AWS Cloudfront Lambda default file URL rewriter

index.js

// See https://medium.com/@chrispointon/default-files-in-s3-subdirectories-using-cloudfront-and-lambda-edge-941100a3c629
// Register this as the viewer-request trigger handler
'use strict';
exports.handler = (event, context, callback) => {    
    // Extract the request from the CloudFront event that is sent to Lambda@Edge 
    var request = event.Records[0].cf.request;

    // Extract the URI and params from the request
    var olduri = request.uri;

    // Match any uri that ends with some combination of 
    // [0-9][a-z][A-Z]_- and append a slash
    var endslashuri = olduri.replace(/(\/[\w\-]+)$/, '$1/');

    //console.log("Old URI: " + olduri);
    //console.log("End slash URI: " + endslashuri);

    if(endslashuri != olduri) {
        // If we changed the uri, 301 to the version with a slash, appending querystring
        var params = '';
        if(('querystring' in request) && (request.querystring.length>0)) {
            params = '?'+request.querystring;
        }
        var newuri = '//' + request.headers.host[0].value + endslashuri + params;
        
        //console.log("Params: " + params);
        //console.log("New URI: " + newuri);

        const response = {
            status: '301',
            statusDescription: 'Permanently moved',
            headers: {
                location: [{
                    key: 'Location',
                    value: newuri
                }]
            }
        };
        return callback(null, response);
    } else {        
        // Return to CloudFront
        return callback(null, request);
    }
};