index.mjs

import { S3Client, GetObjectCommand } from '@aws-sdk/client-s3';
import { resolve } from 'path';

const s3 = new S3Client({ region: 'S3_REGION' });
const s3Params = {
  Bucket: 'S3_BUCKET_NAME',
  Key: 'router-paths.txt'
};
const TTL = 10000;

const pullStream = (body) => {
  return new Promise((resolve, reject) => {
    const buffers = [];
    body.on('data', buf => {
      buffers.push(Buffer.from(buf));
    });
    body.on('end', () => {
      resolve(Buffer.concat(buffers));
    });
    body.on('error', reject);
  });
};

let paths;
const getPaths = async () => {
  if (paths) {
    return paths;
  }
  const response = await s3.send(new GetObjectCommand(s3Params));
  paths = (await pullStream(response.Body)).toString();
  setTimeout(() => {
    paths = undefined;
  }, TTL);
  return paths;
};

const getOriginPath = async clientPath => {
  const clientParts = clientPath.substring(1).split('/');
  const paths = (await getPaths())
    .split('\n')
    .map(s => s.trim())
    .filter(s => s);
  for (let i = 0; i < paths.length; ++i) {
    const parts = paths[i].split('.html')[0].split('/');
    const match =
      parts.length === clientParts.length &&
      parts.reduce(
        (match, part, index) =>
          match &&
          clientParts[index].length &&
          (part === clientParts[index] || part.startsWith('@') || part.startsWith('[')),
        true
      );
    if (match) {
      return '/' + paths[i];
    }
  }
  return '/404.html';
};

export const handler = (event, context, callback) => {
  context.callbackWaitsForEmptyEventLoop = false;
  
  const request = event.Records[0].cf.request;
  const headers = request.headers;
  const origin = request.origin;

  // Ignore static files.
  if (request.uri.indexOf('.') >= 0) {
    callback(null, request);
    return;
  }
  
  // Return 404 page for the paths config file.
  if (request.uri === '/router-paths.txt') {
    request.uri = '/404.html';
    callback(null, request);
    return;
  }

  // Homepage.
  if (request.uri === '/') {
    request.uri = '/index.html';
    callback(null, request);
    return;
  }

  // Remove trailing slashes by redirect.
  if (request.uri.endsWith('/')) {
    const response = {
      status: '302',
      statusDescription: 'Found',
      headers: {
        location: [
          {
            key: 'Location',
            value: request.uri.substring(0, request.uri.length - 1)
          }
        ]
      }
    };
    callback(null, response);
    return;
  }

  // Lookup path in paths text file.
  getOriginPath(request.uri)
    .then(path => {
      request.uri = path;
      callback(null, request);
      return;
    })
    .catch(err => {
      console.log(err);
      callback(null, request);
    });
};