AcceptContentNegotiation

index.js

'use strict';

const {
    ValueTuple,
    awsPerformEncodingNegotiation,
    awsPerformTypeNegotiation } = require('http_content_negotiation');

const SERVER_ENCODINGS = [
    new ValueTuple('br', new Map([['q', 1]])),
    new ValueTuple('gzip', new Map([['q', 0.9]])),
    new ValueTuple('identity', new Map([['q', 0.1]]))];

const SERVER_IMAGE_TYPES = [
    new ValueTuple('image/webp', new Map([['q', 1]])),
    new ValueTuple('image/jpeg', new Map([['q', 0.5]]))];

const SERVER_IMAGE_WHITELIST = new Set([
    'image/jpeg',
]);

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

    if (request.uri.endsWith('.jpg')) {
        const type = awsPerformTypeNegotiation(
            request.headers, SERVER_IMAGE_TYPES, SERVER_IMAGE_WHITELIST);
        if (type) {
            const uriWithoutExtension = request.uri.slice(0, -3);
            switch (type.value) {
                case 'image/webp':
                    request.uri = uriWithoutExtension + 'webp';
                    break;

                case 'image/jpeg':
                    // Nothing to do
                    break;
            }
        }
    } else if (!request.uri.startsWith('/gzip/') &&
            !request.uri.startsWith('/br/')) {
        const encoding = awsPerformEncodingNegotiation(request.headers, SERVER_ENCODINGS);
        if (encoding && encoding.value !== 'identity') {
            request.uri = '/' + encoding.value + request.uri;
        }
    }

    callback(null, request);
};