javascript amazon lambda for https://oulipo.link/

oulipo-link-shrinker.js

'use strict';

const AWS = require('aws-sdk');
const url = require('url');

const bucket = process.env["OULIPO_BUCKET"];
const region = process.env["OULIPO_REGION"];
const prefix = process.env["OULIPO_PREFIX"];
const cdn_prefix = process.env["OULIPO_CDN_PREFIX"];

const chars = "abcdfghjijklmnopqrstuvWXYZABCDFGHIJKLMNOPQRSTUVWXYZ0123456789";

const generate = () =>
    'xxxxxx'.replace(/x/g, (c) => chars[Math.random()*chars.length|0]);

exports.handler = (event, context, callback) => {
    const s3 = new AWS.S3({ region: region });
    const url_long = event.url_long;

    let retry = 0;
    
    const done = (url_short, error) => {
        if (error) {
            callback(null, {error: error})
        } else {
            callback(null, {url_long: url_long, url_short: url_short});
        }
    }

    const check_and_create_s3_redirect = (s3_bucket, id_short) => {
      const key_short = prefix + "/" + id_short;
      s3.headObject({ Bucket: bucket, Key: key_short }, (err, data) => {
        if (err) {
          // we should normally have a NotFound error showing that the id is not already in use
          if (err.code === "NotFound") {
            // normal execution path
            s3.putObject({ Bucket: s3_bucket, Key: key_short, Body: "", WebsiteRedirectLocation: url_long, ContentType: "text/plain", CacheControl: "public, max-age=315576000" }, 
              (err, data) => {
                if (err) {
                    done("", err.message);
                } else {
                  const ret_url = "https://" + cdn_prefix + "/" + id_short;
                  console.log("Okay! short_url = " + ret_url);
                  done(ret_url, "");
                }
              });
          } else {
            // treat all other errors as fatal
            done("", "Could not find an okay string: " + err.code);
          }
        } else {
          // we found a duplicate, let's retry a limited number of times
          retry += 1;
          if (retry <= 3) {
            check_and_create_s3_redirect(bucket, generate(), url_long);
          } else {
            // abort after 3 tries
            done("", "Cannot find a good short id, aborting." );
          }
        }
      });
    }
    
    if (event.cdn_prefix !== cdn_prefix) {
        return done("", "Invalid CDN location");
    }
    const url_check = url.parse(url_long);
    if (!(url_check && url_check.host && ["https:", "http:"].indexOf(url_check.protocol) >= 0)) {
        return done("", "Invalid URL format");
    }
    console.log("shrinking " + url_long);
    check_and_create_s3_redirect(bucket, generate(), url_long);
};

using this doc as a basis, but with a handful of adaptations