Simplify control flow in `AuthService` using async/await

Make several functions/methods in the service easier to follow by
refactoring promise chains to async/await.

src/sidebar/services/auth.js

@@ -116,24 +116,6 @@ export class AuthService extends TinyEmitter {
       localStorage.setObject(storageKey(), token);
     }
 
-    /**
-     * Exchange the refresh token for a new access token and refresh token pair.
-     *
-     * @param {string} refreshToken
-     * @param {RefreshOptions} options
-     * @return {Promise<TokenInfo>} Promise for the new access token
-     */
-    function refreshAccessToken(refreshToken, options) {
-      return oauthClient()
-        .then(client => client.refreshToken(refreshToken))
-        .then(tokenInfo => {
-          if (options.persist) {
-            saveToken(tokenInfo);
-          }
-          return tokenInfo;
-        });
-    }
-
     /**
      * Listen for updated access & refresh tokens saved by other instances of the
      * client.
@@ -149,11 +131,11 @@ export class AuthService extends TinyEmitter {
       });
     };
 
-    function oauthClient() {
+    const oauthClient = async () => {
       if (client) {
-        return Promise.resolve(client);
+        return client;
       }
-      return apiRoutes.links().then(links => {
+      const links = await apiRoutes.links();
       client = new OAuthClient({
         clientId: settings.oauthClientId,
         authorizationEndpoint: links['oauth.authorize'],
@@ -161,15 +143,30 @@ export class AuthService extends TinyEmitter {
         tokenEndpoint: tokenUrl,
       });
       return client;
-      });
+    };
+
+    /**
+     * Exchange the refresh token for a new access token and refresh token pair.
+     *
+     * @param {string} refreshToken
+     * @param {RefreshOptions} options
+     * @return {Promise<TokenInfo>} Promise for the new access token
+     */
+    const refreshAccessToken = async (refreshToken, options) => {
+      const client = await oauthClient();
+      const tokenInfo = await client.refreshToken(refreshToken);
+      if (options.persist) {
+        saveToken(tokenInfo);
       }
+      return tokenInfo;
+    };
 
     /**
      * Retrieve an access token for the API.
      *
      * @return {Promise<string|null>} The API access token or `null` if not logged in.
      */
-    function tokenGetter() {
+    const tokenGetter = async () => {
       if (!tokenInfoPromise) {
         const cfg = serviceConfig(settings);
 
@@ -209,9 +206,8 @@ export class AuthService extends TinyEmitter {
       }
 
       const origToken = tokenInfoPromise;
+      const token = await tokenInfoPromise;
 
-      return /** @type {Promise<TokenInfo|null>} */ (tokenInfoPromise).then(
-        token => {
       if (!token) {
         // No token available. User will need to log in.
         return null;
@@ -252,13 +248,11 @@ export class AuthService extends TinyEmitter {
           });
 
         return tokenGetter();
-          } else {
-            return token.accessToken;
-          }
-        }
-      );
       }
 
+      return token.accessToken;
+    };
+
     /**
      * Login to the annotation service using OAuth.
      *
@@ -266,18 +260,15 @@ export class AuthService extends TinyEmitter {
      * (if necessary) and then responds with an auth code which the client can
      * then exchange for access and refresh tokens.
      */
-    function login() {
+    async function login() {
       const authWindow = OAuthClient.openAuthPopupWindow($window);
-      return oauthClient()
-        .then(client => {
-          return client.authorize($window, authWindow);
-        })
-        .then(code => {
+      const client = await oauthClient();
+      const code = await client.authorize($window, authWindow);
+
       // Save the auth code. It will be exchanged for an access token when the
       // next API request is made.
       authCode = code;
       tokenInfoPromise = null;
-        });
     }
 
     /**