Fix lint nits

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

@@ -6,7 +6,7 @@ export function userActionHandler(event: Event): void {
   const target = button.dataset.target;
   const action = button.dataset.action;
 
-  let targetURL = action === 'follow' ? '/ajax/create_friend' : '/ajax/destroy_friend';
+  const targetURL = action === 'follow' ? '/ajax/create_friend' : '/ajax/destroy_friend';
   let success = false;
 
   Rails.ajax({

app/javascript/retrospring/utilities/on.ts

@@ -1,7 +1,9 @@
-export function on(type: string, selector: string, callback: Function): void {
+type OnCallbackFunction = (event: Event) => void;
+
+export function on(type: string, selector: string, callback: OnCallbackFunction): void {
   document.addEventListener(type, (event: Event) => {
     if ((event.target as HTMLElement).matches(selector)) {
       callback(event);
     }
   });
-};
+}