lambda_append_index_CFN.js

'use strict';

/** 
 *  this is useful if you want to use S3 endpoint over S3 Webserver Endpoint for Cloudfront Origin because
 *  OAI is not possible with S3 Webserver Origin (You can restrict access by using secrets in headers but 
 *  you'd have to leave the bucket public - this might be against company policy or does not fit your security 
 *  architecture)
 *  
 Downside if using S3 endpoint origin is lack of support for paths in URI. e.g
 *  www.example.com/path/ will return empty object. What we actually want is to 
 *  serve the index..html existing in the origin subdirectory. You'd have to type expplicitly
 *  www.example.com/path/index.html. It is ugly and there's still the issue with the empty download  
 *
 reason for this behaviour is that CF uses S3 Object API instead of treating S3 as a Webserver.
 *  configure this lambda to intercept the request from CF to S3 object API in 
 *  the behaviour configuration for the path.
 *  uses S3 object API 
 *  adds "index.html" to the end of a uri with a path
 * Extract request from a  CloudFront event that is 
 * sent to Lambda@Edge and replaces 
 */
exports.handler = (event, context, callback) => {
    
    var request = event.Records[0].cf.request;
   
    var olduri = request.uri;
    var newuri = olduri.replace(/\/$/, '\/index.html');
   
    request.uri = newuri;
    return callback(null, request);
};