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:

  {
    // 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:

  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 functions 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.