Port (un)following from CoffeeScript to TypeScript

app/javascript/retrospring/features/user/action.ts

@@ -0,0 +1,45 @@
+import Rails from '@rails/ujs';
+import I18n from '../../../legacy/i18n';
+
+export function userActionHandler(event: Event): void {
+  const button: HTMLButtonElement = event.target as HTMLButtonElement;
+  const target = button.dataset.target;
+  const action = button.dataset.action;
+
+  let targetURL = action === 'follow' ? '/ajax/create_friend' : '/ajax/destroy_friend';
+  let success = false;
+
+  Rails.ajax({
+    url: targetURL,
+    type: 'POST',
+    data: new URLSearchParams({
+      screen_name: target
+    }).toString(),
+    success: (data) => {
+      success = data.success;
+      window['showNotification'](data.message, data.success);
+    },
+    error: (data, status, xhr) => {
+      console.log(data, status, xhr);
+      window['showNotification'](I18n.translate('frontend.error.message'), false);
+    },
+    complete: () => {
+      if (!success) return;
+
+      switch (action) {
+        case 'follow':
+          button.dataset.action = 'unfollow';
+          button.innerText = I18n.translate('views.actions.unfollow');
+          button.classList.remove('btn-primary');
+          button.classList.add('btn-default');
+          break;
+        case 'unfollow':
+          button.dataset.action = 'follow';
+          button.innerText = I18n.translate('views.actions.follow');
+          button.classList.remove('btn-default');
+          button.classList.add('btn-primary');
+          break;
+      }
+     }
+  });
+}