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

# Transpile a file (requires --dir mount); writes app.js next to app.ts
wasmrun exec --dir /path/to/src swc-73.wasm -- /path/to/src/app.ts

# Multiple inputs, TSX included
wasmrun exec --dir /path/to/src swc-73.wasm -- /path/to/src/app.ts /path/to/src/view.tsx

# A tsconfig with experimentalDecorators, target and the automatic JSX runtime
wasmrun exec --dir /path/to/src swc-73.wasm -- \
  --decorators --target es2020 --jsx automatic --source-map \
  /path/to/src/app.tsx

# Then run the output with the Node.js runtime
wasmrun exec --dir /path/to/src nodejs-20.wasm -- run /path/to/src/app.js

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

This is the one build that deliberately deviates from scripts/build-rust.sh: rustc 1.82+ emits post-MVP WASM instructions by default (multi-value, sign-ext, bulk-memory, nontrapping-fptoint), which downstream interpreters — notably wasmrun's exec mode, the primary consumer — reject at parse time. The build script therefore:

1. Rebuilds std with nightly -Zbuild-std under -C target-cpu=mvp
2. Runs wasm-opt with the plain MVP feature set, which doubles as validation that nothing post-MVP slipped through

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)