mni - A World-Class Minifier

A blazing-fast, production-ready minifier for JavaScript, CSS, JSON, HTML, and SVG written in Rust.

Built on industry-leading libraries:

• JavaScript: SWC (powers Next.js, Deno, Vercel)
• CSS: LightningCSS (100x faster than cssnano, powers Parcel)
• HTML: minify-html (also minifies inline CSS/JS)
• SVG: oxvg (Rust port of SVGO, correctness-first safe preset)
• JSON: serde_json (Rust standard)

Features

• Production-Ready: Built on battle-tested libraries used in production by millions
• Blazing Fast: SWC + LightningCSS performance (7x faster than Terser)
• High Compression: 30-45% compression with correctness guarantees
• Safe by Default: Conservative optimizations that never produce invalid code
• Multi-format: JavaScript (ES5-ESNext), CSS, JSON, HTML, SVG
• Smart Detection: Auto-detects file format
• Source Maps: JS (via SWC) and CSS (via LightningCSS)
• Batch Mode: Parallel multi-file minification with --outdir
• Watch Mode: --watch re-runs on filesystem changes
• Config Files: .minirc.json partial overlay with CLI override precedence
• Rich CLI: Comprehensive command-line interface
• Configurable: Presets and fine-grained control

Installation

cargo install mni

Or build from source:

git clone https://github.com/epistates/mni
cd mni
cargo build --release

Usage

Basic Usage

# Minify JavaScript
mni input.js -o output.min.js

# Minify CSS
mni styles.css -o styles.min.css

# Minify JSON
mni data.json -o data.min.json

# Auto-detect format and show stats
mni input.js --stats

From stdin/stdout

# Read from stdin, write to stdout
echo "const x = 1 + 2;" | mni

# Pipe through mni
cat input.js | mni > output.min.js

Presets

# Development preset (fast, readable, source maps)
mni input.js --preset dev -o output.js

# Production preset (balanced compression and speed)
mni input.js --preset prod -o output.js

# Aggressive preset (maximum compression, slower)
mni input.js --preset aggressive -o output.js

Advanced Options

# Customize minification
mni input.js \
  --target es2020 \
  --mangle \
  --compress \
  --drop-console \
  --keep-fnames \
  --passes 2 \
  --stats

Batch Mode

# Minify many files in parallel into an output directory
mni src/a.js src/b.js src/c.css --outdir dist --source-map --stats

# Disable parallelism (sequential fallback)
mni src/*.js --outdir dist --no-parallel

Batch mode requires --outdir. Each input file is written to <outdir>/<basename> along with a sibling .map file when --source-map is enabled. Failed files are reported individually and the process exits non-zero if any file errored. With --stats, a per-file table of sizes/reductions/times is printed alongside totals.

Watch Mode

# Re-run minification whenever an input changes
mni src/app.js --outdir dist --watch
mni src/*.{js,css,html} --outdir dist --watch --source-map

--watch performs an initial build, then monitors the input files for changes using a platform-native filesystem watcher (via notify). Rebuild events are debounced (150ms) to coalesce bursts from editors that save in multiple steps. Errors during rebuilds are reported but do not stop the watcher — press Ctrl-C to exit.

Config File

mni will auto-discover .minirc.json (or mni.config.json) in the current directory, or you can point at one explicitly with --config <path>. The file is a partial JSON overlay of MinifyOptions — you only specify the fields you want to change.

{
  "keep_fnames": true,
  "compress_options": {
    "drop_console": true,
    "passes": 2
  }
}

Precedence (lowest to highest): built-in defaults → --preset → config file → explicit CLI flags. Use --no-config to skip auto-discovery.

Configuration Options

Option	Description	Default
--target	ECMAScript version (es5, es2015, es2020, esnext)	es2020
--mangle	Enable identifier mangling	true
--compress	Enable compression optimizations	true
--source-map	Generate source maps	false
--keep-fnames	Preserve function names	false
--keep-classnames	Preserve class names	false
--drop-console	Remove console.* statements	false
--drop-debugger	Remove debugger statements	true
--passes	Number of compression passes	1
--stats	Show minification statistics	false

Library Usage

use mni::{Minifier, MinifyOptions, Target};

fn main() -> anyhow::Result<()> {
    let code = r#"
        function hello(name) {
            console.log("Hello, " + name + "!");
        }
    "#;

    let options = MinifyOptions {
        target: Target::ES2020,
        mangle: true,
        compress: true,
        ..Default::default()
    };

    let minifier = Minifier::new(options);
    let result = minifier.minify_js(code)?;

    println!("Original: {} bytes", result.stats.original_size);
    println!("Minified: {} bytes", result.stats.minified_size);
    println!("Reduction: {:.1}%", result.stats.compression_ratio * 100.0);
    println!("Time: {} ms", result.stats.time_ms);
    println!("\n{}", result.code);

    Ok(())
}

Performance

Benchmarks on real-world files:

Format	Original	Minified	Reduction	Time
JavaScript (1.3KB)	1,307 bytes	757 bytes	42.1%	18ms
CSS (1.7KB)	1,657 bytes	1,218 bytes	26.5%	6ms
JSON (671B)	671 bytes	519 bytes	22.7%	<1ms

Comparison with Other Tools

mni leverages SWC which is:

• 7x faster than Terser
• 20x faster than Babel on single thread
• 70x faster than Babel on 4 cores

CSS minification via LightningCSS is:

• 100x faster than cssnano
• 2.7+ million lines/sec throughput

Optimization Techniques

JavaScript (via SWC)

• Identifier mangling with frequency analysis
• Dead code elimination
• Constant folding and propagation
• Boolean and comparison optimizations
• Unreachable code removal
• Unused variable elimination
• Scope hoisting
• Arrow function optimization
• Template literal optimization

CSS (via LightningCSS)

• Whitespace removal
• Comment removal
• Color minification (#ffffff → #fff)
• Length optimization (0px → 0)
• Property merging
• Vendor prefix optimization
• Calc() optimization
• Custom property optimization

HTML (via minify-html)

• Whitespace and comment removal
• Attribute minification
• Inline <style> minification (delegates to LightningCSS)
• Inline <script> minification (delegates to minify-js)
• DOCTYPE and optional tag omission

SVG (via oxvg — Rust SVGO port)

Uses oxvg's correctness-first safe preset, which skips transformations that can visually change the document. Typical optimizations applied:

• Whitespace, comment, metadata, and editor-namespace stripping
• Shape → path conversion (<rect> / <ellipse> etc.)
• Path data normalization and compression
• Color minification (#ffffff → #fff, named colors)
• Default attribute removal, useless defs/stroke/fill removal
• Group flattening where safe

JSON (via serde_json)

• Whitespace removal
• Key ordering (optional)
• UTF-8 optimization

Examples

See the examples/ directory for sample files:

# JavaScript minification
cargo run -- examples/sample.js --stats

# CSS minification
cargo run -- examples/sample.css --stats

# JSON minification
cargo run -- examples/sample.json --stats

Development

# Build
cargo build

# Run tests
cargo test

# Run benchmarks
cargo bench

# Format code
cargo fmt

# Lint
cargo clippy

Safety and Correctness

mni prioritizes correctness over compression. We've disabled certain aggressive SWC optimizations that can produce invalid JavaScript in edge cases:

• collapse_vars - Can create invalid left-hand assignments
• inline - Aggressive inlining can break code semantics
• sequences - Comma sequence optimization can produce invalid syntax

Result: All output is guaranteed to be valid JavaScript that re-parses correctly.

See BUGS.md for detailed information about known issues and fixes.

Roadmap

Completed:

• JavaScript minification (SWC)
• CSS minification (LightningCSS)
• HTML minification (minify-html, with inline CSS/JS)
• SVG minification (oxvg safe preset)
• JSON minification
• CLI interface
• Auto-format detection
• Safe compression (correctness first)
• Source map generation (JS via SWC, CSS via LightningCSS)
• Batch file processing with --outdir
• Parallel processing via rayon (disable with --no-parallel)
• Per-file compression statistics comparison (--stats in batch mode)
• Config file support (.minirc.json / mni.config.json / --config)
• Watch mode (--watch) via notify
• Comprehensive test suite

Planned:

• Integration with build tools (Vite/webpack/esbuild plugins)

Architecture

mni is designed as a unified orchestrator around best-in-class libraries:

┌─────────────────────────────────────────┐
│              mni CLI                    │
│  (Unified interface & orchestration)    │
└─────────────────────────────────────────┘
                  │
    ┌─────────────┼─────────────┐
    │             │             │
┌───▼───┐    ┌───▼────┐   ┌───▼──────┐
│  SWC  │    │Lightning│   │serde_json│
│  JS   │    │CSS CSS  │   │   JSON   │
└───────┘    └─────────┘   └──────────┘

This approach:

• Leverages production-proven libraries
• Provides consistent API across formats
• Enables future extensibility
• Maintains high performance

Why mni?

1. Production-Ready: Built on libraries powering the world's largest applications
2. Unified Interface: One tool for all your minification needs
3. Optimal Performance: Best-in-class speed for each format
4. Modern Rust: Memory-safe, fast, and reliable
5. Battle-Tested: Based on SWC (Next.js, Deno) and LightningCSS (Parcel)

License

MIT

Contributing

Contributions welcome! Please read our contributing guidelines and submit PRs.

Acknowledgments

Built with:

• SWC - Speedy Web Compiler
• LightningCSS - Fast CSS parser and transformer
• serde_json - JSON serialization

Support

• GitHub Issues
• Documentation