fix(i18n): redirect based on Referer header (#40512)

Co-authored-by: Oliver Eyton-Williams <ojeytonwilliams@gmail.com>

api-server/server/boot/challenge.js

@@ -12,12 +12,15 @@ import { ObjectID } from 'mongodb';
 import isNumeric from 'validator/lib/isNumeric';
 import isURL from 'validator/lib/isURL';
 
-import { homeLocation } from '../../../config/env';
-
 import { ifNoUserSend } from '../utils/middleware';
 import { dasherize } from '../../../utils/slugs';
 import { fixCompletedChallengeItem } from '../../common/utils';
 import { getChallenges } from '../utils/get-curriculum';
+import {
+  getParamsFromReq,
+  getRedirectBase,
+  normalizeParams
+} from '../utils/get-return-to';
 
 const log = debug('fcc:boot:challenges');
 
@@ -29,7 +32,9 @@ export default async function bootChallenge(app, done) {
     await getChallenges()
   );
   const redirectToCurrentChallenge = createRedirectToCurrentChallenge(
-    challengeUrlResolver
+    challengeUrlResolver,
+    normalizeParams,
+    getParamsFromReq
   );
 
   api.post(
@@ -59,7 +64,6 @@ export default async function bootChallenge(app, done) {
   app.use(router);
   done();
 }
-const learnURL = `${homeLocation}/learn`;
 
 const jsProjects = [
   'aaa48de84e1ecc7c742e1124',
@@ -327,15 +331,22 @@ function backendChallengeCompleted(req, res, next) {
     .subscribe(() => {}, next);
 }
 
+// TODO: extend tests to cover www.freecodecamp.org/language and
+// chinese.freecodecamp.org
 export function createRedirectToCurrentChallenge(
   challengeUrlResolver,
-  { _homeLocation = homeLocation, _learnUrl = learnURL } = {}
+  normalizeParams,
+  getParamsFromReq
 ) {
   return async function redirectToCurrentChallenge(req, res, next) {
     const { user } = req;
+    const { origin, pathPrefix } = normalizeParams(getParamsFromReq(req));
+
+    const redirectBase = getRedirectBase(origin, pathPrefix);
     if (!user) {
-      return res.redirect(_learnUrl);
+      return res.redirect(redirectBase + '/learn');
     }
+
     const challengeId = user && user.currentChallengeId;
     const challengeUrl = await challengeUrlResolver(challengeId).catch(next);
     if (challengeUrl === '/learn') {
@@ -346,6 +357,6 @@ export function createRedirectToCurrentChallenge(
         db may not be properly seeded.
       `);
     }
-    return res.redirect(`${_homeLocation}${challengeUrl}`);
+    return res.redirect(`${redirectBase}${challengeUrl}`);
   };
 }