swc transpiler

Status

Alpha — swc-73.wasm is available. A tool artifact rather than a language runtime: a small WASI CLI over swc_core that transpiles TypeScript to JavaScript. It exists to feed the Node.js runtime, which only speaks CommonJS.

At a glance

Capabilities

• version — print transpiler version
• swc [options] <file.ts|file.tsx>... — transpile each input, writing a sibling .js file (directories preserved)
• Types stripped
• TSX lowered, to React.createElement or to the automatic JSX runtime
• ES modules lowered to CommonJS with interop helpers inlined into the output (no @swc/helpers dependency at runtime)
• Parse errors — including recoverable ones — go to stderr as error: <file>:<line>:<col>: <message> referencing the original TypeScript source, and any failure exits non-zero

Inputs must end in .ts or .tsx, so filenames can never collide with the version subcommand.

Options

Values may be written --flag value or --flag=value, and -- ends option parsing.

Limitations

• Transpile only — no type checking
• Output is CommonJS only (by design, for the Node.js runtime)
• Reads and writes through the WASI filesystem, so the host must pre-open the directory containing the inputs
• Decorators are lowered only when asked for: the transform is --decorators, and only the legacy semantics are implemented

Install

wasmhub get swc 73

Usage examples

# Print version info
wasmrun exec swc-73.wasm version

Transpiling reads and writes files, so it needs a host that gives the program a directory. wasmrun exec gives it none; wasmtime's --dir does. Preopen the project root and name inputs under it:

# Transpile a file; writes app.js next to app.ts
wasmtime run --dir . swc-73.wasm ./src/app.ts

# Multiple inputs, TSX included
wasmtime run --dir . swc-73.wasm ./src/app.ts ./src/view.tsx

# A tsconfig with experimentalDecorators, target and the automatic JSX runtime
# (which imports react/jsx-runtime unless --jsx-import-source says otherwise)
wasmtime run --dir . swc-73.wasm \
  --decorators --target es2020 --jsx automatic --source-map \
  ./src/app.tsx

# Then run the output with the Node.js runtime
wasmtime run --dir . nodejs-20.wasm run ./src/app.js

Preopening the directory itself (--dir ./src) and naming ./src/app.ts does not work: the transpiler cannot resolve a path through a preopen named after a subdirectory, so it reports the file as missing.

wasmrun's agent mode runs this transpiler for you: an exec with "language": "typescript" transpiles every .ts/.tsx file in the session with it, maps the project's tsconfig.json onto these flags, and runs the result with the Node.js runtime.

Use from Rust

use wasmhub::{RuntimeLoader, Language};

let loader = RuntimeLoader::new()?;
let swc = loader.get_runtime(Language::Swc, "73").await?;
// Pass swc.path to your WASM runtime (wasmtime, wasmrun, etc.)

Building from source

./scripts/build-swc.sh runtimes/swc

Or inside Docker via the aggregate build: BUILD_SWC=true ./scripts/build-all.sh.

Technical notes

It is a plain stable wasm32-wasip1 build. Through v0.5 it was MVP-lowered (nightly -Zbuild-std under -C target-cpu=mvp, then wasm-opt's memory.copy/memory.fill lowering), because wasmrun's interpreter rejected the post-MVP instructions rustc emits by default. wasmrun 0.23.0 runs them (multi-value, sign-ext, nontrapping float-to-int, bulk memory, reference types), and wasmrun pins the wasmhub release it uses, so an older wasmrun never fetches this build.

wasm-opt still runs with exactly that feature set enabled and nothing else, which validates the output: a toolchain that starts emitting something wasmrun does not run yet (SIMD, tail calls, GC) fails the build rather than a sandbox.

The version label is 73 — the swc_core major — following the upstream-major naming of nodejs-20 and rust-1.82.

Roadmap

• [x] Source map output
• [x] Automatic JSX runtime and a configurable import source
• [x] Decorators and --target down-levelling
• [ ] Configurable JSX pragma for the classic runtime
• [ ] ESM output mode (once the Node.js runtime supports ES modules)