# Optimize JS Library Size

> Optimize JavaScript library size.

- Skill: `tomeraberbach/optimize-js-library-size` (Agent Skill)
- Install (CLI): `npx skillmds@latest add tomeraberbach/optimize-js-library-size`
- Raw SKILL.md: https://api.skillmd.com/api/skills/tomeraberbach/optimize-js-library-size/raw
- Safety review: pending (external: skill-scanner PASS, skillspector PASS)
- Works with: Claude Code, Claude.ai, OpenAI Codex
- Category: Coding & Dev Tools
- Author: TomerAberbach (https://skillmd.com/u/tomeraberbach)
- Updated: 2026-09-22
- Page: https://skillmd.com/skills/tomeraberbach/optimize-js-library-size

---


Reduce the size of this JavaScript library.

$ARGUMENTS

# Goals

- Reduce the minified and gzipped/brotli minified size
- Don't reduce minified size at the expense of gzipped/brotli minified size
- Preserve the public API
- Focus on large structural changes instead of small savings

# Workflow

1. Read and understand the library structure
2. Run relevant tests and note which ones already fail. Leave them failing
3. Measure size
4. Optimize
5. Rebuild
6. Measure size and compare: if size didn't improve, then revert and go back to
   step 4
7. Run relevant tests: if tests are newly failing, then revert and go back to
   step 4
8. Repeat from step 4

# Tools

- Ensure `terser` is configured:

  ```js
  {
    // Assume modern JavaScript
    ecma: 2020,
    module: true,
    toplevel: true,
    // Run the compressor multiple times
    compress: {
      passes: 3,
    },
    // Mangle underscore prefixed properties
    mangle: {
      properties: {
        regex: `^_[^_]+`,
      },
    },
  }
  ```

- Use `node` to measure size:

  ```bash
  node --input-type=module << 'EOF'
    import { readFileSync } from 'fs'
    import { gzipSync, brotliCompressSync } from 'zlib'
    const raw = readFileSync('REPLACE_ME.js', 'utf8')
    const minified = Buffer.from(raw.replace(/^\/\/# sourceMappingURL=[^\n]+$/m, '').trimEnd())
    console.log(`Minified: ${minified.length} | Gzipped: ${gzipSync(minified).length} | Brotli: ${brotliCompressSync(minified).length}`)
  EOF
  ```

# Techniques

## High-level

- Delete dead code
- Unify repeated or similar logic
- Simplify complex logic without changing behavior
- Extract repeated code to variables or functions

## Low-level

- Mangle internal objects using private class fields or underscore prefixed
  properties
- Replace internal classes with closures returning objects
- Inline single-use functions that don't add clarity
- Restructure single-use functions that add clarity so `terser` can inline them
- Replace `function`s with arrow functions
- Replace `switch` with `if` and `else`
- Replace `===`/`!==` with `==`/`!=` when it doesn't change behavior
- Replace strict checks with truthy/falsy checks when it doesn't change behavior

These aren't exhaustive. Reason from first principles when none fits.