Example Edge@Lambda in AWS Cloudfront. Change URI based on host header

gistfile1.txt

'use strict';

/*
 * HTTP headers are case-insensitive. So 'Host' is a valid header, but so
 * is 'host' or 'hOst'. To make sure we don't miss a header we first lowerCase
 * all of them and then check for the lowercase version of the header
 */
const normaliseHeaders = (headers) => {
    const normalisedHeaders = {};
    const fields = Object.keys(headers);
    for(let i = 0, len = fields.length; i < len; i++ ) {
        normalisedHeaders[fields[i].toLowerCase()] = headers[fields[i]];
    }
    return normalisedHeaders;
};

/*
 * We have all static assets for all of our customers in one S3 Bucket
 * but in separate folders. If people come through the linc-app.co domain
 * we need to prepend their sitename to the URI so Cloudfront can find
 * the assets in the S3 Bucket.
 * So: https://myreactapp.linc-app.co/_assets/js/main.abcde.js becomes
 * /myreactapp.linc-app.co/_assets/js/main.abcde.js when looking up the 
 * file in S3.
 */
exports.handler = (event, context, callback) => {
    const request = event.Records[0].cf.request;
    const headers = normaliseHeaders(request.headers);
    const host = Array.isArray(headers.host) ? headers.host[0] : headers.host;
    if(host) {
        const subdomains = host.split('.');
        if(subdomains && subdomains[0]) {
            request.uri = `/${subdomains[0]}${request.uri}`;
            return callback(null, request);
        }
    }
    
    console.log(`Couldn't find a suitable Host header: ${host}`);
    callback("Couldn't find a suitable Host header");
};