Document IEvent.properties, fix IWelcomeScreenLoad

IEvent.properties is a placeholder that needs to be overriden by extenders for type validation to take place.

IWelcomeScreenLoad should have had properties declared for it. Because it didn't, a faulty call using it was possible.

src/PosthogAnalytics.ts

@@ -35,12 +35,12 @@ import SettingsStore from './settings/SettingsStore';
  */
 
 interface IEvent {
-    // The event name that will be used by PostHog.
+    // The event name that will be used by PostHog. Event names should use snake_case.
-    // TODO: standard format (camel case? snake? UpperCase?)
     eventName: string;
 
-    // The properties of the event that will be stored in PostHog.
+    // The properties of the event that will be stored in PostHog. This is just a placeholder,
-    properties: {};
+    // extending interfaces must override this with a concrete definition to do type validation.
+    properties: {}
 }
 
 export enum Anonymity {
@@ -73,6 +73,7 @@ interface IPageView extends IAnonymousEvent {
 
 export interface IWelcomeScreenLoad extends IAnonymousEvent {
     eventName: "welcome_screen_load";
+    properties: Record<any, never>;
 }
 
 const hashHex = async (input: string): Promise<string> => {

src/components/views/auth/Welcome.tsx

@@ -76,6 +76,6 @@ export default class Welcome extends React.PureComponent<IProps> {
     }
 
     componentDidMount() {
-        getAnalytics().trackAnonymousEvent<IWelcomeScreenLoad>("welcome_screen_load", { foo: "bar" });
+        getAnalytics().trackAnonymousEvent<IWelcomeScreenLoad>("welcome_screen_load", {});
     }
 }