diff --git a/js/src/common/Session.js b/js/src/common/Session.js
deleted file mode 100644
index ca89f310a9..0000000000
--- a/js/src/common/Session.js
+++ /dev/null
@@ -1,56 +0,0 @@
-import app from '../common/app';
-
-/**
- * The `Session` class defines the current user session. It stores a reference
- * to the current authenticated user, and provides methods to log in/out.
- */
-export default class Session {
-  constructor(user, csrfToken) {
-    /**
-     * The current authenticated user.
-     *
-     * @type {User|null}
-     * @public
-     */
-    this.user = user;
-
-    /**
-     * The CSRF token.
-     *
-     * @type {String|null}
-     * @public
-     */
-    this.csrfToken = csrfToken;
-  }
-
-  /**
-   * Attempt to log in a user.
-   *
-   * @param {String} identification The username/email.
-   * @param {String} password
-   * @param {Object} [options]
-   * @return {Promise}
-   * @public
-   */
-  login(body, options = {}) {
-    return app.request(
-      Object.assign(
-        {
-          method: 'POST',
-          url: `${app.forum.attribute('baseUrl')}/login`,
-          body,
-        },
-        options
-      )
-    );
-  }
-
-  /**
-   * Log the user out.
-   *
-   * @public
-   */
-  logout() {
-    window.location = `${app.forum.attribute('baseUrl')}/logout?token=${this.csrfToken}`;
-  }
-}
diff --git a/js/src/common/Session.ts b/js/src/common/Session.ts
new file mode 100644
index 0000000000..0997bf99fc
--- /dev/null
+++ b/js/src/common/Session.ts
@@ -0,0 +1,55 @@
+import app from '../common/app';
+import User from './models/User';
+import { FlarumRequestOptions } from './Application';
+
+export type LoginParams = {
+  /**
+   * The username/email
+   */
+  identification: string;
+
+  /**
+   * Password
+   */
+  password: string;
+};
+
+/**
+ * The `Session` class defines the current user session. It stores a reference
+ * to the current authenticated user, and provides methods to log in/out.
+ */
+export default class Session {
+  /**
+   * The current authenticated user.
+   */
+  user: User | null;
+
+  /**
+   * The CSRF token.
+   */
+  csrfToken: string;
+
+  constructor(user: User | null, csrfToken: string) {
+    this.user = user;
+    this.csrfToken = csrfToken;
+  }
+
+  /**
+   * Attempt to log in a user.
+   */
+  login(body: LoginParams, options: Omit<FlarumRequestOptions<any>, 'url' | 'body' | 'method'> = {}) {
+    return app.request({
+      method: 'POST',
+      url: `${app.forum.attribute('baseUrl')}/login`,
+      body,
+      ...options,
+    });
+  }
+
+  /**
+   * Log the user out.
+   */
+  logout() {
+    window.location.href = `${app.forum.attribute('baseUrl')}/logout?token=${this.csrfToken}`;
+  }
+}
diff --git a/js/src/common/states/AlertManagerState.ts b/js/src/common/states/AlertManagerState.ts
index 2ed21e5e08..076322ac16 100644
--- a/js/src/common/states/AlertManagerState.ts
+++ b/js/src/common/states/AlertManagerState.ts
@@ -55,7 +55,7 @@ export default class AlertManagerState {
   /**
    * Dismiss an alert.
    */
-  dismiss(key: AlertIdentifier): void {
+  dismiss(key: AlertIdentifier | null): void {
     if (!key || !(key in this.activeAlerts)) return;
 
     delete this.activeAlerts[key];
diff --git a/js/src/common/utils/extractText.js b/js/src/common/utils/extractText.ts
similarity index 52%
rename from js/src/common/utils/extractText.js
rename to js/src/common/utils/extractText.ts
index 9604d6a16c..e405f3c7c4 100644
--- a/js/src/common/utils/extractText.js
+++ b/js/src/common/utils/extractText.ts
@@ -1,15 +1,14 @@
+import type Mithril from 'mithril';
+
 /**
  * Extract the text nodes from a virtual element.
- *
- * @param {VirtualElement} vdom
- * @return {String}
  */
-export default function extractText(vdom) {
+export default function extractText(vdom: Mithril.Children): string {
   if (vdom instanceof Array) {
     return vdom.map((element) => extractText(element)).join('');
   } else if (typeof vdom === 'object' && vdom !== null) {
-    return vdom.children ? extractText(vdom.children) : vdom.text;
+    return vdom.children ? extractText(vdom.children) : String(vdom.text);
   } else {
-    return vdom;
+    return String(vdom);
   }
 }