable reference to it if you want to modify the data.
But there’s at most one owner/mutable reference, so it means if you have a mutable reference to a value, other code cannot modify the value.Every update operation should performed by the code whichowns the value or has a mutable reference to it.So, some of babel APIs likenode.delete is super tricky to implement.As your code has ownership or mutable refernce tosome part of AST, SWC cannot modify the AST.
Tricky operations
Deleting node
Let’s say, we want to drop the variable namedbar in the code below.
var foo= 1;var bar= 1;There are two ways to do this.
Mark & Delete
The first way is to mark it asinvalid and delete it later.This is typically more convenient.
use swc_core::ast::*;use swc_core::visit::{VisitMut,VisitMutWith};impl VisitMut for Remover { fn visit_mut_var_declarator(&mut self, v: &mut VarDeclarator) { // This is not required in this example, but you typically need this. v.visit_mut_children_with(self); // v.name is `Pat`. // See https://rustdoc.swc.rs/swc_ecma_ast/enum.Pat.html match v.name { // If we want to delete the node, we should return false. // // Note the `&*` before i.sym. // The type of symbol is `JsWord`, which is an interned string. Pat::Ident(i)=> { if &*i.sym== "bar" { // Take::take() is a helper function, which stores invalid value in the node. // For Pat, it's `Pat::Invalid`. v.name.take(); } } _=> { // Noop if we don't want to delete the node. } } } fn visit_mut_var_declarators(&mut self, vars: &mut Vec<VarDeclarator>) { vars.visit_mut_children_with(self); vars.retain(|node| { // We want to remove the node, so we should return false. if node.name.is_invalid() { return false } // Return true if we want to keep the node. true }); } fn visit_mut_stmt(&mut self, s: &mut Stmt) { s.visit_mut_children_with(self); match s { Stmt::Decl(Decl::Var(var))=> { if var.decls.is_empty() { // Variable declaration without declarator is invalid. // // After this, `s` becomes `Stmt::Empty`. s.take(); } } _=> {} } } fn visit_mut_stmts(&mut self, stmts: &mut Vec<Stmt>) { stmts.visit_mut_children_with(self); // We remove `Stmt::Empty` from the statement list. // This is optional, but it's required if you don't want extra `;` in output. stmts.retain(|s| { // We use `matches` macro as this match is trivial. !matches!(s,Stmt::Empty(..)) }); } fn visit_mut_module_items(&mut self, stmts: &mut Vec<ModuleItem>) { stmts.visit_mut_children_with(self); // This is also required, because top-level statements are stored in `Vec<ModuleItem>`. stmts.retain(|s| { // We use `matches` macro as this match is trivial. !matches!(s,ModuleItem::Stmt(Stmt::Empty(..))) }); }}Delete from the parent handler
Another way to delete the node is deleting it from the parent handler.This can be useful if you want to delete the node only if the parent node is specific type.
e.g. You don’t want to touch the variables in for loops while deleting free variable statements.
use swc_core::ast::*;use swc_core::visit::{VisitMut,VsiitMutWith};struct Remover;impl VisitMut for Remover { fn visit_mut_stmt(&mut self, s: &mut Stmt) { // This is not required in this example, but just to show that you typically need this. s.visit_mut_children_with(self); match s { Stmt::Decl(Decl::Var(var))=> { if var.decls.len()== 1 { match var.decls[0].name { Pat::Ident(i)=> { if &*i.sym== "bar" { s.take(); } } } } } _=> {} } } fn visit_mut_stmts(&mut self, stmts: &mut Vec<Stmt>) { stmts.visit_mut_children_with(self); // We do same thing here. stmts.retain(|s| { !matches!(s,Stmt::Empty(..)) }); } fn visit_mut_module_items(&mut self, stmts: &mut Vec<ModuleItem>) { stmts.visit_mut_children_with(self); // We do same thing here. stmts.retain(|s| { !matches!(s,ModuleItem::Stmt(Stmt::Empty(..))) }); }}Referencing parent node from handler of child node
This includes usage ofpaths andscope.
Caching some information about an AST node
You have two way to use informantion from a parent node.For first, you can precompute information from the parent node handler.Alternatively, you can clone the parent node and use it in the child node handler.
Alternatives for babel APIs
generateUidIdentifier
This returns a unique identifier with a monotonically increasing integer suffix.swc does not provide API to do this, because there’s a very easy way to do this.You can store an integer field in transformer type and use it while callingquote_ident! orprivate_ident!.
struct Example { // You don't need to share counter. cnt: usize}impl Example { /// For properties, it's okay to use `quote_ident`. pub fn next_property_id(&mut self)-> Ident { self.cnt+= 1; quote_ident!(format!("$_css_{}",self.cnt)) } /// If you want to create a safe variable, you should use `private_ident` pub fn next_variable_id(&mut self)-> Ident { self.cnt+= 1; private_ident!(format!("$_css_{}",self.cnt)) }}path.find
Upward traversal is not supported byswc.It’s because upward traversal requires storing information about parent at children nodes, which requires using types likeArc orMutex in rust.
Instead of traversing upward, you should make it top-down.For example, if you want to infer name of a jsx component from variable assignments or assignments, you can storename of component while visitingVarDecl and/orAssignExpr and use it from the component handler.
state.file.get/state.file.set
You can simply store the value in the transform struct as an instance of transform struct only process one file.