index.js

'use strict';

const animated = require('animated-gif-detector'),
      aws = require('aws-sdk'),
      sharp = require('sharp'),
      s3 = new aws.S3({ apiVersion: '2006-03-01' });

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

        if ('200' !== response.status || !request.origin || !request.origin.s3 || !request.origin.s3.domainName) {
            return callback(null, response);
        }

        const match = request.origin.s3.domainName.match(/([^.]*)\.s3(\.[^.]*)?\.amazonaws\.com/i);

        if (!match || !match[1] || 'string' !== typeof match[1]) {
            return callback(null, response);
        }

        const allowedContentTypes = ['image/gif', 'image/jpeg', 'image/png'];
        const bucket = match[1];
        const key = decodeURIComponent(request.uri.substring(1));
        const object = await s3.getObject({ Bucket: bucket, Key: key }).promise();
        
        if (!object.ContentType 
          || !allowedContentTypes.includes(object.ContentType)
          || ('image/gif' === object.ContentType && animated(object.Body))
        ) {
            return callback(null, response);
        }
        
        let contentType = null;
        const image = sharp(object.Body);
        const params = new URLSearchParams(request.querystring);

        if ('image/gif' === object.ContentType) {
            image.png();
            contentType = [{ key: 'Content-Type', value: 'image/png' }];
        }

        if (request.headers['accept'] && request.headers['accept'][0].value.match('image/webp')) {
            image.webp({ quality: Math.round(Math.min(Math.max(parseInt(params.get('quality'), 10) || 82, 0), 100)) });
            contentType = [{ key: 'Content-Type', value: 'image/webp' }];
        }

        if (params.has('width') || params.has('height')) {
            image.resize({
                width: parseInt(params.get('width'), 10) || null,
                height: parseInt(params.get('height'), 10) || null,
                fit: params.has('cropped') ? sharp.fit.cover : sharp.fit.inside,
                withoutEnlargement: true,
            });
        }

        const buffer = await image.toBuffer();
        const responseBody = buffer.toString('base64');

        if (1330000 < Buffer.byteLength(responseBody)) {
            return callback(null, response);
        }

        if (contentType) {
            response.headers['content-type'] = contentType;
        }

        response.body = responseBody;
        response.bodyEncoding = 'base64';

        callback(null, response);
    } catch (error) {
        console.log(error);
    }
};