SWC architecture

This document gives a high level overview of SWC internals. You may find it useful if you want to contribute to SWC or if you are interested in the inner workings of SWC.

Macros

SWC uses proc macro extensively to reduce work. Please see links below to know what each macro do.

And some adhoc-macros are used.

These macro breaks macro hygiene.

Structure

`/crates/swc_atoms`

Handle string interning for the SWC project. The crate depends on string_cache from servo.

`/crates/swc_common`

Contains code related to span, hygiene and error reporting.

Also, it contains / re-exports codes for visitor pattern. Visit<T> is non-mutating visitor, while Fold<T> is a mutating visitor.

`/crates/swc_ecma_ast`

Contains AST nodes for javascript and typescript.

`/crates/swc_ecma_codegen`

Converts javascript AST into javascript code.

`/crates/swc_ecma_parser`

Parses javascript and typescript

`/crates/swc_ecma_transforms_base`

Theres are three core transforms named resolver, hygiene, fixer. Other transforms depends on them.

`/crates/swc_ecma_transforms_base/src/resolver`

This pass resolves and marks all identifiers in the file.

e.g.

let a = 1;
{
    let a = 1;
}

becomes

let a#0 = 1;
{
  let a#1 = 1;
}

where number after # denotes the hygiene id. If two identifiers have same symbol but different hygiene id, it's different.

`/crates/swc_ecma_transforms_base/src/hygiene`

Hygiene pass actually changes symbol of identifiers with same symbol but different hygiene id.

let a#0 = 1;
{
  let a#1 = 2;
}

becomes

let a = 1;
{
    let a1 = 2;
}

`/crates/swc_ecma_transforms_base/src/fixer`

Fixes broken AST. This allow us to simply fold types like BinExpr without caring about operator precedence.

It means,

let v = BinExpr {
    left: "1 + 2",
    op: "*",
    right: "3",
};

(other passes generates AST like this)

is converted into

let v = BinExpr {
    left: "(1 + 2)",
    op: "*",
    right: "3",
};

and printed as

(1 + 2) * 3;

Tests

SWC uses the official ecmascript conformance test suite called test262 for testing.

Parser tests ensures that parsed result of test262/pass is identical with test262/pass-explicit.

Codegen tests ensures that generated code is equivalent to golden fixture files located at tests/references.

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SWC architecture

Macros

Structure

Tests

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