jsmeld

A library-only wrapper around SWC for compiling and bundling JavaScript in Rust and Python.

Rust usage

Both compile and bundle take a file path and a JSMeldOptions struct:

use jsmeld::{compile, bundle, JSMeldOptions};

// Compile a single file
let output = compile(
    "./src/app.ts".to_string(),
    JSMeldOptions {
        target: "es2020".to_string(),
        minify: true,
        ..Default::default()
    },
)?;

// Bundle an entry point
let output = bundle(
    "./src/index.js".to_string(),
    JSMeldOptions {
        target: "es2020".to_string(),
        externals: vec!["react".to_string()],
        ..Default::default()
    },
)?;

JSMeldOptions

A single unified options struct used by both compilation and bundling.

Style hooks

JSMeldOptions supports style transformation hooks keyed by file extension. Hooks are executed in order when a matching style file is loaded during bundling.

Each key is a file extension (e.g. "css", "less") and each value is an ordered list of hook closures.

Hook type:

type StyleTransformHook = Arc<dyn Fn(&Path, &str) -> Result<String, String> + Send + Sync>;

Registering hooks in JSMeldOptions

use std::sync::Arc;
use std::collections::HashMap;
use jsmeld::{Bundler, JSMeldOptions};

let mut options = JSMeldOptions::default();

options.style_hooks.insert(
    "less".to_string(),
    vec![Arc::new(|_path, source| {
        // transform LESS -> CSS here
        Ok(source.to_string())
    })],
);

options.style_hooks.entry("css".to_string()).or_default().push(
    Arc::new(|_path, source| {
        // post-process CSS here
        Ok(source.to_string())
    }),
);

let bundler = Bundler::new(options);
let output = bundler.bundle("./src/index.js")?;

Registering hooks via Bundler

If you initialize a Bundler with options, you can append hooks per extension:

use std::sync::Arc;
use jsmeld::{Bundler, JSMeldOptions};

let mut bundler = Bundler::new(JSMeldOptions::default());
bundler.add_style_hook("css", Arc::new(|_path, source| {
    Ok(source.to_string())
}));

Notes:

• Extensions are normalized to lowercase.
• Leading dots are allowed (e.g. ".css").
• Hooks run only for matching style-file extensions.

Python usage

The Python package exposes two top-level functions. Both accept an optional options dictionary with the same keys as JSMeldOptions above.

import jsmeld

# Compile a single file (all options are optional)
output: str = jsmeld.compile("src/app.ts", {
    "target": "es2020",
    "minify": True,
})

# Bundle an entry point
output: str = jsmeld.bundle("src/index.js", {
    "target": "es2020",
    "externals": ["react"],
})

Style hooks from Python

style_hooks can be passed as a dictionary mapping file extensions to lists of callables (path: str, source: str) -> str:

def add_banner(path: str, source: str) -> str:
    return f"/* bundled by jsmeld */\n{source}"

output = jsmeld.bundle("src/index.js", {
    "style_hooks": {
        "css": [add_banner],
    },
})