Make some small style improvements

-    let data = await InvidiousAPI.getVideo(id);
+    const data = await InvidiousAPI.getVideo(id);

I try to use `const` whenever I don't have to use `let`. While it's not a
guarantee as strong as what e.g. Rust can provide, I think it helps reducing
cognitive load — we know that the symbol will always reference the same piece of data.

-    if (prefs.redirectYoutube === 'Auto') {
+    if (prefs.redirectYoutube == RedirectType.Auto) {

Me nitpicking again. The string value of the RedirectYoutube enum is mainly
there for the radio buttons in the options page; in the code, it's better to use
the actual RedirectType.Auto symbol.

-    return {};
+    throw new Error('No results.');

Under the hood, an `async` function returns a promise, which resolves on return
and is rejected on throw.

src/background.ts

@@ -17,7 +17,7 @@
 import * as _ from 'lodash/fp';
 import * as browser from 'webextension-polyfill';
 
-import { MessageKind } from './types';
+import { MessageKind, RedirectType } from './types';
 import constants from './constants';
 import Preferences from './preferences';
 import InvidiousAPI from './invidious-api';
@@ -27,7 +27,7 @@ import { getPeertubeVideoURL } from './util';
  * Retrieve the title of the video using Invidious API.
  */
 const getTitle = async (id: string) => {
-    let data = await InvidiousAPI.getVideo(id);
+    const data = await InvidiousAPI.getVideo(id);
     return data.title;
 }
 
@@ -38,19 +38,19 @@ const getTitle = async (id: string) => {
 const preventYoutube = async (r) => {
     const prefs = await Preferences.getPreferences();
 
-    if (prefs.redirectYoutube === 'Auto') {
+    if (prefs.redirectYoutube == RedirectType.Auto) {
         const query = new URLSearchParams(r.url.substring(r.url.indexOf('?') + 1));
 
-        let title = await getTitle(query.get('v'));
+        const title = await getTitle(query.get('v'));
-        let video = await searchByName(title);
+        const video = await searchByName(title);
-        let url = getPeertubeVideoURL(video, prefs);
+        const url = getPeertubeVideoURL(video, prefs);
 
         return {
             redirectUrl: url
         };
     }
 
-    return {};
+    throw new Error('No results.');
 };
 
 const buildSearchByNameURL = (instance: string, query: string): string => `https://${instance}/api/v1/search/videos?search=${encodeURIComponent(query)}`;

src/invidious-api.ts

@@ -3,7 +3,7 @@ import constants from './constants';
 
 export default class InvidiousAPI {
     static async _fetchAPI(action: string, params: any) {
-        let paramString = typeof params == 'string'
+        const paramString = typeof params == 'string'
             ? params
             : Object.keys(params).map(function(key) {
                 return encodeURIComponent(key) + '=' + encodeURIComponent(params[key]);
@@ -16,6 +16,7 @@ export default class InvidiousAPI {
                 + e.message
             ));
     }
+
     static async getVideo(id: string) {
         return this._fetchAPI(constants.invidiousAPI.videos, id);
     }