My first cloudfront edge function

well-known-redirect.js

// note that a lot of code choices here are due to the fact that cloudfront edge uses
// a hilariously backwards version of javascript:
//
// "const", you say? what's that?!

// deal with the painful structure that cloudfront edge functions use for query params
function getURLSearchParamsString(obj) {
  var str = [];
  for (var param in obj) {
    if (obj[param].multiValue) {
      str.push(
        obj[param].multiValue.map((item) => param + "=" + item.value).join("&")
      );
    } else if (obj[param].value === "") {
      str.push(param);
    } else {
      str.push(param + "=" + obj[param].value);
    }
  }
  return str.join("&");
}

// redirect the following URL sub-paths to somewhere else
// this is for my fancy personalized mastodon setup; see https://til.simonwillison.net/mastodon/custom-domain-mastodon
// and the articles it links to. most of those happen to deploy their site with Django which... would have been
// easier than writing, testing, and installing this function into CloudFront. But I persist for now! :-)
function handler(event) {
    var paths = ["/.well-known/host-meta", "/.well-known/webfinger", "/.well-known/nodeinfo"];
    var baseUrl = "https://mastodon.davepeck.org";
    var request = event.request;
    var uri = request.uri;
    var matches = paths.filter(path => uri.startsWith(path));
    if (matches.length === 1) {
        var qs = getURLSearchParamsString(request.querystring);
        var qp = qs ? `?${qs}` : "";
        var location = `${baseUrl}${matches[0]}${qp}`;
        var response = {
            statusCode: 301,
            statusDescription: "Found",
            headers: {"location": {"value": location}}
        };
        return response;
    }
    return request;
}