AWS Lambda@Edge for rewriting request URI - reverse proxy for static sites and apps hosted with S3 + CloudFront CDN

index.js

'use strict'

/**
 * Minimal implementation of an AWS 'Lambda@Edge' solution for rewriting a request URI.
 *
 * This Edge Lambda can support static sites + front-end apps hosted on AWS via S3 + CloudFront CDN,
 * including those built by popular frameworks such as NextJS (e.g. via NextJS' `next export` static HTML export feature).
 *
 * An Edge Lambda provides control and can help meet real-world requirements that arise in the development of non-trivial
 * front-end web applications, e.g. involving headers, with query strings, etc. that may not be supported (or at least
 * elegantly supported) by "shortcut" AWS services such as AWS Amplify or app hosting services such as Netlify.
 *
 * Edge Lambdas like this example improve the behaviour of S3's static website hosting feature, which has potentially
 * undesirable behaviours by design, e.g. issuing an HTTP 302 response to requests that omit a trailing slash.
 *
 * @see {@link https://aws.amazon.com/blogs/compute/implementing-default-directory-indexes-in-amazon-s3-backed-amazon-cloudfront-origins-using-lambdaedge/}
 * @see {@link https://github.com/aws-samples/aws-lambda-edge-workshops/blob/master/Workshop1/Lab4_PrettyUrls/README.md}
 * @see {@link https://docs.aws.amazon.com/AmazonCloudFront/latest/DeveloperGuide/lambda-event-structure.html#lambda-event-structure-request}
 */
exports.handler = (event, _context, callback) => {
  const hasEndingSlashRegex = /\/$/
  const hasFileExtensionRegex = /(.+)\.[a-zA-Z0-9]{2,5}$/

  // extract request from the CloudFront event
  const request = event.Records[0].cf.request
  const requestUri = request.uri || ''

  // if logic involving http headers is required...
  // const headers = request.headers // array of request headers

  // if logic involving the query string is required...
  // const params = querystring.parse(request.querystring) // requires import `const querystring = require('querystring')`

  // if the request uri ends in '/' then replace the trailing slash with /index.html
  if (requestUri.match(hasEndingSlashRegex)) {
    request.uri = requestUri.replace(/\/$/, '/index.html')
  }

  // if the request uri is not for a file (i.e. no file extension) then append /index.html
  if (!requestUri.match(hasFileExtensionRegex)) {
    request.uri = requestUri + '/index.html'
  }

  if (requestUri !== request.uri) {
    console.log(`Rewrite request uri '${requestUri}' to '${request.uri}'`)
  }

  return callback(null, request)
}