Lambda@Edge Compression Script Generator

/**
 * Lambda@Edge Compression Script
 * Event type: origin-response
 * Runtime: nodejs20.x
 * Generated by AbackTools — https://abacktools.com
 */

'use strict';

// Content types that benefit from compression
const COMPRESSIBLE_TYPES = [
  'text/html', 'text/css', 'text/plain', 'text/xml', 'text/javascript',
  'application/javascript', 'application/x-javascript', 'application/json',
  'application/ld+json', 'application/xml', 'application/rss+xml',
  'application/atom+xml', 'image/svg+xml', 'font/ttf', 'font/otf',
];

// Already-compressed formats — skip to avoid overhead
const SKIP_TYPES = [
  'image/jpeg', 'image/png', 'image/webp', 'image/avif', 'image/gif',
  'image/bmp', 'video/mp4', 'video/webm', 'audio/mpeg', 'audio/ogg',
  'application/zip', 'application/gzip', 'application/x-brotli',
  'font/woff2',
];

exports.handler = async (event) => {
  const { request, response } = event.Records[0].cf;

  // Get the Accept-Encoding header from the viewer request
  const acceptEncoding =
    (request.headers['accept-encoding'] || [{ value: '' }])[0].value.toLowerCase();

  // Get the Content-Type of the response
  const contentType =
    (response.headers['content-type'] || [{ value: '' }])[0].value.split(';')[0].trim().toLowerCase();

  // Skip if already compressed
  const alreadyEncoded = response.headers['content-encoding'];
  if (alreadyEncoded && alreadyEncoded[0]?.value) {
    return response;
  }

  // Skip already-compressed content types
  if (SKIP_TYPES.some(t => contentType.startsWith(t))) {
    return response;
  }

  // Only compress compressible content types
  const isCompressible = COMPRESSIBLE_TYPES.some(t => contentType.startsWith(t));
  if (!isCompressible) {
    return response;
  }

  // Skip small responses (compression overhead not worth it)
  const bodyLength = response.body ? Buffer.byteLength(response.body, 'utf8') : 0;
  if (bodyLength < 1024) {
    return response;
  }

  // Determine the best encoding to use
  let encoding = null;
  if (acceptEncoding.includes('br')) {
    encoding = 'br';
  } else if (acceptEncoding.includes('gzip')) {
    encoding = 'gzip';
  }

  if (!encoding) {
    return response;
  }

  // Compress the response body
  try {
    const zlib = require('zlib');
    const body = Buffer.from(
      response.body,
      response.bodyEncoding === 'base64' ? 'base64' : 'utf8'
    );

    let compressed;
    if (encoding === 'br') {
      compressed = zlib.brotliCompressSync(body, {
        params: { [zlib.constants.BROTLI_PARAM_QUALITY]: 6 },
      });
    } else {
      compressed = zlib.gzipSync(body, { level: 6 });
    }

    // Only use compressed version if it's actually smaller
    if (compressed.length < body.length) {
      response.body = compressed.toString('base64');
      response.bodyEncoding = 'base64';
      response.headers['content-encoding'] = [{ key: 'Content-Encoding', value: encoding }];
      response.headers['content-length'] = [{ key: 'Content-Length', value: String(compressed.length) }];
    }
  } catch (err) {
    console.error('Compression failed:', err);
    // Return uncompressed response on error
  }

  // Add Vary: Accept-Encoding so CloudFront caches separate variants
  const existingVary = (response.headers['vary'] || [{ value: '' }])[0].value;
  if (!existingVary.toLowerCase().includes('accept-encoding')) {
    response.headers['vary'] = [{
      key: 'Vary',
      value: existingVary ? `${existingVary}, Accept-Encoding` : 'Accept-Encoding',
    }];
  }

  // Set Cache-Control for compressed assets
  if (!response.headers['cache-control']) {
    response.headers['cache-control'] = [{
      key: 'Cache-Control',
      value: 'public, max-age=31536000, stale-while-revalidate=86400',
    }];
  }

  return response;
};