Lambda@Edge Function for Accessing React Apps in Sub-Directories of Multiple S3 Buckets

index.js

// Import the data that maps the origins to each product.
const bucketData = require("./bucketData.js");

exports.handler = (event, context, callback) => {
  
  // Get the request object.
  const request = event.Records[0].cf.request;
  
  // Get the host from the request and take out "www." from the host if it exists.
  let host = request.headers.host[0].value;
  host = host.replace(/^www\./, "");
  
  // Parse the host to identify the product and the directory where the React app's static files are stored based on the subdomain's identifier (marketing, portal, customer, support).
  const [product, dir] = host.split(".")[0].split("-");
  
  // Get the path to the directory.  If the host doesn't have a subdomain with a hyphen (eg. example.com and kimchibeanz.example.com), return an empty string as the files will be at the root of the bucket.
  const entryPoint = dir ? `/${dir}` : "";
  
  // Find the product in your data's list of origins.
  const match = bucketData.find(dataSet => product === dataSet.product);
  
  if (match) {
  const domain = match.origin;
  
  // If the product is in the list of origins, instruct to send the request to the matching origin, specifying for it to look for content within the sub-directory or at the root.
  // The key here is the 'path' property. It specifies the entry point.  It does not affect the request URI (eg. /login). 
  request.origin = {
    custom: {
      domainName: domain,
      port: 80,
      protocol: "http",
      path: entryPoint,
      sslProtocols: ["TLSv1.1", "TLSv1.2"],
      readTimeout: 5,
      keepaliveTimeout: 5,
      customHeaders: {
        // Set a referer request header to gain access to read objects in the S3 bucket.
        referer : [{ key: "referer", value: `http://${host}/` }]
      }
    }
  };

  // Change the host in the request headers to match the origin's website endpoint.
  request.headers["host"] = [{ key: "host", value: domain }];
  callback(null, request);
  } else {
    
  // If the product was not in the list of origins, send the request without any alterations.
    callback(null, request);
  }
};